Home
BEGINNING URBAN SKYWATCHING
By Bob Parvin, an Urban Skywatcher
Urban skywatching?! I'll bet you're thinking that urban skywatching is about as exciting as urban fly casting. I live near the heart of San Francisco, and I will concede that when I step out on my north-facing balcony I may only see the crescent Moon or a planet near the horizon and one or two stars: Capella in the winter or Vega in the summer or both in the spring and fall. However, if I could only see these objects, I would still be an urban skywatcher. It's fun getting to know something about the major objects and watching them come and go through the night and through the seasons. Fortunately, I can go to the south side of our building where I can watch the Moon as it progresses through its phases, check on whichever of the five naked-eye planets are up, and enjoy several impressive stars and star patterns. Even in my light-polluted sky I can see some of the best and brightest things with the naked eye and much more, as we shall see later, with the aid of binoculars.
I was reared on a farm in Eastern Washington 80 miles from the nearest city and 8 miles from the nearest dimly-lit small town. When the Moon was not out, the sky was so dark we could barely find our way to the outhouse. Since it was during the depression, skywatching would have been a great entertainment that we could have afforded, and there were few alternatives anyway. The only star we knew was the "North Star," and the only pattern we recognized was the Big Dipper. When I did have a dark sky, I didn't appreciate it; now I appreciate it but don't have a dark sky. (Isn't that the way life goes?) But as you will soon find out, there is plenty to see even in the city.
I wish every child could have the opportunity to learn about the night sky. If you would like to introduce a child to skywatching, pull up Jack Horkheimer: Star Gazer. If an interest is sparked, check out Resources for Beginners. Children need some good readable nature books to increase their knowledge of their world and spark an interest in recreational reading, and astronomy may be just the ticket.
I encourage people of all ages to check out skywatching and see if it is something they might enjoy. I didn't take up skywatching until I retired, and it has greatly enriched my retirement years. Some people may be content to scan the night sky with the naked eye in the course of their other outdoor activities. Others may do a little intentional skywatching with the aid of binoculars in their backyard or at a vacation spot. Still others may join an amateur astronomy club, meet some friendly helpful people, buy a telescope, make skywatching a full-fledged hobby, and live happily ever afterwards.
For a quick delightful introduction to the universe check this out: Galaxy
If you would like to see what skywatching is all about and learn more about the objects in the night sky, I invite you to read on and skim over whatever makes your eyes glaze over. Whenever you want more extensive and authoritative information, there is usually a link that will take you to it through the magic of the Web.
When you are ready to try a little skywatching, read the "Getting Started" section. Then click on The Evening Sky Map to see a handy monthly two-page guide that you can print out. (Choose the hemisphere, month, and PDF.) It lists naked eye, binocular, and telescope objects that you can see and has symbols for them on the detailed chart. "Don't leave home without it!"
Feel free to print this Web page for your own use. If you only want to print a section of it and you are using Microsoft Internet Explorer, select (highlight) the section, click File, Print, in the Print dialog box under Print Range click Selection, and OK. There are also several links to things such as sky charts that you may want to print.
Before you chide me for not having an index to this on-line booklet, remember that the fast "find" function in your browser is better than an index. With the Internet Explorer, for example, you click "Edit" and "Find" or just Ctrl+F and fill in a few letters for the term you want. You will quickly be taken to the first place where that string of letters occurs and then to subsequent places.
If you have any easy questions or have comments, suggestions, elaborations, criticisms, or corrections to offer, please e-mail them to me, Bob Parvin: bandcparvinXhotmail.com (Substitute @ for X. I'm trying to hide my address from spammers.)
.
The Moon is fascinating because we can see so much interesting detail with binoculars or a small telescope. (I like 12-power binoculars or better or a telescope to view it.) No, I don't see the "man in the Moon" or the "lady with the big hairdo," but we can see rugged mountains, all of those fabulous craters, an "ocean," lots of placid "seas," and a few "bays," "lakes," and "marshes."
The Moon has a diameter of 2,160 miles at the equator and averages nearly 240,000 miles from Earth. Which object would appear to be the largest: the Sun or the Moon, if you were foolish enough to look at the Sun? Actually from our vantage point they both have an angular width of 1/2 degree, which is half the width of your little finger held at arm's length.
The Moon orbits from west to east 1/2 degree (its width) per hour causing it to rise about 50 minutes later every day, so why does it appear to rise in the east and travel west? The Earth turns from west to east at the rate of 15 degrees per hour, so it overtakes the Moon and makes the Moon appear to be going backwards to the west just as a slow moving car seems to us to be moving backwards when we overtake it in a faster moving car.
You are quite aware of the fact that the Sun's apparent path goes high overhead in the summer at mid-northern latitudes and lower to the south in the winter due to the fact that the Earth is tipped 23.5 degrees on its axis. But have you noticed that when the Sun's path is high the Moon's path is low? The reason is that in the summer when the northern hemisphere is tipped toward the Sun, it is tipped away from the Moon and planets when they are on the opposite side of Earth from the Sun. Since the June Moon is lower in the southern sky, it appears larger and more yellowish making it more romantic.
Moon Phases
More often than not you won't notice the Moon because it may be up mostly during the day or it may rise after your bedtime. The time of the Moon's rising depends upon where it is in its orbit or, in other words, the phase of the Moon. It orbits the Earth once in a lunar month of about four weeks during which its appearance from Earth goes through these phases:
- New Moon: It rises and sets with the Sun and is between Earth and Sun, so we can't see any of the sunlit side at night. (Three celestial bodies lined up in a row is called a "sysygy" /SIZ-uh-jee/. I couldn't resist getting in that three "y" word.)
- Waxing (growing) Crescent Moon: It follows closely behind the Sun all day long, but it isn't brightly lit until sunset shortly before it sets in the west. (See the picture at the top of the page taken October 29, 2000, from my home in San Francisco.) The horns of the crescent are pointed to the left. When the crescent is still thin, notice that the unlit part of the Moon is very faintly illuminated by light reflected from Earth called "earthshine."
- First Quarter: Its face is half lit, so why isn't it the first half Moon? The first quarter refers to the fact that it is one-quarter of the way through its phases. It rises around noon and appears high in the southern sky at sunset and sets in the west about midnight.
- Waxing Gibbous Moon: It rises in the afternoon and sets in the west in the early morning.
- Full Moon: It rises at sunset and sets at sunrise. The Full Moon nearest the autumnal equinox is called the Harvest Moon. The second Full Moon in the same calendar month is called a Blue Moon, and it only happens "once in a Blue Moon" (an average of every 33 months). A full Moon is good if you want to spoon but not if you want to appreciate its topography because there are no shadows.
- Waning Gibbous Moon: It rises between sunset and midnight.
- Last Quarter: It rises at midnight and sets the following noon.
- Waning Crescent Moon: It rises between midnight and dawn and sets in the afternoon. The horns of the crescent are pointed to the right.
Did you ever think about this question: Since the Earth is in between the full Moon and the Sun, why isn't every full Moon eclipsed? The Moon's orbit is tipped five degrees from the plane of Earth's orbit, so there are only two points where an eclipse is possible and that is where the Moon's orbit crosses the plane of the Earth's orbit. However, you are much more likely to see a lunar eclipse than a solar eclipse because the Earth's shadow can completely cover the Moon. You can see a full eclipse any place on Earth. On a solar eclipse the Moon's shadow only covers a narrow strip across the Earth, so you usually have to pack your bags and travel somewhere to see it.
Would you like to be able to determine the date of Easter? It falls between March 22 and April 25, and it is said that "Easter Sunday falls on the first Sunday after the first full Moon on or after the March Equinox." However, there are important qualifications. See The Date of Easter.
To see how celestial observation figured into the development of the calendar go to The Stories of Calendars.
To check the dates of the Moon phases, go to the excellent Moon Phase.
Here is an intriguing puzzle: The Moon makes one complete orbit in 27.3 days (sidereal month), but it takes 29.5 days (synodic month) to complete its phases. How come? The reason is that the Earth has moved in its orbit around the Sun, so it takes two more days for the Moon, Earth, and Sun to line up again for a new Moon. To see how this works and to see the phases animated go to Lunar Puzzlers.
|
Some people will enjoy keeping an observing journal to record what they have seen in the night sky. When making observations about the Moon, it is much more explicit to record the Moon's "age," i.e., the number of days since the new Moon, rather than its phase.
Lunar Features
At the city star parties held by our San Francisco Amateur Astronomers, visitors upon seeing the rugged topography of the Moon through a telescope or binoculars for the first time often respond with an exuberant "Wow!" or "Cool!" depending upon their age. They see the awesome effects of eons of bombardment by objects from space. Earth suffered the same bombardment, but erosion, deposition, and other geologic processes have neatly erased the scars. Since the Moon has no atmosphere, it has no weather except for heat and cold and no water or wind erosion. (Having no atmosphere also means that the Moon's sky is pitch black instead of blue. In the daytime it's like being in a baseball stadium at night. The ground is lit up, but the sky is black. To see why the Moon's sky is black and why our sky is blue go to Black Sky?
The most conspicuous lunar features that we can see with the naked eye are large, dark, circular areas that are called "seas" or maria in Latin. Even Galileo upon first seeing them through his crude telescope thought they were indeed seas. The maria were caused by the impacts of very large objects, and the impact basins or craters were subsequently filled by lava making dark lava plains. The southerly lunar highlands are densely cratered without having been flooded with lava and have, therefore, a lighter tone.
For a look at the most conspicuous features of the Moon go to Skywatcher's Guide to the Moon, which you might want to print before you go moonwatching. The labeled features and other features that you may eventually learn to recognize are as follows:
- On a three-day Moon you can get your first look at the roundish Sea of Crises (Mare /MAR-ay/ Crisium). It is the only completely isolated mare. You can use the roughly 300 mile-wide Crisium as a unit of measure describing the location of other features. Below Crisium you will see the Sea of Fertility (Mare Fecunditatis).
- On a six-day Moon look a little to the southwest of Crisium for the Sea of Tranquility (Mare Tranquillitatis), which was less tranquil on that day in 1969 when Apollo 11 made the first manned Moon landing near its western "shore." Look at the medium gray, broad pie-shaped area on the eastern side of Tranquility near Crisium. It is the Marsh of Sleep (Palus Somni; palus means a marsh. Yeah, it does look marshy if you can think of Tranquility as a sea). Now look for Crater Theophilus situated in the southern extension of the Sea of Tranquility near the southwest edge and adjacent to the northwest edge of the Sea of Nectar (Mare Nectaris). Theophilus has a diameter of 65 miles, is very round, is about three miles deep, and has a pronounced central multiple peak caused by the crust rebounding after the impact. Craters are especially impressive when they are fairly close to the terminator (the changing margin between what is lit and unlit).
Look 1-1/2 Crisium widths northwest (left) of Crisium for the Sea of Serenity (Mare Serenitatis). It is bounded on the southwest by the Haemus Mountains and on the northwest by the Caucasus Mountains. When the Moon is 5 days and 18 hours old, both of these ranges catch the light beyond the terminator, so they appear to poke into the darkness. The rugged Apennines range lie just to the northwest of the Haemus Mountains, and they are perhaps the most outstanding range on the Moon. One of its peaks, Mt. Huygens, is the highest peak on the Moon. It rises up to about 25,000 feet. To the north and a little to the east of the Sea of Serenity is another "watery" feature: Lake of Sleep (Lacus Somniorum).
- On an eight-day Moon you can see about half of the large Sea of Rains (Mare Imbrium), just to the west of Serenity. Notice the rugged lunar Alps along the northeast edge. Look just above the Sea of Rain's north edge for an elegant small, round, dark, lava-filled crater (a small walled plain) called Plato.
