Observing Stars: Stargazing has been a favorite pastime of people since cave man first crawled out of his cave. As we gaze into the nighttime sky, there appear to be a countless myriad of stars overhead. Actually, with the unaided eye, we can see only from 3,500 to 4,000 stars at any one time under the best viewing conditions. Some stars seem brighter than others; some are red, others are blue, yellow, orange, or white. They all seem to twinkle as they shine in the night-time sky. Stars twinkle because we see them through the Earth's atmosphere, which is constantly moving and scattering light. Stars are far enough away that even through a telescope they appear as tiny points of light. The planets, which are much closer, appear to shine more steadily than the stars, because their relatively large disks are large enough that the effect is much smaller.
Size: Stars vary a great deal in size. Some are 1,000 times larger than the Sun, while a few are as small as the Earth. The biggest stars have diameters 400,000 times the diameter of the smallest star. Our Sun is an medium sized star about 1,392 ,000 kilometers across.
Distance: Distances to the stars are measured in light years. A light year is the distance light travels in one year. The speed of light is 300,000 km/second (186,000 miles/second), so a light year is about 9.46 trillion km (6 trillion miles). I n one second, light could travel around the Earth 8 times. The nearest star our Sun is Proxima Centauri, is 4 1/3 light years away, while the farthest known stars are nearly 8 billion light years distant. Astronomers measure the distance to nearby stars using parallax. This is a way of measuring the amount a star seems to move against a background of more distant stars as seen from the Earth as we travel around the Sun once a year. The closer a star is, the more it seems to move against the background of more distant stars.
Magnitude: The apparent magnitude of a star is a measure of how bright it is as seen from the Earth. The brightness is measured by a number scale, with higher numbers indicating dimmer stars. The brightest star in the night sky is Sirius, with a -1.5 magnitude. The dimmest stars you can see in the sky without a telescope are around magnitude +6. There are dimmer stars with higher magnitudes, but they can only be seen through binoculars or a telescope. For comparison, the apparent magnitude of the Sun is -26.8. The apparent magnitude of a star depends on its distance from the Earth as well as the real brightness of the star itself.
Motion: As we look at the night-time sky, all of the stars seem to move together. This motion is due to the daily rotation (spin) of the Earth, the same motion that makes the Sun rise and set each day. Over weeks and months, the stars also move t ogether due to the yearly revolution (orbit) of the Earth. Other than those motions, the stars appear firmly fixed in the sky, never moving with respect to each other. However, each star does have its own motion in the sky, called proper motion. We can not see this motion with the naked eye because the stars move very slowly in relation to their distance from us. Only with telescopes and good measuring tools can an astronomer detect the motion. That is why the arrangement of the stars has looked the s ame to people for thousands of years. Many of the patterns we see in the stars, called constellations, have been with us for just as long. In fact, you would see the same constellations in the sky no matter what planet in our solar system you happened t o b e standing on. The stars are so far away that even if you moved a few hundred million miles, they would not seem to change their relative positions at all.
Composition: Stars are made of many different chemical elements. By using a spectroscope, an astronomer can find out what a particular star is made of by looking at its color "fingerprint." Whenever a material gets very hot, it starts to glow in a particular way. A spectroscope can spread the light out into a rainbow-colored spectrum so that a scientist can see in which colors the material glows, and which colors it doesn't. The spectrum of a hot material appears as a set of bright and dark li nes. That pattern of lines is distinct and unique for every chemical element. Thus, an astronomer can tell what a star is made of just by looking at its spectrum. Stars are mostly composed of hydrogen and helium.
Age: Stars are being born and dying all of the time. Most of the new stars are very far away and cannot easily be seen, even with strong telescopes. However, one easily visible star birthplace is the Orion Nebula. Astronomers watch dying stars often. In many cases, dying stars blow themselves up in what are called supernovae. Sometimes, smaller stars just burn out into cold cinders. Some stars are so big and hot that they burn their fuel quickly and live only a few million years. Smaller st ars may last tens of billions of years. Our Sun is middle aged at about 5 billion years. It should continue to shine for another 5 billion years.
Information Source: http://www.moreheadplanetarium.org/index.cfm