OBSERVING OUR SKIES
Today, constellations divide the sky into easy-to-remember pieces. Modern astronomy recognizes a total of 88 constellations. Twelve of these form a belt (the "zodiac") and are probably the oldest constellations. Within this belt the Sun, the Moon, and the planets always appear to move.
Try to locate some of these constellations, where they are in the sky, and what times of the year they are visible. Since the night sky changes from month to month, a chart for the current month will be very helpful. One choice for the beginning sky watcher is a planisphere. This is a cardboard star chart with a rotating window that can be set for the date and time of your observing session. Another easy way to obtain a current chart is to purchase one of the three nationally published magazines: Sky and Telescope, Astronomy, and Night Sky. The first two magazines are published monthly and the third one is published bimonthly. Night Sky concentrates on the observational aspects of our hobby and is an ideal publication for the new sky watcher.
As you start observing, you will find that there are two broad categories of things you will want to look at. Our own solar system with its planets, the Sun, the Moon, and other bodies forms one category. Objects that lie outside of our solar system form the other category, often referred to as "deep sky objects".
The Solar System
The Sun
Never look directly at the Sun with a telescope, binoculars, or your naked eye! You can severely damage your eyes by looking at the Sun without proper safety precautions!
But don't give up! The Sun can be safely studied either by using a well-made, white light solar filter or by projecting the image of the Sun on a screen where it may be viewed indirectly. With these simple precautions, you will be able to see and follow sunspots on the Sun's surface. It would be best to talk to some experienced IAS observers about how to observe the Sun with your equipment. If you become interested in solar observing, you may eventually want to purchase a hydrogen alpha filter system, which will reveal solar prominences and much detail on the surface of the Sun. However, these filters are difficult to manufacture and therefore are very expensive.
The Moon
The Earth's natural satellite, the Moon, is the most obvious and the easiest target to observe. When first looking at the Moon through a telescope, you will be surprised at how bright it is. Most observers use a neutral density filter to reduce the lunar brightness.
The best times to observe our "neighbor" is when it is near its First or Last Quarter phase (the Full Moon phase is completely frontal lit and offers no shadows, making it harder to resolve detail). When observing the partially lit Moon, follow along the lunar terminator, the line that separates night and day. It is along this line that you will see mountains, craters, scarps, and rilles in the sharpest relief. The Moon responds well to higher magnifications, so you can crank the power up and search for really small details. A number of amateurs like to look for the Apollo mission landing sites. Unfortunately, our scopes and even the largest professional telescopes do not have sufficient resolution to spot the Apollo artifacts or the American flag!
The Planets
With a large enough telescope all of the nine planets are possible targets for you, but three of them--Mars, Jupiter, and Saturn--will be planets that you will want to observe again and again.
Jupiter, the largest planet, will appear to have belts across its disk. These are actually the tops of cloud belts, and you can sometimes observe some smaller swirls and festoons in these belts. There are the four bright Galilean satellites nearby and, if you look closely, you will be able to follow the daily motions of these bodies as they change positions while orbiting Jupiter.
Saturn is always a spectacular sight in any telescope. You will see a small globe surrounded by a beautiful system of rings. Several of Saturn's moons, especially Titan, are also visible in small and medium aperture telescopes.
Mars is another inviting target to observe when it is in opposition, or close, to the earth. It is, however, a challenging planet when trying to discern surface detail. Look for its polar caps and some of the larger dark areas such as Syrtis Major. Mars requires a medium aperture scope with good optics and, as always, plenty of patience.
The two other naked-eye planets, Mercury and Venus, have orbits that lie within the earth's orbit and are called interior planets. Because they are interior planets, we never see them as round disks, unlike the outer planets. Mercury and Venus exhibit phases like our Moon.
Venus is often the brightest object visible in the early evening or early morning hours. It is about the size of the earth and has been referred to as our "sister" planet. Even though it is very bright, no surface detail can be seen due to its dense cloud cover. Using your telescope, you will be surprised to find that when Venus is at its brightest it is actually crescent shaped. Occasionally, Venus crosses in front of the Sun's disk. When this occurs, it is called a transit of Venus and we see Venus as a silhouetted dot against the Sun's surface-this is a very rare event, the last one occurring in June, 2004.
Mercury is a difficult target because it is small and orbits close to the Sun. This means that it can only be viewed shortly after sunset or shortly before sunrise a few times during the year.
The remaining planets--Uranus, Neptune, and Pluto--are very dim and much harder to find. Uranus and Neptune can be viewed with binoculars, but a telescope is needed to see their planetary disks. Uranus appears as a greenish disk and Neptune takes on a bluish color. Pluto requires a moderately large telescope and, even then, is just a pinpoint like a star. It has to be observed on several successive nights in order to identify it as it moves across the background star field.
Comets
Comets are small members of the solar system, and most of them travel in elongated, elliptical orbits around the Sun. They originate in the Oort Cloud, an area far beyond the orbit of Pluto. Comets are bodies of ices and dust and are left over from the formation of our solar system. When these bodies come into the inner solar system, the heat from the Sun vaporizes some of the ices, creating a very large luminous head ("coma") and tail to the comet. Each year a few comets can be seen with binoculars or a small telescope as their orbits bring them close to the Sun. Occasionally, one of them becomes a bright, very spectacular feature in the night sky.
The Deep Sky
Big as it seems to us, the solar system is just Earth's backyard. "Deep Sky" is a term which astronomers use to describe the realm of all things that lie beyond our solar system. It includes star clusters and nebulas within our galaxy (the Milky Way) and other galaxies that lie far beyond the Milky Way in the outer reaches of our universe.
