In this course we shall frequently refer to the location of a celestial object by identifying the "constellation" or group of stars in which it is located. For early astronomers the collection of stars comprising a constellation represented some figure. Thus, the stars which we refer to as "the big dipper" were part of a constellation depicting a big bear, Ursa Major. While few modern people admit being able to recognize these figures, the collections of stars which comprise them are well worth learning.
If one mounts a camera facing toward the north, and takes a time exposure on a clear dark night, the star images appear as circular arcs. The star "Polaris" appears to move in a tiny circle about 1 degree in radius. Actually, of course, it is the rotation of the earth which leads to this effect.
Suppose we look at the sky on March 20th. The "vernal equinox" occurs on that date this year. That means that the Sun is directly over the earth's equator, and our days and nights are equally long. Later the Sun will move north of the equator and our days will be longer than our nights. If on March 20th one looks north at midnight, one can easily identify the big dipper (Ursa Major), the little dipper (Ursa Minor) and the W-shaped constellation Cassiopeia. The big dipper will be almost overhead (at the "zenith") while Cassiopeia will be low on the horizon. Notice how the "pointer stars" b and a in the big dipper point the way to Polaris, the north star, which is the last star in the handle of the little dipper. Also take a good look at the star z Ursae Majoris, for it is a visual double. How do you suppose one might tell whether or not these two stars, Alcor and Mizar, are actually close to one another?
Now suppose you turn around and look south. The constellation Leo the Lion is not hard to recognize, and one should be able to spot the bright star Spica in the constellation Virgo. The line marked "ecliptic" portrays the apparent path of the Sun from right to left during the month of September. On September 23rd the Sun will cross the celestial equator. That will be the time of the "autumnal equinox." The point in the sky at which the Sun is located then is also called the "autumnal equinox." This point is located within the constellation Virgo.
On March 20th the Sun is at the vernal equinox. It is now moving through the constellation of Pisces the fish. None of these constellations is now visible, for they are obscured by the brilliant Sun. Thus, you cannot see the giant square of Pegasus, the flying horse, and you cannot see the constellation Andromeda, in which is located the famous Andromeda Galaxy M31. In spite of the fact that this galaxy is one of the closest to us, it takes light 2 million years to make the journey from our neighbor. How do you suppose the distance to M31 was determined?
A couple hours after Sunset the stars should be visible, assuming the weather is agreeable. Low in the western sky it should be possible to pick out the bright stars Capella, Aldebaran, Betelgeuse and Rigel. See, however, if you can find Mira, the wonder star, located near the constellation Pisces. For almost half a year this remarkable star is conspicuous, while for another half year you won't be able to see it without a good telescope. Why do you suppose a star would behave in this strange way? The Hyades in the constellation Taurus the bull should not be hard to find, and the same applies to the Pleiades, or Seven Sisters. Through a large telescope one can observe nebulosity associated with these stars.
I also want you to become familiar with the constellation Orion the hunter, which any novice can spot. The Milky Way passes through the vicinity of Orion. In addition to the conspicuous stars comprising the constellation there are many others which may be photographed through a wide angle telescope. In the sword of Orion, hanging from the hunter's belt, is the famous Orion nebula M42. Through a small telescope you get a vague hint of nebulosity, but primarily you see four stars called the Trapezium, which provide the source of light for the nebula. The famous Crab nebula M1 represents the remnants of a stellar explosion which was observed in 1054 by people all over the world (except in Western Europe). M1 is located very close to z Tauri.
North of the Pleiades you should be able to locate the constellation Perseus. The star b Persei is named Algol, the Demon Star. It blinks with a period 2.87 days. Why do you suppose it would do that? Watch this star for a few days, comparing it with other stars in the vicinity. For about five hours you may note a significant decrease in its brightness.
Somewhat further to the east, and quite low in the sky, the star Sirius should be conspicuous, for this brilliant jewel is the brightest star in the sky. The constellation in which it appears is Canis Major, the great dog. Sirius is commonly called the "dog star." It's quite close to our Sun, only 8 light years distant. For comparison it takes only 8 minutes for light to reach us from the Sun. How do you suppose the distance to Sirius was determined? On the same chart we can see Procyon, the brightest star in Canis Minor, as well as the moderately bright stars Castor and Pollux in the constellation of Gemini, the Twins.
As the night presses on Leo the Lion and other constellations we considered before rise and proceed toward the west. By midnight the star Antares in Scorpius has risen in the east. This star like Betelgeuse is a "red giant," a star with a diameter so large that the entire orbit of the earth would be swallowed easily within either star. If you look at either of these stars you will perceive its redness compared with many other stars. How do you suppose the immense size of these red giants was inferred?
Further north on the same chart you will find the constellations of Boötes and Hercules. The brightest star of the former constellation is called Arcturus. The constellation itself always reminded me of a kite. About 1/3 the way from h to z Herculis one can locate the globular cluster M13, which is located within our own galaxy.
Shortly before dawn arise the constellations Cygnus the Swan and Lyra, which constellations will be prominent in the Summer sky later this year. The swan really looks like a cross, but if you use your imagination you might see the long neck of the swan, its tail, and its outspread wings, as it flies south along the Milky Way. The star b Cygni, at the head of the swan, is a beautiful double star through a small telescope. One component is blue while the other is orange, so it is quite striking. Are these stars really close?
The bright star in Lyra is Vega. e nearly is a double-double, which is worth viewing through a small telescope, such as our 8 inch Celestron. While you are surveying the constellation Lyra with a telescope, don't miss the Ring Nebula M57. While less spectacular than the Crab Nebula, this also represents a stage in the demise of a star, which can be seen at the center of the ring.
The constellation Cygnus is in the direction of the Milky Way. Through a large telescope a myriad of stars can be seen. The redness of the North American Nebula is due to the preferential scattering of red light by galactic dust. The Pacific Northwest has been short-changed, but the rest of the North American continent is fairly represented.
The center of our own galaxy is located in the direction of the constellation Sagittarius. With field glasses the Milky Way presents a spectacular impression.
While you are enrolled in this course I hope you will take advantage of clear nights to become better acquainted with the night sky. Periodically we can get together to use our 8 inch Celestron reflector, and members of the class are encouraged to bring their own telescopes as well.