STAR (AS. steorra,OHG.sterro, sterna, Ger. Stern., Goth. stairm5, star, connected with Lat. steno, Gk. do-rhp, aster, ('orn., Bret. stereo, Arm. astX, Skt. tura, star. Av. star.), star, and possibly with Skt. star, to strew). One of those heavenly bodies which remain apparently im movable with respect to one another. Hence they were early called fixed stars. a name which they still retain, although their perfect fixity has been completely disproved in numerous cases, and is no longer believed in regard to any. or scintillation is mark which stars from planets. The first thing that strikes the observer is the apparent daily motions of the stars. The greater part appear to rise in the east, describe smaller or greater arcs in the heavens, and set in the west; while others describe complete circles around the pole of the heavens. These apparent motions arise from the rotation of the earth on its axis. (See Email.) With few exceptions the distance of the fixed stars is still unknown, and must in all cases be enormously great. Since the time of Bradley ninny attempts have been made to measure what is called the annual parallax (q.v.) of the stars, and thus determine their distances. When we consider that the motion of the earth round the sun alters our position in space a whole diameter of its orbit (185, 000,000 miles) in six months, we should ex pect a change in the relative positions of certain stars as seen from two opposite points of the orbit. But no such change is seen to take place, and this was one of the early objections to the theory of Copernicus (q.v.). The only answer that the Copernicans conld give was that the distance of the stars from us is so great that the diameter of the earth's orbit shrinks into insignificance when compared with it. The de tection of the parallax of the fixed stars de pended upon the perfection of instruments. The parallax of a star is the minute angle contained by two lines drawn from it, the one to the sun, the other to the earth. If that angle amounted to a second the distance of the star would be at least 206,000 times that of the sun; and when the measurement of angles Caine to be reliable to a second and still no parallax was discernible astronomers could say that the distance of the nearest stars must be more than 206,000 times that of the sun, i.e. 206,000 times 93,000,000
miles.
The method now in use for measuring stellar parallaxes was first applied successfully by Bes se] (q.v.) in 1838. He employed in his observa tions a remarkably fine heliometer (q.v.) and adopted what is called the differential method. having selected a star which he suspected might be near us, and therefore have a parallax large enough to measure, he proceeded to determine every clear night its position in the sky rela tively to two neighboring very small stars. relative (differential) determinations of posi tions can be made with far greater accuracy than measures with the meridian circle (q.v.) ; and Besse] judged rightly that the two minute com parison stars were really so immeasurably far away that they would themselves suffer no ap preciable parallactic displacement. Sure enough, he found his 'parallax star:' 61 Cygni, slowly de scribing day after day a tiny oval curve in the sky, reproducing there the earth's orbit in space. But the comparison stars did not move. The proof was complete that 61 Cygni had an appa rent motion due to the real motion of our earth, and that it was meaRurable. This observation of Bessel's is one of the most famous in the annals of astronomy.
When other astronomers had succeeded in re peating Bessel's observations, and quite a number of stars came to have known parallaxes. their distances were found to be too great to be ex pressed conveniently in miles. or any other linear unit. Therefore astronomers invented a new unit, the 'light-year,' being a distance equal to the space traversed by light in one year. As light moves about 180.000 miles per Jecond, it will be seen that the light-year is a unit of stupendous magnitude, and will be fitted to measure the profound distances of stellar space. A few• of the larger stellar parallaxes at present known are as follows: In this table the second column gives the 'magnitude' of the stars. The third column gives the parallax, or angle subtended by the earth's orbit-radius at the star. The corresponding dis tance in light-years is in the last column. Thus we do not see the stars as they are to-day, but as they appeared so many years ago.