- On a nine-day Moon look a little below the south edge of the Sea of Rains for Crater Copernicus, which is 60 miles in diameter and 12,600 feet deep. On Sea of Rain's northwest side there is the semicircular range of the Jura Mountains. This range may be the remaining rim of an ancient crater forming the "shoreline" of the Bay of Rainbows or Sinus Iridum. (Sinus in Latin means curve or hollow explaining the name for those pesky hollows in the front of our skulls.) The Jura Mts. rise up to 12,700 feet above the plain, which is over twice the height of the highest peak in the Jura Mts. in France in case you're wondering about that. On the ninth day past the new Moon at just the right time these high mountains reach up beyond the terminator and catch the light and look like a huge teacup handle jutting out from the Moon. You can't catch it at the right time every month. Incidentally, to find the "age" of the Moon at the current time, go to The Moon. Using this source, the age of the Moon was 10 days and 8 hours old when I last saw the "handle" at the perfect time. At the bottom of the Moon notice the rugged crater-riddled Lunar Highlands.
- On the fourteen-day full Moon you can see the full 2,160 mile diameter (or 3,300 surface miles of Moonscape). Along the western edge see the largest lowland lava plain of all, the sprawling Ocean of Storms (Oceanus Procellarum). In it is the brightest spot on the Moon, Crater Aristarchus. Perhaps the darkest spot on the Moon is Crater Grimaldi, which is a little larger than Plato and is two Crisiums below Aristarchus near the limb or edge of the Moon's apparent disk. You can find Crater Kepler one Crisium west of Copernicus.
Below the bottom of the Ocean of Storms look for the Sea of Moisture (Mare Humorum), which is a southwestern look-alike for the Sea of Crises. Just to the east lies the irregular-shaped Sea of Clouds (Mare Nubium). Just to its south is Crater Tycho, a close match for Copernicus. Tycho is 56 miles in diameter and 13,800 feet deep. It has the most conspicuous rays of all craters. They radiate out like longitudinal lines on a tipped globe. These rays or splash marks are a powdery material cast out upon the impact of a large object. Since the rays are still light-colored, the crater is inferred to be relatively young. For an excellent map of the Moon with detailed labeling of features go to Moon Map, which you can print.
Did you know that the Moon turns on its axis? It manages to always show us the same face by turning exactly once in one orbit around the Earth. We didn't know what the far side of the Moon looked like until the Russians sent a spacecraft to photograph it.
|
Get more information about the Moon by clicking The Moon.
Back to the Beginning
You learned in grade school that there are 9 planets, right? The problem was that planets had not been officially defined. On August 24, 2006, the International Astronomical Union finally decided that a planet must meet three criteria: 1) It must orbit the Sun. 2) It must be big enough for gravity to mold it into a round ball. 3) It must have cleared other things out of the way in its orbital neighborhood. By that definition there are only 8 planets. Those objects that orbit the Sun that only meet the size criterion are now called "dwarf planets." At present only three are known: Ceres in the Asteroid Belt and Pluto and "Xena" in the Kuipper Belt. See Eight Planets and New Solar System Designations.
Rather than placing so much importance on the number of planets, Dr. Neil de Grasse Tyson, director of the Hayden Planetarium at the Rose Center for Earth and Space thinks it is much more instructive to think about the five broad families of solar system objects: the four rocky terrestrial planets, the Asteroid Belt, the four gassy Jovian planets, the Kuiper Belt of comets and the Oort Cloud of (long-period) comets. (See Dr. Neil de Grasse Tyson on planets. We don't think of Pluto as a comet, but it is an icy object that would have a tail if it moved in closer to the Sun.
Where would you look for a planet? Remember that the Solar system is "flat," i.e., the orbits of the planets and our Moon are all in about the same plane like peas on a flat glass plate. If we look at the peas from the edge of the plate, they appear in a line. Likewise, when we look "up" at the Moon or the planets, we are looking out into the plane of the orbits rather than looking at the orbits from above or below; therefore, the planets all appear to be lined up. The plane of Earth's orbit is called the ecliptic /ih-KLIP-tik/. Since the Sun is in the plane of Earth's orbit, the ecliptic is also defined as the apparent path of the Sun. Since the Moon's path is close to the ecliptic, look for planets along the path of the Moon.
To see the ecliptic on a sky chart, go to The Evening Sky Map referred to above and click PDF under the current month under the appropriate hemisphere. Look at the chart, and you will see the dashed line for the ecliptic. Notice that if there are any planets up they are along that line. Also notice that the ecliptic cuts through all of the constellations of the Zodiac. That is the only thing that is special about the Zodiac. To read more about the ecliptic and to see the seasonal paths taken by the Sun, go to The Path of the Sun. You will notice that the Sun doesn't rise straight up overhead in our mid-northern latitudes as it does at the equator.
For another excellent sky chart to find the planets and stars and one for which you can specify the time go to Heavens-Above. On first use you must register by completing the form including your country and town so that you will not have to repeat it on subsequent log-ins. You can switch the chart to black on white and print it out. This is a fantastic resource!
Another good site for locating the planets is The Night Sky This Week. To learn more about the planets go to The Nine Planets, which is an excellent site.
All of the planets move from our west to east in their orbits around the sun, but since Earth's rotation overtakes them as it does the Moon, they also appear to be moving westward.
The planets that are between the Sun and Earth are called inferior planets. However, this does not mean that they are not up to snuff; it is just a matter of location. Those that are farther from the Sun than Earth are superior planets. When superior planets are directly opposite Earth from the Sun, they are at opposition, and this is where they are in full phase and the closest and brightest. At opposition they rise at sunset and set at sunrise just as the full Moon does.
Mercury and Venus
The first "rocky" planet out from the Sun is yellowish, sizzling hot Mercury. When it is visible, which is not long at a time, look for it near the horizon in the west shortly after sunset or in the east if you are an early riser. You may need binoculars to find it. The length of Mercury's day relative to the stars (the sidereal /si-DEER-ee-ul/ day) is 52 Earth days and its year is 88 Earth days.
Have you ever noticed the "evening star" that looks like the headlight of an airplane coming in from the west? That's Venus, the second "rocky" planet from the Sun. (See the picture at the beginning of this Web page.) When it is visible, it is the brightest object in the night sky except for the Moon. Although it is a rocky planet, it is so bright because it is covered with very reflective clouds. It is extremely hot because it is close to the Sun and because the carbon dioxide clouds trap the heat. Talk about global warming from the greenhouse effect! Here is an interesting thing about Venus: It's sidereal orbital period is 225 Earth days, and its day is 243 Earth days long. So Venusians are really ready for bed after an incredibly long, hellishly hot Venus day. Since Mercury and Venus orbit between Earth and Sun, they can be seen periodically either before sunrise in the east or in the early evening in the west but never overhead. Due to the position of their orbits, they both have phases. I observe a crescent Venus by using a small telescope with a polarizing filter to reduce the glare.
Earth
Earth, is the "third rock" from the sun. Here are some of its metrics:
- Its diameter is 7,928 miles at the equator. (It is a little flattened at the poles.)
- Its distance from the Sun averages 93 million miles, and this distance is called 1 AU (Astronomical Unit), which is a useful unit of measure within the solar system.
- Its orbit changes from circular to 5% elliptical over a period of 100,000 years. This may be one cause of periodic glaciation.
- Its axial tilt, which causes the changes in seasons is currently 23.4 degrees. This goes from 21.5 to 24.5 over a period of 41,000 years, and this has seasonality consequences.
- It has a wobble called precession, which we will discuss later.
The Earth has two major motions. It rotates on its axis once in 23.93 hours, but for the Sun to return to the same position in the sky after Earth's rotation it takes 24 hours (the Solar day). Why the difference? It is because of the Earth's second motion: it revolves around the Sun in 365.25 days. It takes 4 minutes longer than the sidereal day for the Sun to get back in its same apparent position in our sky. To see how this works go to Time and Seasons.
We say that the Moon orbits Earth, but actually both the Moon and Earth orbit a center of mass between them. For an analogy picture a big fat guy and a petite lady dancing, and they are swinging around and around a pivot point between them as they dance around the perimeter of a slightly elliptical dance floor. She swings way out, but he stays close to the pivot point. The mass of Earth is 81 times that of the Moon, so the Earth makes very small circles. Speaking of Earth's mass, are you wondering how many one gram paper clips it would take to balance the mass of Earth? The answer is 6 followed by 27 zeroes.
A unique feature of Earth is that it has a very hot core that drives the tectonic cycle, which causes ocean floor spreading and ocean plate destruction. This cycle moves the continents since they ride on the tectonic plates. By contrast the Moon has a very stable crust because it has a cooler core.
Mars
Known for ages as the "red star," Mars is the fourth "rocky" planet, and it is an average of 1.5 AU from the Sun. It is about half the diameter of Earth, which makes it small in our eyepieces, but the ice caps can be seen with clear, dark skies with a good small telescope. It is best seen during opposition. It requires 687 Earth days to make one revolution around its orbit in relation to the stars. Its day is 24.6 hours, which is remarkably close to our day. It has two very tiny, potato-shaped moons (Phobos and Deimos, Greek for "fear" and "panic," named after the chariot horses of the Greek God of War) but no Martians or canals. It does appear to have dry riverbeds.
If you are planning a trip to Mars, you must plan way ahead because the proper alignment for lift-off only occurs every 26 months, and you must stay there about three months before returning. You also better check the weather reports and pack your warmest clothes. See What is the typical temperature on Mars?
Asteroid Belt
The asteroid belt is a bunch of rocks orbiting between Mars and Jupiter. The belt has families of asteroids orbiting in different strands and having different compositions. Three of these families pose some danger to Earth. The first asteroid discovered was Ceres, which is by far the largest asteroid with a diameter of about 625 miles. Now we know that there are billions of automobile-sized and larger asteroids. On February 13, 2001, an American spacecraft called NEAR orbited and landed on the asteroid Eros, a potato-shaped "rock" about 25 miles long. Several of the larger asteroids can be seen through small telescopes, but it is difficult to find them without something like the Observer's Handbook. For more information on asteroids go to Asteroids.
The Four Big Gassy Jovian Planets
Jupiter is way out 5.2 AU from the Sun. It requires almost 12 Earth years to make it around its orbit. The farther a planet is from the Sun the longer it takes to make it around its orbit, in accordance with Kepler's third law of planetary motion. Jupiter's year is long, but due to its extremely rapid rotation its day is short, only 9.9 hours. It's the largest planet and has a diameter that is 11 times that of Earth. Through a small telescope the most apparent feature is two brownish cloud bands circling the planet. With a good large telescope and good seeing we can see the Great Red Spot hurricane swirling between two opposing wind currents. If you see a bright white planet high overhead, it is Jupiter, not Venus.
Jupiter has four large moons plus a large number of tiny moons and more are being discovered. With 10-power binoculars on a tripod we can see the four Galilean moons, which were large enough to have been discovered by Galileo with his crude telescope in 1610. They are, from the inside out, Io, Europa, Ganymede, and Callisto. In our binoculars the moons look tiny but be aware that the largest moon, Ganymede, is the largest moon in the Solar system; it is even larger than Mercury. The innermost of the four moons, volcano-belching Io, is 20% more massive than our Moon and has a larger orbit, which it scoots around in only 1.8 days. You will notice that from our perspective the moons always appear in a straight line because they all orbit in the same plane, and we are looking into the edge of the plane of their orbits rather than face on. The lineup of the moons from our perspective is constantly changing. Quite often one will be hidden behind Jupiter or passing in front.
The next planet is gorgeous Saturn, which is almost as large as Jupiter but almost twice as far away. The only person looking at Saturn through one of my telescopes that didn't make an exclamation was a blase 10-year old reared on video games. Saturn has a conspicuous thin ring system consisting mostly of pieces of ice, rock, and dust. I can sometimes barely see some of the gap between the ring and the sphere with 20-power binoculars, but 40-power on my small telescope gives a much better view. There are many gaps in the ring system, and the largest is called the Cassini Division, which can be seen with a small telescope. Of Saturn's many moons, Titan, can be seen with medium-powered binoculars.
