Star

a heavenly body shining with its own light. This definition excludes the planets and their satellites, but includes the sun, which is supposed to differ from other stars mainly in our being near it. It is worth while to know that the stars twinkle but the planets do not. Many of the stars are larger and much hotter than the sun; some are smaller and cooler. Six or seven thousand stars are visible to the average eye. About 2,500 are visible at any one time. No one can see all the stars without observing in both hemispheres. About 100,000 stars may be seen with a common opera glass. Three times that number are visible through a small telescope. A telescope forty inches in diameter reveals possibly 100,000,000 stars. Stars not visible to the eye through the most powerful telescope may be photographed by the delicate instruments now in use by astronomers. It is impossible to say whether the stars thus noted are practically all there are in the universe, or whether we have only made a beginning in seeing them.

The entire space of the heavens is charted in sixty-seven areas called constellations. This custom has descended to modern astronomers from the ancients, who were wont to decorate their charts with the figures of various animals and goddesses whose names were given to the constellations. The groups are thoroughly artificial, but are very useful in locating a star. Some of the brightest stars have individual names, but the great majority are designated by the name of the constellation and a Greek letter. Even this system fails in the case of telescopic stars, because the number of letters is not sufficient to name all the stars within one constellation. Astronomers have therefore resorted to numbers, each star being given its own catalogue number. The largest number in any one catalogue thus published is about one-third of a million. Of late charts have been formed by telescopic photography; that is to say, by a combination of the telescope and photographic camera.

Harvard observatory employs fifty assistants and has accumulated many thousand photographs taken at Cambridge for the northern hemisphere and in Peru for the southern. An ingenious mechanical arrangement enables an observer to put his camera under control of clockwork so that the camera follows a particular spot in the heavens at exactly the same speed with which the earth would turn it away from that spot. If a luminous body in the area under observation moves, it creates a trail or path on the photographic lens, calling attention to its movement instantly. A series of such photographs furnishes a complete history of the movements of heavenly bodies. Stars are classified according to their brightness. The ancients recognized six magnitudes. The first magnitude includes about twenty of the brightest. It is estimated that all together the stars give about one-sixtieth as much light on a clear night as a full moon. Some heat is doubtless received from them, but the amount is so slight that it has been impossible thus far to detect it. As distinguished from the planets, the true stars are sometimes called fixed stars, but it is certain that many and perhaps all are in motion like our sun. See Arcturus; Orion; Pleiades; Milky Way; Telescope; Observatory