Distance in deep space is much greater than within the solar system, so astronomers use a measurement called a light year. This is the distance that light travels in one year at the speed of 186,000 miles per second or roughly 6 trillion miles, which is an enormous distance. If the distance between Earth and the Sun were one inch, a light year would be an equivalent of one mile. The star nearest the Sun, Alpha Centauri, is about 4.3 light years away, while the most distant galaxy detected is 12 to 15 billion light years away.
When you explore the deep sky, you will need good star maps and access to a sky that is not too severely light-polluted. The sky charts that were discussed on p.11 will not have sufficient detail to find many of the deep sky objects. To find these faint objects, you will need a deep sky atlas that has a series of charts plotting the sky in great detail. Two recommended atlases that can get you started are by Wil Tirion: The Cambridge Star Atlas and The Bright Star Atlas. The Cambridge Star Atlas plots stars down to 6.5 magnitude, while clusters, nebulas, and galaxies are plotted to 10+ magnitude. This atlas has a page opposite each star chart that lists star clusters, nebulas, galaxies, and other objects of interest on that chart. The Bright Star Atlas is similar but somewhat less detailed. Another well-known atlas is Nortons 2000.0, which plots stars to magnitude 6.5 and has 150 pages of tables and references in addition to the sky charts. There are also several astronomical applications that can be installed on a desktop or laptop computer.
When reading about astronomy or when using a star atlas, you will find that many deep sky objects are referred to with an "M" or "NGC" number - like M42 or NGC 1976. The "M" numbers refer to a list of 110 "fuzzy" objects, published in 1781 by Charles Messier, whose primary interest was discovering comets. He compiled this list so that he would know whether the fuzzy object he was looking at was a comet or some "undesirable" faint nebula. For amateurs, this list is popular because the Messier objects are relatively bright and most can be seen with binoculars or small telescopes. The NGC (New General Catalogue) was compiled in the late 1800's. It is a much larger list that includes many fainter objects. In addition to these two catalogues, you will encounter other listings, but these two are the most common.
Stars
Stars are really luminous balls of gas scattered throughout space. They generate their heat and light by nuclear fusion, turning light elements such as hydrogen into heavier elements like helium. This is the same type of reaction that powers a hydrogen bomb. Depending on their temperature, stars can have different colors from very hot, massive, blue stars to cool, giant, red stars. The hot blue stars, while large, burn their fuel at a prodigious rate and have relatively short lives. Cooler stars like the Sun, although smaller, have very long lives, which for us on the planet Earth is a good thing.
The apparent brightness of a star or other celestial object is reported as its apparent magnitude, which is defined using a logarithmic scale with larger numbers indicating fainter stars. The naked eye limit is about +6 magnitude under a dark sky. A 1st or 0 magnitude star is a bright star. A few stars are still brighter and these are indicated by a negative magnitude. The brightest star in the night sky is Sirius; its magnitude is -1.5. Our Sun has a magnitude of -26.8, the Moon is a -12.6 magnitude object, and -4.4 is the maximum brightness of Venus. Kitt Peak Observatory in Arizona, with its ground base telescope, can see +24.5 magnitude stars. The Hubble Space Telescope can do better; it can see to +28 magnitude. You must remember that magnitude only tells us how bright something appears to us on the Earth. The magnitude number does not tell us how bright a star or galaxy actually is. A very big bright star may appear faint to us because it is at a great distance.
Many amateurs like to observe double stars, with their contrasting colors. The double star called Albireo, in the constellation Cygnus, is a classic example. The larger star is gold or yellow, while its circling companion is blue. Their colors are clearly visible through a telescope.
All of the stars that we see in the night sky are members of the Milky Way, which is a very large disk with spiral arms that contain billions of stars. We are located out toward the edge of the disk on one of these spiral arms. When we look up into a dark sky and see the Milky Way, we are looking at the unresolved stars in our galaxy. Using binoculars or a telescope at low power will resolve many of these stars. Cruising the Milky Way with some form of optical aid will reveal many star clusters and gas nebulas.
Star Clusters, Nebulas, and Galaxies
Star clusters are among the amateur astronomer's favorite things to observe. There are two types of star clusters: open clusters containing up to a few hundred stars often loosely organized, and globular clusters, which are dense balls containing 100,000 stars or more. A well-known example of an open cluster is the Pleiades (M45). A classic example of a globular cluster is M13 in Hercules. Both globular clusters and open clusters are wonderful sights in your telescope.
Nebulas are concentrations of dust and gas and occur in many forms. Absorption or dark nebulas absorb light from stars and appear as a dark cloud. Reflection nebulas are gas and dust clouds that shine by reflected light from nearby stars. Emission nebulas absorb ultraviolet light from nearby stars and re-emit this light at visible wavelengths. An example of a nebula that is a favorite of amateur astronomers is the Orion Nebula (M42), which is illuminated by several very hot stars within it. These stars are very young, having just been born in this nebula.
While M42 is an example of a nebula giving birth to new stars, a planetary nebula is created by a dying star. The planetary nebula has nothing to do with a planet but was so named because it sometimes appears as a small disk (much like a distant planet) in a telescope. The Ring Nebula (M57) in Lyra is an example of a planetary nebula.
The previously mentioned star clusters and nebulas all lie within our galaxy. You can use your telescope to observe other galaxies in the night sky. The most famous of these is the Andromeda galaxy (M31). This galaxy lies approximately 2.5 million light years from us and can be seen from a dark sky site with the naked eye. This is the most distant object in the universe that can be seen with the unaided eye.
©2005 Indiana Astronomical Society