The next two "gassy" planets are Uranus and Neptune. They are big, but since they are so far away, they are faint. With binoculars and a chart you may be able to find and see greenish-blue Uranus in the city, but blue Neptune is more difficult.
THE PARADE OF THE PLANETS! The five naked-eye planets lined up in the west to give us some extraordinary sights during April and May, 2002. The night of April 17 is one I will long remember. A while after sundown I saw Mercury, Venus, Mars, Saturn, a crescent Moon, and Jupiter equidistant apart parading down the Zodiacal boulevard to the horizon! The frosting on the cake was that the planets were routed down through the west side of the spectacular Winter Hexagon, which we will discuss later. It just doesn't get much better than this, and it won't happen again until 2040!
|
Kuiper Belt and the Oort Cloud
Pluto is the best known Kuiper Belt icy object of which there are many, but one, officially named Eris, has been found that is larger than Pluto. Pluto has a close relatively large companion (Charon), and they orbit around a common center of mass. Pluto is in a tipped orbit between 30 and 50 AU from the Sun. Its eccentric orbit sweeps inside of Neptune's orbit at times.
The Oort cloud of comets is out beyond the Kuiper Belt. It is probably the home region of perhaps 100 billion potential long-period comets, and who knows what else may be found out there.
Are there Earth-like planets around other stars in our Galaxy? According to Wikipedia, "As of June 2009, 353 exoplanets are listed in the Extrasolar Planets Encyclopaedia." More are being added almost monthly. Many massive planets have been detected by the way their stars wobble. (Remember the fat guy circling with his partner?) Another method used to detect an Earth-sized planet is to measure the reduction of light occurring when the planet moves across the face of star. Planets around two stars have been directly observed. A planet orbiting the bright star Fomalhaut (Fomalhaut b) was observed using the Hubble space telescope.
Back to the Beginning
Comets may look like fuzzy stars moving so slowly that you won't notice any movement. They are huge "dirty snowballs" consisting of ices, rocks, and dust. The nucleus may be several kilometers in diameter. It is surrounded by the glowing coma that may be 200,000 kilometers in diameter. The visible tail that billows out when the comet nears the Sun may be millions of kilometers long. Comets and asteroids that wander into the Earth's orbit can be hazardous to our health as the dinosaurs discovered. Astronomers were astounded on July 16, 1994, to see the impact areas on Jupiter made by fragments of Comet Shoemaker-Levy 9. If you would like to know the probability of a wayward asteroid ruining your day, go to Current Impact Risks. (Look at the Torino Scale that rates the risk from 0 to 10.)
Long-period comets are thought to come from the Oort comet cloud. They may be 20,000 to 100,000 AUs (as much as a couple of light years) from the Sun, and they come by no oftener than 200 years and as infrequent as millions of years.
Short-period comets may come from the closer Kuiper Belt about 35 to 1,000 AUs from the Sun. The "Jupiter family" of comets have periods of less than 20 years, and the "Halley family" has periods of from 20 to 200 years.
There are about 150 predictable or periodic comets such as the most famous Halley's Comet. There have been many more unpredictable comets. For more interesting information go to Comets.
Meteoroids are pieces or particles of rock most of which are the debris of comets and asteroids. When meteoroids enter our atmosphere and burn, their trails are called meteors or falling or shooting stars. The few that hit the ground are then called meteorites. Comets leave debris behind in their orbits, and when Earth periodically passes through their orbits, we experience "meteor showers" that are named according to the constellation from which they appear to be coming. The Perseid (circle August 11 on your calendar and check exact date), Geminid, and Leonid meteor showers can be awesome events when the Moon is not up. To find out more, go to Meteorites.
Back to the Beginning
What are stars? They are other suns, but they may be larger or smaller or at a different point in their life cycles. They are so far away they appear to be only twinkling points of light rather than disks like the Sun. They may not look very impressive, but wait until you learn more about them. |
To get in the right mood for stargazing, read a delightful poem, Stars, by Sara Teasdale.
How many stars can be seen with the naked eye? It depends of how good your eyes are and how clear and dark the sky is. Most people under a dark sky can't see more than roughly 6,000 of which only half are above the horizon at any one time. This is an infinitesimal sample of the billions of stars, perhaps 200 billion, in our Milky Way Galaxy.
"Milky Way" is also used in a much older sense to mean the milky-looking band streaming across a relatively dark sky. It is a "cross-sectional" view into the disk of our Galaxy. In the summer it is brightest toward the southern horizon because we are looking in toward the center of the Galaxy.
Ours is a spiral galaxy proportioned like a compact disk except that the Galaxy has a bulge at the center. It has six coiling arms in the spiral. The Solar System is located at the inside edge of the short Orion Arm a little over half way out from the center of the Galaxy. All the stars that you see on a starry night with the naked eye are in our galactic suburb, and few of them are much beyond a thousand LY from us.
Among galaxies there are many variations on the spiral, but two-thirds of the galaxies are ellipticals and a smattering are irregulars. The disk of our Galaxy is over 100,000 LY (light years) wide and about 2,000 LY thick except at the central bulge where it is thicker. (A light year is the distance light travels in a vacuum in one year and is equal to nearly 6 trillion miles.)
Enclosing the disk is a very sparse spherical region above and below the plane of the Galaxy called the halo where only a few stars reside except for globular clusters.
At the beginning of the Twentieth Century our Milky Way Galaxy was generally thought to be synonymous with the Universe. In 1923 Edwin Hubble proved that there are other galaxies. Even he would have been astounded to know just how many! There are in the order of 100 billion other galaxies. He studied the spectra of galaxies from his vantage point at the Mt. Wilson Observatory near Pasadena and found that most galaxies are moving away from us. Not only that, but he also found that the farther away they are the faster they are moving. Therefore, the Universe is not only incomprehensibly large, but it is also expanding. If we "run the film backwards," we can deduce that the Universe began at a point and started its expansion with the Big Bang. The Universe is not only expanding, but it is expanding at an increasing rate! Why?? It may be because of that mysterious "dark energy" that you may have heard about.
Part of the Big Bang theory was that there would be a Big Bang afterglow consisting of microwave radiation. Evidence of this radiation was discovered in 1965. Part of the "snow" that you see on your TV screen when off channel and part of the "sh---" we hear between FM stations is caused by this radiation. By studying this radiation astronomers have determined that the age of the Universe is 13.7 billion years plus or minus 1%, which is an astounding level of accuracy.
Galaxies are not scattered around the Universe at random; there is structure. To better understand this "bubble-ridden" structure, go to Galaxies, Clusters, and Superclusters. Our Galaxy and our sister galaxy, Andromeda, dominate a group of some 20-plus galaxies in what is prosaically named "The Local Group." It is part of a larger group called the Virgo Cluster that in turn is part of "The Local Supercluster," aka The Virgo Supercluster. For more go to Virgo Cluster of Galaxies. Astronomy has come a long way: Not too long ago it was strongly believed that the Earth was the whole enchilada under crystal spheres decorated with little "lights."
In case you are planning a tour of the local galaxies here is a nice map for you: The Universe within 500,000 Light Years, The Satellite Galaxies.
Star Movement
From our spinning Earth, stars appear to move from east to west about 15 degrees per hour just as the planets seem to move. When half way across the sky, a star culminates or reaches the meridian, which is an imaginary line midway between the eastern and western horizons. The dates and time of culmination are given for the 15 brightest stars in the Northern Hemisphere in the star table in the APPENDIX. Since the Earth also moves in its orbit around the Sun, our view of the stars shifts one-degree per day to the east. This also means that a given star rises four minutes earlier each day.
Do stars move with respect to the Sun? They do, but not enough for you to notice. Their movement (space velocity) has two components. The first is toward or away from us (radial velocity) measured in kilometers per second. The second component is across our line of sight (tangential velocity) and the angular displacement in a year's time is called the star's proper motion expressed in arc seconds. Our North Star, Polaris, is approaching the Sun with a radial velocity of 10 miles per second. Its proper motion is a scant 0.046 arc seconds per year toward the true north celestial pole.
These measurements are made relative to the Sun, but what is the Sun's motion? By averaging the motions and velocities of stars around us, the Sun was found to be moving 20 Km/Sec in about the direction of Vega.
How is it possible to measure the movement of stars and galaxies toward or away from us? Well, astronomers can analyze a star's light, which can be separated into its spectrum or "rainbow" of colors of different wavelengths. Under certain conditions the spectrum shows dark lines crossing the continuous band of colors. When a star is moving toward us, the light waves bunch up and the dark lines shift toward the blue (blueshift). When the star is moving away from us, the light waves are stretched out and the dark lines shift toward the red (redshift). By the way, the spectral lines also reveal the chemical composition of a star, but that is another interesting story that we will get to later.
Star Brightness
The most noticeable characteristic of stars is that they vary in apparent brightness. Originally the brightest stars were called stars of the first magnitude. Since some of these stars turned out to be much brighter than others, the brightest ones were given minus magnitudes. Each magnitude is 2.5 times brighter than the next lower magnitude. A minus 1 magnitude is 2.5 times brighter than a 0.0 magnitude star, which is 2.5 times brighter than a 1st magnitude star. A 1st magnitude star is about 100 times brighter than a 6th magnitude star, which for most people is about the limit for the naked eye in clear dark skies away from cities. (2.5 raised to the fifth power is 98.)
The apparent brightness of a star depends upon its innate luminosity and its distance from us. One measure of luminosity is a comparison to the Sun, e.g., 10 Suns. Another measure is absolute magnitude, which is what the apparent magnitude would be if the star were 10 parsecs (32.6 LY) away. The apparent magnitude of the Sun is a blinding minus 26.7, but its absolute magnitude is a piddling +4.8.
Distances to the Stars
How far away are the stars that we can see? Except for our Sun they are incredibly far away! The distance to the closest star out from the Sun, Proxima Centauri, is 4.3 LY away. When we look at the stars, we are not only looking far out into space, but we are also looking way back in time, which is extraordinary when you think about it. When we see a star, we see it as it was when the light from it left on its long journey to our eyes.
You are probably wondering how they measure the distance to a star. Direct measurements can only be made by the parallax method. To understand parallax, hold up a finger at arm's length against a background such as the wall. Close one eye and notice where the finger is with respect to the background. Now close the other eye and look again and see how far the finger shifted against the background. Now repeat the experiment with the finger 6 inches in front of your nose. Notice that the angular shift against the background is much more.
Astronomers note the position of a subject star with respect to a distant background star. Then, instead of shifting from one eye to the other, they wait six months to when Earth is at the other side of its orbit and record the apparent shift of the subject star in respect to the more distant star. The parallax angle is one-half of this angular shift. The greater the parallax angle, the closer the subject star is. The distance of a star having a parallax angle of one second of arc is one parsec, which equals 3.26 LY. The Hipparcos spacecraft (1989-93) produced much more accurate parallax measurements than previous land based measurements, but the distances to stars over 500 LY away are increasingly crude. My planetarium computer program indicates that the distance to Deneb, the star in the tail of The Swan, is 3,261.6 LY away. That is a calculated figure carried out to a meaningless precision. The distance is probably closer to 1,500 LY. To see illustrations of parallax go to The ABC's of Distances, and when it gets to complicated skip on to the other sections. For a simpler page on distances, go to Distances to the Sun and Stars.
To measure distances to other galaxies, there are other indirect methods. There is a type of variable star called Cepheid variables (after Delta Cepheid) that changes in brightness over a fixed period. It was discovered that the longer the period the greater the luminosity. By comparing the apparent magnitude of a star to the calculated luminosity astronomers are able to estimate the star's distance. By the way, Polaris is a Cepheid variable with a small fluctuation in luminosity. Another indirect method of estimating distance is to look at a stars spectrum and determine the innate luminosity, compare that with the apparent magnitude, and infer the distance.
