Stars Essay Research Paper Star a glowing

Stars Essay, Research Paper

Star, a glowing body of gases that also emits heat and other forms of energy that derive ultimately from thermonuclear reactions taking place in its interior. Stars and the vast collections of stars known as galaxies are the building blocks of the universe. Although the characteristics of individual stars vary greatly, our own sun may be described as a fairly typical star. This article deals with the nature of stars and the ways in which their properties are determined, the source of stellar energy, and the birth and death of stars. Related articles in The Encyclopedia Americana include Astronomy, Astrophysics, Cosmology, Galaxy, and Universe. In ancient times the stars were an unresolvable mystery, because no means existed for estimating or even guessing at their true nature. It is not surprising that stars were once considered to be lights on a crystal sphere surrounding the earth, or to be holes on a dark surface through which shone the brilliance of a celestial realm. The first solid foundation for the hypothesis that the stars are in fact distant suns did not exist until the 16th century and the establishment of the Copernican concept that the earth revolves around the sun. Toward the end of that century, astronomers such as Tycho Brahe made measurements of the stars that revealed no observable parallax effects. That is, a given star was observed first from one side of the earth’s orbital path and then, half a year later, from the opposite side of the path. When this was done, no shift in the star’s position relative to other stars could be observed. The implication was that the star lay at a very great distance from the solar system, just as the most distant objects in a landscape on earth seem to remain fixed relative to small changes in the position of an observer. It was not until the 19th century that more precise instruments enabled the shift in position of a relatively nearby star to be measured for the first time. 1. Basic Facts about the Stars On a clear and moonless night, away from the lights of cities, it may seem as though the number of stars shining in the sky is infinite. In fact the number is surprisingly low. No more than about 6,000 individual stars are visible to the naked eye on the entire celestial sphere, and no more than half of that sphere is seen by an observer at one time. Even with the aid of a good pair of binoculars, no more than about 100,000 stars could be counted, and if the entire sky were photographed by means of powerful telescopes, the number appearing on the photographs would amount to about one billion. All the stars seen with the naked eye and practically all those observed through telescopes belong to our own collection of stars, known as the Milky Way galaxy. There are many other galaxies in the universe, but except for the nearest of these the stars they contain are too distant to be distinguished as individual points of light. Our galaxy contains about 100 billion stars, and 100 million galaxies are visible. The Twinkling of Stars One visual characteristic of stars distinguishes them from the five planets of the solar systemMercury, Venus, Mars, Jupiter, and Saturnthat can be seen with the naked eye. The stars twinkle, whereas the planets shine with a steady light except when they are viewed close to the horizon. The twinkling is a combined effect of the great distance of the stars and the turbulence of the earth’s atmosphere. That is, the stars are so far away that even though they are large they act as point sources of light rather than as disks, whereas the smaller planets are relatively close to the earth and are tiny disks in the sky, although this is not discernible to the naked eye. The light coming from the stars and planets fluctuates rapidly in apparent brightness because changes of density in the earth’s atmosphere refract the light and produce twinkling effects. Because the stars are point sources of light, they appear to twinkle. Because the planets are disks, however, the twinkling produced by individual points on the disks is averaged out, and the planets shine with a steady light. Near the horizon, however, the disturbances caused by looking through a greater thickness of atmosphere are sufficient to cause the planets to twinkle as well. Stellar Distances The stars are indeed very far from the earth, but astronomers have developed methods for estimating these distances. Some sense of stellar distances may be gained by comparing them to the greatest distances in the solar system. Light moves at a velocity of about 186,282 miles (299,776 km) per second. At this rate it takes light reflected from Pluto, the outermost planet, about five hours to reach the earth. So vast is space, however, that the time light takes to travel from the stars is spoken of not in terms of hours or days but of years. The light even from the nearest known star, Alpha Centauri, takes a little longer than four years to reach our solar system. It would take a ray of light approximately 100,000 years to pass from one side of our Milky Way galaxy to the other, and distances to other galaxies must be measured in millions of light-years. Range of Stellar Characteristics Although stars appear as point sources of light because they are so distant, man has learned that they are in fact large bodies of glowing gases like our sun. The stars vary greatly in color, depending on their temperatures. Some of them shine steadily, while oth