In order to explain these singular changes, astrono mers have supposed that the stars are suns, having parts of their surface occupied by large black spots, which, in the course of their rotation about an axis, present themselves to us, and thus diminish the bril liancy of the star. Some astronomers suppose the black spots to be permanent ; but others are of opinion, that the luminous surface of these bodies is subject to per petual changes, which sometimes increase their light, and at other times extinguish it. The same phenomena have been explained, with less plausibility, by suppo sing, that the stars are extremely flat, and that they are more or less brilliant according as their flat or sharp side turns to the eye. The variation in the light of the stars . _ has also been ascribed to the interposition of the planetP which revolve around them ; but it is not probable that the planets are sufficiently large to produce any sensible effect : Even when seen from the earth, the light of our own sun is not perceptibly impaired when Mercury or Venus are passing over his disc. A complete list of new and variable stars, with a full account of their changes, will be given in a tabular form among the Ta bles at the end of this article.
See Hevelius, Phil. Trans. 1665, p. 372 ; 1670, p. 2087 ; 1671, p. 2197. Bullialdus, Phil. Trans. vol. i. p. 381. Anthelme, Phil. Trans. 1670, vol. v. p; 2092. Tycho, Progymnasmata, ant de nova Stella anni 1572. Kepler, de Stella Nova in Pede Serpentarii. Phil.. Trans. 1715, vol. xxix. p. 354. Goodricke, Phil. Trans. 1785, p. 153. Pigott, Phil. Trans. 1785, p. 127 ; 1797, p. 193. Herschel, Phil. Trans. 1780, p. 338 ; 1792, p. 24; 1795, p. 166 ; 1796, p. 452. See also ALGOL, and the references under that article.
On Nebula, and the Milky Way.
If we examine the heavens with a good telescope, we shall find that the stars are not uniformly scattered over the firmament. In some places they are crowded to gether, and in other parts of the sky there are blank spaces where no stars can be seen. Besides these starry groupes where the individual stars are distinctly visible, there are numbers of small luminous spots of a cloudy appearance, called Nebula'. The largest of these nebula is the galaxy or milky way, a white and luminous zone which nearly encircles the heavens, and which appears to be the nearest of all the nebula. Dr Herschel exa mined this interesting part of the heavens with very powerful telescopes, and found that its whiteness was owing to an immense number of small stars. A portion of the milky way, about 15 degrees long, and 2 broad, contained no fewer than 50,000 stars, large enough to be distinctly counted, and the Doctor suspected twice as many more, which, for want of sufficient light in his telescope, he saw only now and then. The attention of
Dr Herschel was next directed to the nebula, which he found to be composed of stars, or at least to contain stars, and to exhibit every other appearance of being entirely composed of them. These nebula were gene rally arranged into strata, and ran on to a great length. One of these nebulous strata was so crowded, that in passing through a section of it in the time of 36 minutes, Dr Herschel discovered 31 nebula distinctly visible, but varying in their shape, situation, and condition. In an other stratum he saw double and treble nebula variously arranged : Some of the large ones were accompanied with others that were smaller ; some of them were long and narrow bright dashes ; others had the shape of a fan, like the electric spark issuing from a lucid point. Others resembled comets, with a nucleus or centre, or cloudy stars, surrounded with a nebulous atmosphere. A different kind contained a milky nebulosity, similar to the light about B Orionis. These various appearances of the nebula are represented in Plate XL, Fig. 8, 9, 10, 11, 12, 13, 14, and in Plate XLI, Fig. 1, 2, 3, 4, 5, 6, 7, 8. Fig. 8, Plate XL, is the 98th "JiIebuleuse sans etoiles," in the Connoissance des Temps for 1784, ac _.
cording to Dr Herschel. Fig. 9. is the 53d of the same catalogue.
The immense size of the milky way, and the circuit which it makes round the heavens, induced Dr Herschel to imagine, that it is a large nebula, in the middle of which our own system is placed. If we suppose this nebula to have the form represented in Fig. 9. which is a section of it in a plane perpendicular to its greatest diameter, and the solar system to be placed at S, it will exhibit all the phenomena which appear in the milky way. The brightness of the milky way will be greatest in the directions Sa, Sb, Sp, where the intervening stars are most numerous. In the lateral directions Sin, Sn, the nebulosity will not appear ; and in the direction Sc, on account of the opening between a and b, there will be an empty space, comprehended between these two branches, where the nebulosity is not observed, as is the case in the milky way between 1.c. Scorpionis in the south, and 9 Cygni in the north, a space. of ahnht 1n2 degrees. The circle drawn round S, in Fig. 9, is at 40 times the distance of the nearest fixed stars, and proba bly comprehends all those which are visible to the naked eye.