The Constellations
Ancient middle-eastern people saw constellations of stars to which they assigned the familiar mythical figures such as Orion the Hunter and created imaginative legends about them. Modern astronomers adopted these star patterns for identifying constellations, but to avoid uncertainty about where things are located, they drew boundaries around each constellation so that they have become precisely delineated sections of the sky. They have officially recognized 88 constellations. Since we can't see their boundaries except on a star chart, we still think of constellations in terms of their signature star patterns rather than as areas of the sky.
There are 12 signs of the Zodiac that were once more or less aligned with the constellations that straddle the ecliptic, which is the apparent path of the Sun. Due to the way in which the modern official constellation boundaries were drawn, the Sun's path actually goes through 13 constellations. Between November 29 and December 17 the Sun is now in Ophiuchus /oh-fee-U-cuss/ the Serpent Bearer rather than Sagittarius. The tip of Ophiuchus slices down between Sagittarius and Scorpius, and in so doing it severs Serpens the Serpent into two separate parts making it the only divided constellation.
|
Stars in a constellation that appear to be close together may be far apart due to differences in their distances from us. For example, in the Big Dog constellation the head of the dog, Sirius, is only 8.6 LY away, but the tail of the dog, Aludra, is roughly 3,000 LY away. So the star patterns that we see from Earth are dependent upon our location in the Galaxy and our lines of sight.
Star patterns that are within a part of a constellation or across constellations are called asterisms. For example, the Big Dipper is an asterism within the Big Bear constellation. The three stars making up the Summer Triangle asterism are in three different constellations.
Star Names
When you look at a star chart, you will see stars named in various ways. The common names may be shown for the brighter stars such as Aldebaran in Taurus. You will see many stars with Greek letter names. Their full Bayer name consist of the Greek letter and the genitive form of the constellation. For example, Aldebaran is Alpha Tauri. The brightest star in the constellation is usually but not always awarded the Alpha designation.
Flamsteed numbers may be shown on the chart for other fainter stars such as 16 Tauri. The numbers rise going from east to west in a constellation.
Colors of Stars, Classes, and the HR Diagram
When you have looked at stars, you noticed differences in brightness, but did you notice the differences in color? Color is important because it is closely correlated with stellar mass. In order to better understand stellar differences we need to have a look at one of the most useful tools in astronomy called the H-R Diagram. Hertzsprung and Russell independently plotted the absolute magnitudes of stars against their spectral classes (which correlate closely with colors and temperatures). These classes are represented by the letters OBAFGKM (Oh Be A Fine Girl Kiss Me). The "O" class includes the bluish (the shortest wavelength), hottest (up to 50,000 Kelvin), and largest hydrogen-burning stars. The other classes in descending order of temperature are: B--bluish-white, A--white, F--white (cooler), G--yellow-white, K--yellow-orange, and M--orange-red.
Most of the stars lie in a central band called the main sequence sloping downward across the HR Diagram. They are still "burning" hydrogen at their cores. Not surprisingly, the hottest stars are the largest, most bluish, and most luminous stars on the main sequence. The coolest main sequence stars, the red dwarfs at the other end of the spectrum, are the least luminous and smallest. What was surprising was bands of stars above the main sequence that were very luminous but cool and reddish. To be so luminous but cool, they have to be very large. They are the orange or red giants and red supergiants. The white dwarfs, which are very hot but not very luminous because of their small size, lie below the main sequence in the H-R Diagram.
Stars have been classified into luminosity classes that correlate with the masses of the stars, and this is depicted by their location on the HR Diagram. The supergiants (class I), the bright giants (class II), giants (class III), subgiants (class IV), main sequence stars (class V), and white dwarfs (class D). Now we can understand the classification of the Sun, which is a G2 V star. This means that it is in the yellowish-white spectral class G (the 2 means its at the hotter end of G) of medium temperature, and it is in luminosity class V, which means that it is a main sequence star. So the Sun is a medium hot star still burning hydrogen at its core.
The H-R Diagram makes a nice graphic summary of the classes of stars and their characteristics. For an extensive treatment of star color and the H-R Diagram go to Dr. Jamie Love's site, Color Designation and the HR Diagram.
The Life Cycle of Stars
Where do stars come from? They are born in celestial nurseries called nebulas consisting of humongous clouds of gas and dust. Under certain conditions the gas and dust condenses into globs that are compressed and heated by gravity. If they are massive enough, the gravitational squeeze is enough to cause hydrogen fusion at the core as in a hydrogen bomb, and a star is born. Hydrogen is converted to helium plus enormous amounts of heat causing an outward pressure that is countered by the inward pull of gravity so that a balance is preserved. If the mass is not quite adequate to start hydrogen fusion, the result is a failed star called a brown dwarf. If it is even smaller, it may become a big gassy object like Jupiter.
So now that we have a newly minted star, how long will it shine? That depends upon how large it is, but the relationship between size and life expectancy may surprise you. Smaller stars like the Sun "burn" their hydrogen fuel frugally and live to a ripe old age of perhaps 10 billion years or more. The Sun may last another 5 billion years or more before it becomes a swollen red giant. The Sun's final act will be to throw off its outer shell, which becomes a planetary nebula (such as the Ring Nebula), which is neither planetary nor a nebula. (For a great collection of Hubble images go to Images of Galactic Planetary Nebulae.) What's left at the core is a very hot stellar "cinder" called a white dwarf, which is so dense that a tablespoon of its matter weighs as much as an automobile. It eventually cools down and becomes a cold stellar cinder or black dwarf.
The largest stars have a much shorter but more spectacular life. They voraciously burn up their core hydrogen and then fuse helium, carbon, oxygen, and heavier elements and swell up and become red supergiants. When iron is formed, the fusion ceases and the stars collapse and then rebounds in an explosion called a supernova that may be as bright as all of the other stars in the Galaxy put together. When the awesome light show is finally over, the outer part blown away becomes a supernova remnant (such as the Crab Nebula). In Type-II supernova explosions the core collapses and becomes a neutron star (protons and electrons are crushed together forming neutrons and strange little neutrinos) or, in the case of larger stars, a dreaded black hole. (If you would like to know about these mind-boggling objects go to Black Holes.) A neutron star is so dense that a tablespoon of its matter would weigh as much as a mountain!
Huge amounts of gas and dust containing heavier elements result from supernova explosions and can be recycled into the next generation of stars. All of the heavier elements including those in our bodies were formed in stars, so we are in fact "star dust," if you're a romantic, or "nuclear waste," if you're a cynic. The romance theme is carried further by the fact that some of the interstellar dust consists of diamonds, rubies, and sapphires. No, they're not gemstones suitable for jewelry, but their chemical makeup is about the same.
When you see a bright orangish or reddish star with the naked eye, it is a red giant or super-giant (like Betelgeuse) nearing the end of its life cycle. When you see a bluish star (like Rigel), it is a large main sequence star rapidly "burning" hydrogen and destined for a short life on the fast track.
For an excellent readable, authoritative, and more complete treatment of stars and their evolution go to
The Natures of Stars by Professor James B. Kaler. He provides lots of good information on individual stars at Stars. Another excellent site is Stars.
Back to the Beginning
Do you happen to know the name of the closest star?
That's a trick question. It is, of course, the Sun, our local star. It is on average 93 million miles away or about 8 light minutes. Although the apparent angular width of the Sun is the same as that of the Moon, one-half degree, almost two diameters of the Moon's entire orbit would fit side by side within the diameter of the sun, which is 868,000 miles.
How long is a day? Well, a solar day is, of course, 24 hours. That is the time that it takes for the Sun to cross the meridian twice. On the other hand, it takes 23 hours and 56 minutes, a sidereal day, for a distant star to cross the meridian twice, which is the time that it takes for Earth to rotate on its axis. However, it takes 4 more minutes to get the Sun back on meridian because Earth has moved along its orbit. (Remember the Moon puzzle?) For an graphic illustration of the difference between solar and sidereal time go to Sidereal Time.
The Sun is a huge atomic nuclear reactor. It started out with a 10 billion year supply of hydrogen and has fused about half of it into helium. Astronomers have taken the Sun's Temperature and found it to be about 5,800 Kelvin or 11,000 degrees F at the surface. (Stellar temperatures range from about 2,000 to 50,000 Kelvin.)
Although the Sun's absolute magnitude is only 4.8 making it unimpressive in relation to the visible stars, it is more luminous than 95% of all stars because there are so many small stars of low luminosity.
| WARNING: Never look at or near the sun with binoculars or a telescope (or telescope finderscope) without an adequate full aperture solar filter. You can be permanently blinded in a nanosecond! Don't let small children use these instruments unattended while the Sun is out. Also don't look at the Sun with the naked eye, as if you didn't know better. |
When we look at the Sun through a telescope with a solar filter, the sun looks like an orange with black blemishes on it. These spots, which appear black only because of the contrast, are sunspots. They result from magnetic disturbances and usually come in pairs or groups. The number of sunspots depends upon where we are in the 11-year sunspot cycle. The year 2002 was a year of maximum sunspots. The number of sunspots is significant to us because the Sun radiates the most energy when they are the most numerous.
Sunspot at the equator indicate that the Sun makes a complete rotation in about 25 days, but the time becomes longer closer to the poles. This neatly demonstrates that the Sun is a ball of gas instead of a solid like an orange. The little molecules in a ball of gas don't move in concert as in a solid body.
You will recall that the ecliptic is the Sun's apparent path across our sky. If you look at a star wheel, you will see that the ecliptic crosses the celestial equator about March 21 at midnight, which is the Vernal Equinox, and about September 23, which is the Autumnal Equinox. On these two occasions day and night are of equal length. The ecliptic reaches its most northerly point at the Summer Solstice about June 21, which is the longest day of the year in the northern hemisphere, and the Sun gets directly overhead all around the Tropic of Cancer. The Sun reaches its most southerly point at the Winter Solstice about December 22, which is the shortest day of the year in the northern hemisphere, and the Sun gets directly overhead all around the Tropic of Capricorn.
For more information about the Sun and the planets go to The Sun.
Back to the Beginning
Go to Heavens-Above, log-on, under "Astronomy" click "Whole Sky Chart," change the time to 9:00 PM Standard Time, under "Option" click "Black on white," and print out a chart. On the printed chart draw a circle with the North Star at the center and the northern horizon on the perimeter of the circle. The stars within the circle are the circumpolar stars at your northern latitude.
|
If you stand at the North Pole and look straight up, you will see Polaris /puh-LAIR-iss/ at the zenith. All of the visible stars will circle around Polaris above the horizon and will be visible all night. Such stars that never set at a given latitude are called circumpolar stars. If you stand on the equator, you will see Polaris sitting on the northern horizon, and none of the other stars are circumpolar since they will periodically sink below the horizon. If you live half way between the North Pole and the equator (45 degrees latitude such as in Minneapolis), Polaris will appear to be half way up from the horizon and all of the stars of the Big Dipper are circumpolar.
The Big Dipper is an asterism that is a part of The Big Bear constellation, whose official Latin name is Ursa Major. The handle of the dipper is the long tail (Did bears once have long tails?) of the bear. As every Boy Scout knows, the two stars at the front of the dipper bowl, Dubhe /DOO-be/ (not a very dignified name for a 1.9 magnitude golden giant) at the top front of the bowl and Merak at the bottom front, are the "pointer stars" that point toward Polaris, the North Star. Six of the dipper stars are 2nd magnitude, and the seventh, the one where the handle joins the bowl, is 3.3. Five of the stars (all except Dubhe and Alkaid at the end of the handle) are at about the same distance (from 78 to 84 LY) and have about the same proper motion which is evidence that they constitute an association or loose cluster of stars that were born together in the same nebula.
