Star

STAR, the general term for the luminous bodies seen in the sky, but more especially applied to the "fixed stars," which main tain practically the same relative positions in the sky and form constant patterns or constellations. The planets or "wandering stars" are accordingly excluded (see ASTRONOMY).

Unlike the planets which shine by reflected light from the sun, the stars are self-luminous. They are bodies of the same nature as the sun ; and the only reason why the sun appears so much more brilliant is that it is comparatively close to us. The following figures speak for themselves: Distance of sun-92,900,000 miles; distance of nearest star-25.500,000,000,000 miles. Each star is a fiery globe enormously larger than the earth. Some stars may have planets but we cannot yet detect such planets if they exist. Moreover, theoretical researches warn us against speculation; the system of planets round the sun may be something of a freak.

Although we have been able to learn indirectly the actual dimen sions of many of the stars, their size is not revealed by telescopic observation. In the largest telescope a star remains a point of light, and there is no possibility of observing surface markings such as we see on the sun. One unexpected feature, however, is seen directly a telescope is used; a considerable number of the stars which appear single to the naked eye, are seen in the telescope to consist of two stars. The occurrence of close pairs of stars is far too frequent to be due to chance ; and, indeed, in some cases the two components have been observed to be revolving round one another. We have to recognize that although single stars like the sun are in the majority, evolution has very frequently taken another course and furnished pairs of luminous globes.

It can be recognized with the naked eye that the stars differ in colour. The colour is not very vivid; but, for example, if the reader is familiar with the constellation Orion, he will easily see that the brighter of the two shoulder stars (Betelgeuse) has a reddish tinge in contrast to all the other bright stars in the con stellation, which are white. The quality of the light (crudely indi cated by the colour, and in a more detailed way by examination with a spectroscope) is a clue to the surface temperature of the star. As we should expect, the "white-hot" stars are the hottest, and the "red-hot" stars the coolest. Another fact which can be

verified with the naked eye, though it requires patient and system atic watching, is that some of the stars give an inconstant light which varies periodically in the course of a few days or months. There are several different kinds of variable stars described below; in one class the variation is a genuine physical change, the star swelling and contracting with regular pulsations.

The Stellar Universe.

Thus the statement that the stars are "suns," whilst giving a correct general idea of their nature, must be amplified to cover a wide variety of objects with many indi vidual characters. They differ greatly in physical condition. Betelgeuse (above-mentioned) is an example of one extreme; it is a gigantic globe so large that the whole orbit of the earth could be put inside it, but so tenuous that its density is less than that of air—many a so-called vacuum is not more ex hausted than this. An example of the other extreme is the com panion of Sirius (see Simms), which is compressed to a density far exceeding anything known on earth, and is in size not so large as the planet Uranus. Besides the study of the stars as individual units we have to consider their organization as a system. The principal indication of this organization is found in their motions. Although we often call them fixed stars, they are moving rapidly with speeds of the order 1 o to 10o m. a second. Notwithstanding their great distance, the resulting changes of position in the sky can be determined by refined measurement ; it is the exception for a bright star to have a motion too small to be detected in, say, 20 years. Further evidence of organization is afforded by the distribution of the stars in the sky. They are crowded towards one special plane, viz., that marked out by the Milky Way, or Galaxy, a band of diffuse light stretching all round the sky which, under telescopic examination, is resolved into millions of faint stars. In general, in speaking of the stars we refer to this galactic system or universe ; but it is believed that the spiral nebulae, of which there are a great number, are island universes coequal with our galactic system. In some of these nebulae individual stars can be detected by aid of the largest telescopes.