When we want to tell another observer how far one object in the sky is from another, we use the angular distance or spread. We can measure the angular spread by using parts of our hand, and we can use the Big Dipper as the standard. The spread between the two pointer stars is 5 degrees or the width of three fingers held at arm's length, and this is about the width of field of most home binoculars. The spread between the two top bowl stars is 10 degrees or the width of the fist. The spread between Dubhe and the first star out in the handle is 15 degrees or the distance between the tip of the forefinger and pinky when spread apart. The spread between Dubhe and Mizar, which is at the bend in the handle, is almost 20 degrees or the distance between the tip of the thumb and the pinky when spread wide apart. The total length of the Big Dipper is about 25 degrees. Our pinky at arm's length is about one-degree in width, and you can check that against the width of the Moon, which is about one-half-degree in angular width. Calibrate your own "measuring tools" with these angular distances. Does this work equally well for children and adults? Yes, and you can figure out why.
Mizar /MY-zar/, a 2nd magnitude star, provides a good lesson in double stars. It has a fainter close neighbor called Alcor, 4th magnitude. If you can split the two with the naked eye in a fairly dark sky, you pass an ancient test of vision. The two stars have been known as "the horse and his rider." Mizar and Alcor illustrate an optical double star, which is a pair of stars that only look close together in our line of sight.
Mizar itself illustrates a true binary star system in which the two stars, Mizar A and B, orbit around a common center of mass. Incredible as it may seem, more than half of the stars that we can see with the naked eye are binaries or multiple star systems. (If Jupiter had been 100 times larger, a nuclear "fire" would have started at its core, and it would be a binary companion of the Sun. Wouldn't that be interesting?) Mizar is the first unquestionable binary ever observed (17th Century), and it can be split with a good small telescope. Mizar A by itself is a close visual binary, which until recently was "too close to call" visually, but it was the first binary ever detected spectroscopically. Mizar B was also found to be a spectroscopic binary. Mizar's final distinction is that it was the first star ever photographed.
Polaris is located in The Little Bear constellation (Ursa Minor) and has the distinction of being our present Northern Hemisphere pole star. It is about three-quarters of a degree away from the northern celestial pole (in the opposite direction from Kochab, see below,) and is getting closer. The distinction of being our pole star is passed around among several stars because of the earth's wobble, precession, that was mentioned earlier. So, the Earth's axis traces out a circle that is repeated every twenty-six thousand years. This is caused by the Moon and Sun tugging on Earth's equatorial bulge. (And to make matters more complicated, there is a little wobble superimposed on precession called nutation.)
Precession changes the time of year that the Sun appears to be in a constellation. The astrology columns say I am an Aquarian, but the Sun was actually in Capricornus when I was born. (Dang! I'm not sure I want to be a Capricorn.) Some astrologers may argue that it is the 30-degree units of the zodiac that happen to be named after constellations that are important. That takes the romance out of being an Aquarian! It's really just a matter of mathematics rather than celestial geography.
Polaris is a double star, but splitting it is a good test of the capability of a small telescope. It is also a variable star, but you would hardly notice its periodic changes in brightness. Polaris is at the end of the handle of the Little Dipper. The handle of the dipper is the tail of the little bear (yeah, another long-tailed bear). In the fall season the Little Dipper appears to be pouring its contents into the Big Dipper, and in the spring it is the other way around.
| To find an object in the night sky, we start from a known bright stars and hop from one star to another. This is called "starhopping." |
The Little Dipper may be a little tricky for a beginner to find in urban skies because most of its stars are fainter than those of the Big Dipper. Here's an easy way: Look for Dubhe at the lip of the bowl of the Big Dipper and at Alkaid at the end of the handle of the Big Dipper. (Refer to Catching the Light and move your mouse pointer over the image.) Then look "above" the Big Dipper for the yellowish star, Kochab /KOE-kab/, that forms an equilateral triangle with the end stars of the Big Dipper. Kochab is the lip star of the Little Dipper, and Polaris is at the end of the handle. Since the bowl of the Little Dipper is about 3 degrees deep and 5 degrees long, you can get the whole bowl in the field of view of wide-angle binoculars.
The Little Dipper's bowl provides a handy scale for judging how good your sky is for observing. (Refer to untitled.) The star at the front lip of the bowl (Kochab) is 2nd magnitude, the brightest of two stars at the front bottom of the bowl is 3rd magnitude, the brightest of two star where the handle joins is 4th magnitude, and the brightest of three stars at the back bottom of the bowl is 5th magnitude. Then the next star beyond Kochab opposite from the handle is a 6th magnitude star. If your eyes are dark-adapted and if you can only see Kochab, the 2nd magnitude star, you have a 2nd magnitude sky as far as your eyes are concerned. If you can see no more than the 3rd magnitude star, you have a 3rd magnitude sky and so on.
It is interesting how Kochab resembles Dubhe in the Big Dipper. Both are 2nd magnitude golden giants. (Their color makes them easy to identify in light-polluted skies.) Each is the brightest star in its bowl. Each is the front rim star in their bowls, and each is the fifth star from the end of the handle. Finally, both are about 125 LY away, so they have about the same luminosity (about 140 times that of the Sun).
On the opposite side of Polaris from the Big Dipper look for The Queen (Cassiopeia /kas-ee-uh-PEE-uh/) whose brightest stars form a "W" opening toward Polaris. (Refer to Cassiopeia.) This constellation lies within the "Milky Way" band. A colorful and popular double star visible in small telescopes in the city is Eta Cass (Cassiopeiae). The larger star is very similar to our sun and is only about 20 LY away. Eta Cass is the brightest star on the left side (inside) of the deepest "V" nearest the bottom star, which is orange Schedar (which also has a faint bluish-white companion star.) The "W" points the way to other objects to be discussed later.
Sitting on his throne on the open side of the Queen's "W" is The King (Cepheus /SEE-fee-oos/) wearing his crown that looks more like a house with a steep roof, which points between the "W" and Polaris. (Refer to Cepheus.) This is not a very impressive naked eye constellation, but, befitting a king, it has three of the most humongous stars in the Galaxy. One is RW Cephei, which is a red supergiant, but since it is so distant, it appears to be a faint seventh magnitude star. Better known is VV Cephei that may be the largest binary system known and, due to its location, you could call it the "door knob" of the Cepheus "house." It consists of a huge red supergiant with a giant blue-white companion that causes eclipsing. The binary is so far away that it is only a fifth magnitude star. If the red supergiant were to replace the Sun, it would swallow up Jupiter and extend almost to Saturn! Still better known is the gigantic Garnet Star (Mu Cephei), which may be the reddest star in the Northern Hemisphere skies. To find it, look in the middle of the "basement" of the house. It is a fourth magnitude star that you can enjoy with binoculars. Cepheus also contains Delta Cephei that is the prototype of "Cepheid variables," which were discussed above.
The circumpolar area at mid-latitudes is like the face of a clock. The Big Dipper makes one complete turn around the Polaris in 24 hours. It also clocks the seasons. If we observe consistently at about 9:30 PM ST, on April 15 the Big Dipper's two pointer stars are pointing straight down, on July 15 straight east, on October 15 straight up, and on January 15 straight west.
To see all of the circumpolar constellations go to Constellations, arranged by season. Click on the stars in red for information.
Back to the Beginning
| For a winter chart, when it is not winter, go to Heavens-Above, log-on, cursor down to "Astronomy" and click "Whole Sky Chart," change the date to February 1 and the time to 9:00 PM Standard Time, and print out a black on white chart. For the current winter month, go to The Evening Sky Map mentioned above and print out the monthly chart.
|
I once hated to see winter coming with it short days and Standard Time. Now I look forward to it because it heralds the arrival of the Winter Hexagon, the most magnificent star pattern in the sky, and with the short days I can see it before dinner. You will find it in the upper southern aspect of our mid-northern latitude sky. It encloses all or parts of six constellations including the popular centerpiece, Orion the Hunter, which we will get back to later. The Hexagon plus Betelgeuse in Orion account for 7 of the 15 brightest stars seen from the Northern Hemisphere.
The first star rising in the northeast in October announcing the arrival of the Winter Hexagon is bright Capella (mag. 0.06) in The Charioteer (Auriga /aw-RYE-juh/). In January in San Francisco I find Capella straight up overhead near the zenith. Capella is the legendary "she-goat," and in the city you can confirm your identification of her by finding the "three kids" that are fainter stars (in the direction of Aldebaran) that form a long, skinny isosceles triangle. However, now we know that all three kids are bigger than their "mother." (The closest one to Capella is a weird eclipsing binary supergiant. To find out why it is so weird go to Almaaz in "Stars" by James Kaler.) I could fix this problem of the big kids with a new legend. Capella is a multiple star system consisting of a matched pair of light yellow stars (Capella A and B) plus Capella H, a pair of red dwarfs. I would call A and B the mother and father goats and would call H their two kids. The only problem is that the family is so "close knit" that we can't see the separate pairs with amateur optics.
About 18 degrees south of Capella is Elnath (mag. 1.6). When you look at Elnath you are looking out into the plane of the Galaxy almost exactly in the opposite direction from the center of the Milky Way Galaxy, in case you want to get your Galactic bearings.
Around the first of November go about 31 degrees southwest to the next corner of the Winter Hexagon, which is Aldebaran /al-DEB-er-un/, a red giant star that is the bloodshot eye of The Bull (Taurus). The other white stars forming the V-shaped head of The Bull are stars in the Hyades open cluster, which is twice as far away as Aldebaran. Open clusters consist of stars that are "litter mates" born together in the same nebula.
Speaking of open clusters, let's take a side trip to the best known and loved of all, the Pleiades /PLEE-uh-deez/, which is dipper-shaped and sometimes confused with the Little Dipper. This delightful little cluster of sparkling stars is also called the Seven Sisters, but you may only see six (ranging from 3rd to 4th magnitude) with the naked eye or 15 or 20 with binoculars depending upon their power. However, there are hundreds of stars in the cluster. To find them, go from Aldebaran across Taurus' head and a little below his right eye and continue on about 10 degrees or a fist width past his head. The six brightest stars form a neat little pitcher about 2 degrees in angular width, so it is a great binocular sight. These stars are close together because they are so young that they have not yet wandered away from their nebular nest. There are still very faint wisps of the nebula left from which they were hatched. The cluster is about 400 LY away. The next time you see a Subaru car look at the nameplate, and you will see the six brightest Pleiades stars. How come? Subaru is the Japanese name for Pleiades. The Persian name for the Pleiades is "Parvin," which may explain why I am so fond of the little cluster. To see a picture and to read more about the Pleiades, go to Messier deep space objects, go down to "Clusters" and then "Open clusters" and click M45.
Messier objects are named after an 18th Century French comet hunter, Charles Messier /Mess-yai/. He made a catalog now containing 110 objects that might be confused with comets. These objects that were not comets turned out to be a good sample of galaxies, nebulas, open clusters of stars, and globular clusters, which amateur astronomers enjoy finding. In marathons they try to find them all in one spring night.
|
Around the middle of November go about 26 degrees south to Rigel /RY-jul/ at the next corner of the Winter Hexagon. Rigel is the right "kneecap" (or "foot," if you prefer) of the Hunter (Orion) and is a hot, blue-white supergiant whose life will be short but impressive. It has an apparent brightness of a 0.18 magnitude, but its absolute magnitude is a blazing minus 7, more or less, which makes it one of the most luminous stars we can see. Rigel also has a couple of neighboring stars off to each side making a threesome like that of Antares and Altair, as we shall see. The left kneecap of The Hunter is Saiph, which is a 2nd magnitude star but also a distant white supergiant.
It isn't known for sure what celestial object could have been "the star of Bethlehem," but we do know that when you ring in the new year you can look up to the south and find the brightest star visible from Earth (a dazzling minus 1.4 magnitude) on meridian (half way between the east and west horizons). It is Sirius /SIR-ee-ous/ in The Big Dog (Canus Major), and it is the bottom star of the Winter Hexagon. One reason Sirius is so bright is that it is "only" 8.6 LY away. (Rigel is nearly a hundred times as far away.) Other reasons are that it is much hotter and twice as large as the Sun. Befitting a Big Dog star, Sirius has a tiny close companion called the "pup" that is a white dwarf star, the hot cinder remnant of a Sun class star.
Sirius gets all of the attention, but it's a flea on the Dog compared to the Big Dog's tail star, Aludra. It is a blue-white supergiant roughly 3,000 LY away, so its apparent magnitude is only 2.4. However, its absolute magnitude is a blazing minus 7 or 8.
Sirius is one of the closest stars seen in our northern skies, but if it has a planet inhabited by little green men, it would take over 60,000 years for them to make a round trip to Earth traveling at the speed of our spacecraft. It would take nearly 20 years traveling at close to the speed of light, which is the absolute speed limit. So there are probably not many little green men lining up to visit us.
From Sirius go about 25 degrees northeast to the next corner of the Winter Hexagon, which is Procyon /PRO-see-ahn/ in The Little Dog (Canus Minor), but it doesn't even look like a stick dog. Coincidentally, Procyon is about the same distance from us as Sirius, and, believe it or not, it also has a white dwarf "pup."
Now go about 23 degrees north to the next corner of the Winter Hexagon, which is Pollux. It is the left head of The Twins (Gemini), and is an orange giant. The white-headed twin, Castor, is 4 degrees beyond toward Capella. Castor didn't quite make the 1st magnitude team, but somehow it acquired the formal name of Alpha Geminorum that should have been bestowed on the brighter Pollux. Was Castor once brighter? It does impressively outdo Pollux in one respect. Castor appears to be one star but turns out to be a six-star system--two pairs of white-hot stars and a pair of red dwarfs!
Go about 34 degrees northwest to the next corner of the Winter Hexagon, and you are back to Capella.
About a fist width above Rigel toward the center of the Hexagon, look for three 2nd magnitude stars close together in a straight row. These three big, hot stars form The Hunter's belt. Incidentally, they line up with Sirius below and Aldebaran above.
The Hunter has a dagger ("Sword of Orion") dangling from the left side of his belt that is formed by what may appear in low-power binoculars to be three pairs of stars in a row. In a dark sky you may see with the naked eye a faint, fuzzy patch around the middle stars. That small patch, which is actually 40 LY wide and about 1,500 LY away, is the Great Orion Nebula. The middle "two" stars (Theta Orionis) are both multiple star systems. (For a great upside-down photo go to Theta 1 and 2. The southeastern (upper right in the photo) star system, Theta 2 Orionis, consists of three bright stars in a row. The northwestern (lower left in the photo) star system, Theta 1 Orionis, consists of a multiple star system of which the brightest four form a trapezoid called the Trapezium that can be seen in a small telescope at 40-power. Two more stars in the system may be seen with a good large telescope. Ultraviolet radiation from the brightest star (C) makes the Orion Nebula visible (a diffuse or emission nebula). The Trapezium may be a part of the Orion Nebula Cluster that includes about 2,300 young stars surrounded by planet-forming disks and includes about 200 stellar embryos that haven't yet formed disks.
Now let's see what a star looks like in old age. Extend the line from Rigel through the belt an equal distance and look for a reddish-orange star. That star is Betelgeuse /BEET-ul-jooz/ or "beetle juice" if you like. This is an Arabic name that can mean "the armpit of the giant," namely The Hunter. It is a red supergiant and is one of the largest stars known. What makes it even more awesome is the fact that it balloons up and then deflates in a 6-year cycle. If it were to replace our Sun, it would fill Earth's orbit when it is at its smallest, and when it swells up, it would fill Jupiter's orbit, which is roughly a billion miles in diameter!
In his left hand The Hunter is holding a club outlined by several faint stars. From Betelgeuse look about one fist to the southwest and see the fainter Bellatrix, which is Orion's other armpit. It has ten times as much hydrogen fuel as the Sun, and it is burning it much faster in a much hotter nuclear fire. It may burn out in a mere 10 million years. Above and in between Betelgeuse and Bellatrix are three fainter 3rd magnitude stars that form The Hunter's head. Go up and to the west from Bellatrix another fist, and there are six faint stars that form a curved shield. What is The Hunter shielding himself from and getting ready to bash with his club? Right, it is Taurus the Bull who is poised to charge down on him.
Now draw lines on your printed Whole Sky Chart connecting the corner stars mentioned above and see what a nice hexagon it makes.
In mid-January just after it gets dark you can see the Summer Triangle (see The Summer Stars below) just above the northwestern horizon, and at the same time you will see the Winter Hexagon enclosing Betelgeuse just above the horizon in the east. In the low southwest you can see Fomalhaut /FOE-muh-low/. That adds up to eleven first magnitude stars visible all at one time in the city sky!
To see all of the winter constellations go to Constellations, arranged by season.
Back to the Beginning
| For a spring chart, when it is not spring, go to Heavens-Above, log-on, cursor down to "Astronomy," and click "Whole Sky Chart," change the date to April 15 and the time to 9:00 PM Standard Time, and print out a black on white chart. For the current spring month go to The Evening Sky Map mentioned above and print out the monthly chart.
|
As The Hunter nears the western horizon, The Lion (Leo) is relaxing overhead. The Lion's front end is the asterism called the Sickle of Leo. At the end of the sickle handle is the regal first magnitude star, Regulus /REG-yuh-lus/. To find it from the Big Dipper, the two stars at the back of the bowl point back toward Regulus, and the angular distance is about 47 degrees. Also, it is the first bright star that you see about 37 degrees east and a little south of Pollux.
Regulus is 77 LY away and has a luminosity of 140 times that of our Sun, but "The Little King" is a commoner compared to the next star up where the handle joins the Sickle blade. That star is Eta Leonis, which the ancients didn't bother to name since it is only a 3rd magnitude star, but it is a white-hot supergiant over 2,000 LY away that is well over 10,000 times as luminous as the Sun. Regulus is almost smack on the ecliptic, so it is fairly often occulted (covered) by the Moon. The Lion is in the direction from which the meteor shower called the Leonids seems to come in the middle of November. Check on the Internet for the best date and time, and if the Moon is nearly new, drive out to the dark countryside and enjoy the show.
The next star above Eta Leonis is Algieba, which is a fine 2nd magnitude close yellow binary. It can be split with a good small telescope.
Look a bit over a third of the way from Pollux to Regulus in the constellation of Cancer for the Beehive Cluster of stars, M44. It is a popular open cluster that can be seen with the naked eye in dark skies and with binoculars in the city. While in the neighborhood, those with small telescopes may want to look for Iota Cancri, which is a beautiful binary consisting of a blue star and a yellow star, very similar to Albireo. M44 forms a triangle (about 3 degrees on a side) with 2 stars to the west. To find Iota Cancri, extend a line through those stars to the north and a tad to the east 7 degrees.
By extending the arc of the handle of the Big Dipper about 31 degrees or a little over another Dipper's length, you can "arc to Arcturus" /ark-TOOR-us/. This minus 0.04 magnitude red giant's first distinction is that it doesn't follow the herd. Most stars move in the plane around the hub of the Galaxy like peas circling around on a flat plate. (The Sun makes one trip around the "plate" in roughly 200 million years.) Arcturus orbits perpendicularly over and under the plane of the Galaxy. Its second distinction is that it is speeding toward us at the rate of over 10,000 miles per hour but will pass us and eventually recede from view. Its third distinction is that it forms a huge triangle with Vega and Capella dividing the sky into roughly thirds.
Arcturus is in The Herdsman (Bootes /boh-OH-teez/), but the star pattern looks like a kite. Arcturus is at the string end. If you are pointing out The Herdsman to a child, you can say that it looks like an ice cream cone, and the second scoop of ice cream has fallen off to left. The scoop is even topped with a cherry-like brighter star, Gemma, Latin for jewel in the crown. The scoop is actually the Northern Crown (Corona Borealis) constellation.
From Arcturus on the same arc of the handle "speed on to Spica /SPY-kuh/" an angular distance of 33 degrees. Spica is a white-hot, first magnitude, massive close double star in The Maiden (Virgo). It culminates May 28 when we are looking out of the plane of the Milky Way Galaxy where we are more likely to see other galaxies. The distant Virgo Cluster of 13,000 Galaxies is mostly in the area west of Spica and Arcturus in the constellations of The Maiden and Berenice's Hair (Coma /KOH-muh/ Berenices /bear-uh-NIE-seez/), which has a V-shaped stick figure. At the west end of the V is the large Coma Star Cluster. It includes several 5th-magnitude stars about 290 LY away making it a nice open cluster for low-power binoculars.
From Spica "curve on" to Corvus the Crow evidenced by a four-sided figure very similar to the Keystone in Hercules and its neighbor, the "head" of Draco, but is a little larger and brighter. There is a mag 3 star at each corner, and the star closest to Spica is Algorab. It has a nearby neighbor that you can see with binoculars, but the viewer with a telescope can also see that Algorab is a pretty binary that may appear to be a pale lilac star with a yellowish companion.
To see all of the spring constellations go to Constellations, arranged by season.
Back to the Beginning
| For a summer chart, when it is not summer, go to Heavens-Above, log-on, cursor down to "Astronomy," click Whole Sky Chart, change the date to August 1 and the time to 9:00 PM Standard Time, and print out a black on white chart. For the current summer month go to The Evening Sky Map mentioned above and print out the monthly chart.
|
That dominating reddish star that you see low in the south near the meridian in July is Antares /an-TARE-eez/, in The Scorpion (Scorpius). You can see the "stinger" curling back to the east. It is more like a fishhook, and Antares is up near the top of the fishhook and is bracketed by two smaller neighbors the way Altair is bracketed. Antares is the summer counterpart of the winter Betelgeuse. Both are red supergiants of about the same incredible diameter, but Antares has a lower mass. Both pulsate like a heart, and Antares is after all "the heart of The Scorpion." Antares can be confused with Mars because you can sometimes find them close together near the ecliptic, and they both have about the same color and sometimes about the same brightness. In fact, the name "Antares" means "like Mars."
From Antares go down to the star where the hook turns east. Just above that star is the Scorpius Jewel Box consisting of two nice star clusters, a nice binocular object. Go east to the "barb" (two stars less than 1/2-degreeapart) on the fishhook, then go east 3 degrees to a mag 3 star and north about 2 degrees to find Ptolemy's Cluster (M7), which is a fairly bright open cluster about a 1,000 LY away. With low-power binoculars it may require a few seconds of concentration for it to pop into view. The fainter Butterfly Cluster (M6) lies about 4 degrees to the northwest, and it is about 2,000 LY away.
In August The Scorpion's place is taken by The Archer (Sagittarius /saj-ih-TARE-ee-us/). It is dominated by the Teapot asterism. Since the Teapot may not be visible with your naked eye in your urban sky and since it is about 13 degrees wide, you may need to sweep across it with your binoculars and look for the spout and the handle. An object of special interest in this neighborhood is the popular Lagoon Nebula, M8, located about 6 degrees northwest of the "knob star" on the lid of the Teapot. In a 3rd magnitude sky with binoculars it may appear to be a fuzzy patch. In a telescope you can see adjoining on the east a nice tight little open cluster of young stars. (Move the telescope about 3 degrees east of the "knob star," and you can see M22, which is a fine example of a globular cluster, which we will discuss later.) For a good picture and details of M8 go to Messier deep space objects, go down to "Nebulae," and click M8. For the skinny on M22 go down to "Clusters" and then down to "Globular clusters" and click M22.
When we look at the Teapot, we are looking in the direction of our Milky Way Galaxy's center, which is mostly obscured by dust. Under dark skies you will notice a bright star cloud that looks like a cloud of steam coming out of the spout of the Teapot. In that star cloud at the end of the spout there is an area called Baade's Window. To see an image of this area go to Baade's Window. Walter Baade found that a powerful telescope could peer in through this window near the center of the Galaxy.
By the way, after looking at the Teapot, look just below it for the Southern Crown, Corona Australis. It is very similar to the Northern Crown but not quite as bright.
When I look straight up to the zenith at 9 PM in the middle of August, the bright star that I see is blue-white Vega /VAY-guh/ or /VEE-guh/ in The Lyre (Lyra). (Vega first appears in May in the northeast mirroring Capella in the northwest. In November they will switch places.) It is the brightest star in the big Summer Triangle and the fifth brightest star in the sky because it is close (25 LY) and quite luminous (50 Suns). In wide-angle binoculars you will see that Vega forms a smaller triangle, about 7 degrees on a side, with two pairs of fainter stars. In between the most widely separated pair of stars there is the faint Ring Nebula, the first planetary nebula discovered. (Recall that this is a remnant of a Sun-sized star.) In a telescope it looks like a tiny smoke ring.
Vega is at the corner of an even smaller triangle visible in binoculars, less than two degrees on a side. The pair of stars at the triangle corner to the northwest of Vega is a double star and each of these stars is also a double, so this system turns out to be the famous "double-double star." The wider double can easily be split with binoculars, but it takes a telescope (and good seeing) to split the close double. The star at the other corner of the small triangle is a double star that can be split nicely with 12x50 binoculars. While you are in the neighborhood taking in double stars, take a look at Albireo /al-BUR-ee-oh/ with 12x50s or larger binoculars. It is a gorgeous jeweled binary--a golden yellow topaz and a blue sapphire. To find it, look for the brightest star half way between Vega and Altair (see below). I enjoy it most in June and October when I don't have to crane my neck so much to see it.
Look 20 degrees or one hand span to the northeast of Vega, and you will see Deneb /DEN-eb/, which is the tail of The Swan (Cygnus). (Albireo is its head.) Deneb is a first magnitude white supergiant, and although it is the dimmest star in the summer Triangle, it is the most distant of the three and the most luminous.
Now look two full finger spreads or about 30 degrees southeast of Vega to the third corner of the Summer Triangle to find Altair /AL-tare/, the head of The Eagle (Aquila /AK-wuh-luh/). Altair is a first magnitude white star, but its claim to fame is that it makes a full rotation on its axis in only 6.5 hours. So at its equator the surface is moving at the speed of 150 miles per second compared to Earth's 1/4 mile per second which is a brisk 1,000 miles per hour. Much like Antares and Rigel, Altair is bracketed by two smaller neighbors, which makes for easy identification.
When you get out under the dark country sky, notice that Deneb is located where a strip of dust appears to divide the Milky Way into two branches. The Swan is flying east along one branch, and The Eagle is flying west along the other branch. They are like two cars about to pass on a divided highway.
When you are out of the city, look a third of the way from Vega toward Arcturus for the four-side Keystone asterism in Hercules consisting of 3rd and 4th magnitude stars and is about 6 degrees on a side. Look along the side facing Arcturus for M13, which is a globular cluster that may look like a fuzz ball. Globulars are very densely packed, very old clusters consisting on average of about 100,000 stars. Whereas open clusters can be found relatively nearby in our galactic disc, globular clusters are found at great distances out in the halo around our Galaxy. M13 is the best known globular cluster in northern skies. It consists of a ball of about a half million stars and is roughly 25,000 LY away. To see a picture of M13 and to read more about it, go to Messier deep space objects. Go down to "Clusters" and then to "Globular clusters" and click M13.
To see all of the summer constellations go to Constellations, arranged by season.
Back to the Beginning
| For a fall chart, when it is not fall, go to Heavens-Above, log-on, cursor down to "Astronomy," click "Whole Sky Chart," change the date to October 15 and the time to 9:00 PM Standard Time, and print out a black on white chart. For the current fall month go to The Evening Sky Map mentioned above and print out the monthly chart.
|
A fall constellation that should not be overlooked is Perseus /PER-see-us/. It lies between Cassiopeia and Taurus. Its most famous inhabitant is the Double Cluster, which is a great telescope object found half way between the Cassiopeia "W" and Mirfak, the brightest star in Perseus. However, around Mirfak is the Perseus Cluster or Alpha Persei Cluster which is a much more impressive low-power binocular sight. It is a bright loose cluster of stars having an angular width of about 3 degrees. Since it is a looser group than an typical open cluster, it is sometimes called an association or group. The Perseids is a major meteor shower that seems to come from the direction of Perseus. To find out about the most interesting star in Perseus, a famous eclipsing binary star, go to ALGOL.
I look forward to the late summer and autumn when I can see the faint, glowing hub of the Andromeda galaxy (M31). One reason that I am especially fond of it is that when I was born in February of 1923 astronomers were arguing about whether or not there were other "island universes" like our Milky Way Galaxy. Within a year of my birth Edwin Hubble proved that M31 was indeed another "island universe," and it was not the only one. So, within my lifetime the known universe has grown from just our Galaxy to over 100 billion galaxies!
It blows my mind to think that from my balcony in the city and with a moonless sky I can look with nothing more than 8X40 binoculars way out beyond our Galaxy and glimpse another galaxy even though I can only see a faint smudge of light from M31's hub. If you go to the country under a dark sky, you may see M31 smudge with the naked eye. It is between two and three million LY away and is the most distant object that can be seen with the naked eye. Its full width is about 3 degrees, and it is tipped rather than being a face-on spiral.
To find M31 with binoculars in the city, start from Cassiopeia. The deepest of its two "Vs" points directly at M31, but if you miss it go on to the red giant Mirach at an angular distance of about 22 degrees from Cassiopeia in the Andromeda constellation. Now come back about four degrees toward Cassiopeia to a fainter star, and then come back about three more degrees to an even fainter star. This star is at the corner of a small triangle formed with another star and with M31 at the corner nearest to Cassiopeia. With a little practice you can go about 15 degrees straight from Cassiopeia to M31. For a good picture and details of the galaxy go to Messier deep space objects and go down the list to "Galaxies" and click on M31.
When you are out of the city in November under a dark sky, try finding the Great Square of Pegasus. First, find Polaris and follow a straight line through Caph, which is the star at the right end of the "W" in Cassiopeia, and continue another 30 degrees. You will intersect the northerly side of the Great Square of Pegasus. It consists of four stars between 2nd and 3rd magnitude making a square that is about 15 degrees on each side. Three of the stars are in The Winged Horse (Pegasus). Look eastward to the northeast corner star, which is just barely in The Chained Princess constellation (Andromeda).
In October the brightest star you will see low in our southern sky is lonely Fomalhaut /FOE-muh-low/ in The Southern Fish (Pisces Austrinus). To confirm your identification, look for two close-together fainter stars 3 degrees below and on a line making a 45-degree angle with the horizon. Fomalhaut is a hot white star of the first magnitude. In September it is in the low southeast, and Antares is in the low southwest.
In November look to the northwest, and you will see the Summer Triangle getting low in the west with Altair and Vega parallel to the horizon and Deneb on top.
To see all of the fall constellations go to Constellations, arranged by season.
Back to the Beginning
Celestial Aids to Navigation
There are three prominent groups of celestial "aids to navigation." The first group is the 15 first magnitude stars seen in the Northern Hemisphere night sky. They serve as our navigational beacons in light-polluted city skies because we can nearly always see them in a clear sky with the naked eye when they are above the horizon. All 15 stars are listed in the table in the APPENDIX.
The second group of aids to navigation are the best known asterisms. We have seen how helpful the Big Dipper is in showing us around. In the winter look for the Winter Hexagon, which is a celestial treasure chest. In the summer look for the Summer Triangle of bright stars that locates Cygnus the Swan, Aquila the Eagle, and Lyra the Harp. Other useful asterisms are the "W" in Cassiopeia, the Square of Pegasus, the Keystone in Hercules, the four-sided "head" of Draco, the Sickle of Leo, and the Teapot in Sagittarius.
The third group of aids to navigation is of course the constellation star patterns. We can gradually learn the important constellations in the course of learning the bright stars and the asterisms as they appear over the eastern horizon. For a useful beginners' guide to several asterisms and constellations by season go to Celestial Photo Album.
Now let's use these "celestial aids to navigation." Suppose you live in the mid-northern latitudes, and it is June. You might go to The Evening Sky Map and print it out. You scan the list of objects "Easily Seen with Binoculars," and the "bright cluster," M25 in Sgr (Sagittarius), catches your eye. To find Sagittarius, first find the 1st mag orange star, Antares, in the south not far above the horizon. Second, look a full hand spread (20 degrees) east to the spout of The Teapot asterism in Sagittarius. Third, focus your binocular on the "lid knob" star and move north and a bit to the east 6 degrees to find M25.
Celestial Addresses
You might wonder if there is a system of celestial coordinates to locate objects in the sky. There are two major systems of coordinates used by amateur astronomers. One is the horizon coordinate system, or altazimuth system. Suppose you want to tell a friend across town how to find a certain object in the sky at that moment. You might say that its azimuth is 90 degrees (due east) and its altitude is 45 degrees (above the horizon). This system is of limited use because it is a local system, and the coordinates are good for one particular time and observing place.
What is needed is a system that is not dependent upon the time or place of the observation. One such system of coordinates is the equatorial system. Let's make a model in which the Earth is enclosed in a much larger celestial sphere on which all of the constellations are shown. (We borrow this idea from the ancients who thought that stars are little lights on the surface of a crystal sphere.) In our model Earth's equator and its lines latitude and longitude are reflected on the celestial sphere. The lines of latitude become lines of declination measured in degrees, either plus (above the celestial equator) or minus (below). The lines of longitude reflected on to the celestial sphere will be called lines of right ascension (RA) measured in hours (zero to 24 hours) instead of degrees. One hour of RA is the equivalent of 15 degrees. Unlike longitude the RA starts at zero and increases to the east all of the way to 24 hours or zero again.
The celestial equator is the zero point for measuring declination, but we have no such obvious zero point for RA. It was arbitrarily decided that the zero point would be where the ecliptic crosses the celestial equator at the vernal equinox. Recall that the reason that the ecliptic does not coincide with the celestial equator is that the Earth's axis is tilted 23.5 degrees. On March 21 at the vernal equinox the ecliptic now crosses the celestial equator in the constellation of Pisces, and this is where zero RA is located. (Due to precession this point gradually changes over the course of years.) So much for our model; now let's go to the real world perspective:
- If you stand on the equator at zero latitude, the celestial equator and zero declination are straight overhead. (The western star in Orion's belt is close to the zero parallel of declination.) Since Polaris has a declination of plus (north) 90 degrees, it will be sitting on the horizon. If you stand at the North Pole, Polaris will be straight overhead and zero declination will be at the true horizon. If you are at 30 degrees North latitude, Polaris will be 30 degrees above the horizon. We have just stumbled on to an important navigational rule: Your northern latitude equal's the angle between Polaris and the horizon.
- At noon on March 21 (spring equinox) the Sun will be on meridian (midway between the eastern and western horizons) in Pisces. Your watch will say that it is 12:00 noon, but it is 0 hours sidereal /sigh-DEAR-eal/ time, which is measured on the 24-hour basis. The Sun's east/west position on the celestial sphere and that of any other star in that direction is 0 hours RA. One of these stars is Caph in Cassiopeia.
Now it is twelve hours later, the Earth has turned 180 degrees inside the celestial sphere, and it is midnight and 12 hours sidereal time. Look up at the two back bowl stars in the Big Dipper. They straddle the 12-hour RA circle, which is now on meridian.
Now it is six months later at midnight on September 23, the autumnal equinox. Earth is at the other side of its orbit in between Pisces and the Sun, so we see Pisces at night. There are no bright stars on the 0 hour RA line in Pisces, so again look up at Caph, which is the most western star of the "W" in Cassiopeia. It lies very close to the zero hour RA circle. Its coordinates are about 0h RA +60deg Dec. If we look to the east (at about latitude 38 north), we will see Betelgeuse clearing the horizon.
Let's find Fomalhaut, a lonely first magnitude star in the southern sky, on a selected star chart or a full sky map (such as Orion's "Deep Map 600") using the star's equatorial coordinates, which are 22h 57m -29deg 37m. Find the -30deg declination parallel on the side and follow it across until you come to the 23h RA line. The closest bright star that you see is Fomalhaut.
The binocular skywatcher doesn't need to know much about these coordinates, but they can help him in locating an object on a star chart. The telescope user with setting circles or a "go to" system can input the coordinates to guide or move his scope to the target. Since the Earth is rotating within the celestial sphere, it is necessary to align setting circles with the stars at any given time. To stay aligned, your mount must have a tracking motor (clock drive) on the RA axis. For another discussion on equatorial coordinates with excellent diagrams go to Celestial Coordinates.
Back to the Beginning
You're looking through your binoculars at your favorite star cluster or whatever and suddenly a yellowish "star" glides right through your field of view. Wow, it's a satellite! I have had that happen twice in one night. Fortunately, you don't have to rely on these fortuitous encounters. You may also track down the satellites by using the information in the great Web site, Heavens-Above, by following these steps if you have completed registration: 1) Log-in. 2) On "Heavens-Above Main Page" under "Miscellaneous" select "What time is it?" to synchronize your watch. 3) Then go back to the Main Page and under "Satellites" and under "Daily predictions for bright satellites---" select the magnitude. (I select mag 3.5 or brighter.) 4) A table is produced showing the search period, your location, and the local time, whether DST or Standard Time. It lists the satellites and shows the brightness and the time, altitude, and direction to look for the start, the point of maximum altitude, and the end. Instead of clicking on a satellite's name, click on its time at "Max. Altitude." 5) You will get a "Visible Pass Chart" showing the path and times for passing various points, which you can print. The window for seeing satellites lasts about two hours after it gets dark enough to see them.
The International Space Station puts on a good show because it is so bright and can often be seen easily without binoculars even though it may be about 230 miles high. On the Heavens-Above Main Page under "Satellites" you can click ISS for a 10-day prediction. Select a flight with the right path, date, and time of day that is suitable. To get a chart and other details, click on the date in the table.
Satellite hunting is a more challenging sport in urban skies. You will see that there are only a few satellites brighter than 3.5 magnitude, so you can't see many of them with the naked eye. When you can't see a satellite with the naked eye, you can check the Heavens-Above chart and find the satellite's path and times of arrival. Focus your binoculars on an identifiable place on the path and wait for it to arrive. The satellites are usually right on time and on course. This is fun!
Satellites are lit by the Sun, so they appear to be yellowish. If they come in and out of sight, they are tumbling. They are easy to distinguish from airplanes, which appear to move more slowly at high altitudes and have flashing lights.
There are geostationary satellites, which stay over one location on Earth at an altitude of 22,300 miles, but the more visible satellites are in low, polar (north/south) or east/west orbits, so they are easier to find and see.
The Iridium communication satellites are the most spectacular when you are in the right location to see the maximum flare which may be of a -8 magnitude. Twenty miles away from the center it may be about 0 magnitude. You can check Heavens-Above in advance to find where the maximum flare is centered and drive to it.
If you want to know more about observing satellites, go to Visual Satellite Observer's Home Page, which is an excellent reference resource.
Back to the Beginning
Now you are ready to start looking up and enjoying the riches of the night sky. At the end under "Resources for Beginners" there are links to appropriate sky guides, but in addition you should buy a plastic star wheel (planisphere) for your latitude. A good one is The Night Sky (8-inch) by David Chandler available from telescope stores and from Amazon.com for $11 (The Night Sky). By rotating the wheel to match the time of day (Standard Time) with the date, you can see which stars are in the sky at that time. It shows the northern and overhead sky on one side and the southern aspect of the sky as seen from the northern hemisphere on the other side. To orient the star wheel, note the compass directions and horizons shown.
To preserve your eyes' dark adaptation while looking at your star wheel or chart, you need a red-filtered light, such as a flashlight with red plastic wrapped over the lens. To shield your eyes from local lights when you are viewing, you can zip up a windbreaker, slip it over your head, and put the collar around your binoculars or telescope diagonal.
Here are the minimum items you will need for skywatching at a location away from home: 1) star wheel, 2) red-filtered light, 3) handheld binoculars, 4) suitable warm clothing, 5) notebook listing objects that you want to view, and 6) a suitable tote bag. Optional items include these: 1) a simple star atlas such as the Edmund Mag 5 Star Atlas or the Field Guide to the Night Sky, 2) hot drink and a cookie or two or three, and 3) a light beach chair with adjustable back on which you can lie back in comfort and look up and enjoy the show.
When you're left out in the dark, look up and marvel at the splendor
of the night sky! |
To find out about binoculars and telescopes for beginners, go to Choosing Optics for Skywatching.
To keep tab on the night sky, you have already found out about The Evening Sky Map, which is a handy monthly two-page guide that you can print out. It lists naked eye, binocular, and telescope objects that you can see and has symbols for them on the detailed chart. You also found out about the Heavens-Above site. Another similar resource is Your Sky. This site features "Sky Map," "Horizon Views," and the "Virtual Telescope." Experiment with options in the control panels to reduce clutter, widen field of view, etc. This site also has a link to "Home Planet," which is a public domain Earth/Space/Sky simulator.
EarthSky Tonight treats a new subject for every night of the month. It is especially good for a beginning skywatcher.
For other resources for keeping tab on the sky, see if your local newspaper's weather page gives the phase of the Moon, the planets that can be seen, and other interesting night sky info. Also go to This Week's Sky at a Glance courtesy of Sky & Telescope magazine. Adults and kids may enjoy the weekly column of Jack Horkheimer, Star Gazer.
Still another way to keep tab on the sky is with a planetarium program. One free sky chart program is Sky Charts. You can also buy a full-featured planetarium program such as Starry Night Pro or the lower-priced Starry Night Backyard Special Edition, and read the included book, Starry Night Companion, and do the exercises using the program. This is like learning astronomy in a planetarium because it is so realistic. If you really want a bargain Starry Night program, see the comments on Discovering the Universe under the astronomy textbooks below.
Here are some more good sites on the Internet dealing with every aspects of amateur astronomy:
Here are some books and guides for the younger set:
- One Small Square, the Night Sky by Donald M. Silver is a good, beautifully illustrated book for a child, say 10 or older.
- New Astronomer subtitled "The Practical Guide to the Skills and Techniques of Skywatching," by Carole Stott is outstanding for its simple explanations and generous illustrations.
- The Young Oxford Book of Astronomy by Simon and Jacqueline Mitton is a good, simple introductory astronomy book with great graphics for the teenager and adult.
- A Walk through the Heavens, A Guide to Stars and Constellations and their Legends by Heifetz and Tirion is a good little starter atlas to help young beginners find their way around the sky.
Here are some books and atlases to help you find your way around the sky and to tell you bout what you are seeing:
- Field Guide to the Night Sky published by the National Audubon Society includes a good introduction to amateur astronomy, good beginner's monthly and constellation charts, and good descriptions of constellations, planets, and other objects.
- 365 Starry Nights by Chet Raymo is a well-illustrated, highly readable book for children, who are good readers, or adults. It shows lots of star patterns and has lots of beginning astronomy. ($16 Amazon.com new or about $5 for good used copy.)
- The Observer's Year by Patrick Moore in England guides you through the 366 nights of leap year and has lots of helpful small charts, tables, and information. It can be used as a field guide. ($34.95 Amazon.com new or about $5 for a good used copy.)
- Edmund Mag 5 Star Atlas by Sam Brown gives more detailed charts and gives lists of selected sky objects keyed to the charts. It is available from Edmund Scientific for $6.95.
- What's out Tonight? by Ken Graun is a 50-year astronomy field guide with monthly star charts, detailed star-hopping charts for deep sky objects, tables telling what planets can be seen, the phase of the Moon, time of sunset and sunrise, and more.
- The Year-Round Messier Marathon Field Guide by Harvard C. Pennington is an excellent guide for finding the Messier objects. It shows their positions in constellations (which Turn Left at Orion doesn't), the finder view, and the eyepiece view. On-line book sellers may not have this, but the publisher, Willmann-Bell, Inc., still lists it in its on-line catalog.
- NightWatch subtitled "A Practical Guide to Viewing the Universe," Third Edition, 1998, by Terrance Dickinson gives a good, well-illustrated overview of what can be seen and how to see it, and it includes good charts locating the most outstanding objects for binoculars and telescopes.
- The Cambridge Star Atlas by Wil Tirion is for the intermediate observer. It shows stars down to magnitude 6.5, but don't expect to find constellation stick figures.
- The Binocular Stargazer subtitled "A beginner's guide to exploring the sky" by Leslie C. Peltier, a noted amateur astronomer, is an update of a classic book.
- Exploring the Moon through Binoculars and Small Telescopes by Ernest H. Cherrington, Jr. is an excellent source of photos and information on the lunar features.
- 40 Nights to Knowing the Sky by Red Schaaf is a good comprehensive book on observational astronomy.
Here are two books that give a simple introduction to astronomy and give charts of the constellations;
- Night Sky is a paperback published by Discovery Books. It gives some basic amateur astronomy stuff along with constellation charts and describes objects of interest.
- Skywatching by David H. Levy is well-illustrated hardback that also covers and has a good chart for each of the 88 constellations.
It is awe inspiring to look up and find familiar planets, stars, and star patterns in the night sky, but it is deeply satisfying to understand what you are seeing and what is going on in the cosmos. For a readable introduction to basic astronomy and the methods of science, nothing beats getting an introductory astronomy textbook intended for non-science majors in college or for capable high school students. To see a list of 15 of these books Google "Introductory Astronomy Textbook Web Sites." They are all understandable and very well-illustrated with full color graphics. The bad news is that they sell for about $90 new, but since the publishers bring out a new edition about every three years to stay competitive, you can buy a good used copy of the next to last edition for peanuts. Here are a couple of examples:
- Discovering the Universe by Neil F. Comins and William Kaufmann III may be the least intimidating of the genre. The math is in "astronomer's toolboxes" so that you are not faced with it in reading the text. This comes with the student version of the Starry Night planetarium program, which is quite realistic. It has some useful features for beginners such as giving the name of an object that you click on. You can buy a good used paperback copy of the 2000 fifth edition for about $4 plus $3.49 for Media Mail. Confirm that it is selling w/CD.
- Astronomy Today by Eric Chaisson and Steve McMillan is the best selling example of this genre. For a great bargain go to Amazon.com, search for "Astronomy Today, 2000 Media Update Edition, 3rd Ed.,1999, hardcover with a CD with the entire book on it, and check the "used and new" listings. Very good used copies sell for about $5-6 plus $3.49 for Media Mail. The new 4th Ed., 2001, sells for $90.
If you would like an excellent read on the history of astronomy leading to the develoment of the Big Bang model, I highly recommend Big Bang, Harper Collins, 2004, by Simon Singh. It is very understandable and has lots of interesting stories about the people involved.
Check out your local planetariums, observatories, and amateur astronomy clubs holding public star parties. Club members love to share the views in their scopes and share their knowledge. To help you find these local resources, go to Clubs and Organizations.
Whatever you do, don't go through most of your life, as I did, missing the pleasure of skywatching even if you live in a city!
Back to the Beginning
APPENDIX
Copyright © 2008 Robert G. Parvin.
