Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/890

818 exist, insomuch that the stars have been divided into four orders, distinguished by their spectra. These are thus presented by Secchi, after examination of the spectra of more than 500 stars:— The first type is represented by a Lyrse, Sirius, &c., and includes most of the stars shining with a white light, as Altair, Regulus, Rigel, the stars (3, y, c,, and 77 of Ursa Major, &c. These give a spectrum showing all the seven colours, and crossed usually by multitudes of dark lines, but always by the four lines of hydrogen, very dark and strong. The breadth of these lines indicates a very deep absorptive stratum at a high temperature and at great pressure. Nearly, half the observed stars showed this spectrum. The second includes most of the yellow stars, as Capella, Pollux, Arcturus, Aldebaran, a Ursae Majoris, Procyon, &amp;lt;fec. The Fraunhofer lines are well seen in the red and blue, but not so well in the yellow. The resemblance of this spectrum to the sun s suggests that stars of this type resemble the sun closely in physical constitution and con dition. About one-third of the observed stars showed this spectrum. The third type includes Antares, α Orionis, α Herculis, β Pegasi, Mira, and most of the stars shining with a red light. The spectra show bands of lines (Secchi s instru ment showed shaded bands, but with superior spectro scopes multitudes of fine lines appear). The spectra resemble somewhat the spectrum of a sun spot, and Secchi infers that these stars are covered in great part by spots like those of the sun. About 100 of the observed stars belong to this type. The fourth type differs only from the last in the arrangement and appearance of the bands, and includes only faint stars. A few stars, as γ Cassiopeise, η Argus, β Lyrae, &c., show the lines of hydrogen bright instead of dark, as though surrounded by hydrogen glowing with a heat more intense than that of the central orb itself. The exact spectroscopic observation of many stars by Huggins and Miller has shown (what Secchi s comparatively rough observations could not effect) that many elements known to us exist in these stars. Thus, in the spectrum of Aldebaran are lines coinciding with the lines of the elements hydrogen, sodium, magnesium, iron, tellurium, antimony, calcium, bismuth, and mercury. In the spec trum of Betelgeuse these physicists recognised in like manner the lines of sodium, magnesium, iron, bismuth, and calcium, but found the lines of hydrogen wanting. Sirius, Pollux, Castor, the stars of Ursa Major, and many others, were examined with similar results.

Many stars are coloured. Thus Antares, Aldebaran, and Betelgeuse are ruddy; Arcturus, Procyon, and Pollux yellow ; Vega and Altair slightly bluish ; while Capella, Sirius, and many other stars are brilliantly white. Among telescopic stars, some are of a deep, almost blood, red, while others are ruddy, orange, yellow, garnet-coloured, and so on. Few single stars show a well-marked blue colour. But among the binary and multiple star-systems, blue, green, indigo, violet, and lilac stars are common, especially as smaller companion stars ; such colours as olive, russet, grey, ash-colour, fawn, and so on, are also observed, though this is probably due to the combined lustre of several very small stars closely set. Complementary colours are not uncommon among double stars, the brighter of a pair (in such cases) usually having a red, orange, or yellow colour, and the smaller showing the respective complementary, green, blue, or purple. Nor are these instances to be explained as due to contrast (though contrast doubtless strengthens the apparent hue of the smaller star); for when the brighter is concealed from view, the fainter is still found to shine with its colour unchanged. Spectro scopic analysis shows that usually the colour of a star is due to the presence of absorptive vapours in the star s atmosphere cutting off certain portions of the light. Thus the orange component of the double star Albireo has a spectrum crossed by several bands in the blue and violet portion, while the blue component has a spectrum showing many strong lines in the red and orange portion, hence the former star looks orange because of the deficiency of blue and violet in its light, and the latter looks blue because its light is deficient in red and orange.

On examining the stars with telescopes of considerable power, many of them are found to be composed of two or more stars placed contiguous to each other, or at distances subtending a very minute angle. This appearance is probably in many cases owing solely to the optical effect of their position relatively to the spectator ; for it is evident that two stars will appear contiguous if they are placed nearly in the same line of vision, although their real distance may be immeasurably great. There are, however, many instances in which the angle of position of the two stars varies in such a manner as to indicate a motion of revolution about a common centre ; and in this case the two stars form a binary system, performing to each other the office of sun and planet, and connected together by gravity or some equivalent principle. The observations of Herschel, Dawes, South, and Struve have placed this fact beyond doubt. Motions have been detected which are so rapid as to be measurable within very short periods of time ; and in certain cases the smaller star has been observed to disappear, either on passing behind or before its primary, or by approaching so near to it that its light has been absorbed by that of the other. The most remark able instance of a regular revolution of this sort is that of the double star f Ursse Majoris, in which the angular velocity is 6 4: annually, so that the two stars complete a revolution about one another in the space of 60 years, and about a circuit and a half have been already described since its discovery in 1781. The double star 70 Ophiuchi presents a similar phenomenon, a revolution being com pleted in a period of about 80 years, a Castoris, y Virginia, Cancri, Bootis, 8 Serpentis, and that remarkable double star 61 Cygni, together with several others, exhibit similar variations in their respective angles of position. Sir W. Herschel observed iu all 2400 double stars. The catalogue of Struve of Dorpat contains 3063 of the most remark able. The object of these catalogues is not merely to fix the place of the star within such limits as will enable it to be easily discovered at any future time, but also to record a description of the appearance, position, and mutual dis tances of the individual stars composing the system, in order that subsequent observers may have the means of detecting their connected motions, or any changes by which they may be affected. There are many triple, quadruple, and quintuple stars. Every region of the heavens furnishes examples of these curious phenomena; but it is remarked that some parts of the heavens contain very few, while others present them in great abundance.

Aggregations of stars more and more complex appear as we extend our survey, and wider and wider regions of the heavens are occupied by groups of associated stars. The most complex and extensive of all such gatherings is the galaxy or milky-way, a zone-shaped region surrounding the whole sphere of the heavens, in which stars innumerable so combine their lustre as to present a milky luminosity. We speak of a zone-shaped region, be it noticed, not of a zone, for the milky-way does not form a complete zone, but simply occupies a zone within which appear streams, and nodules, and irregular clusterings of stars. Along part of the region there is but one well-marked stream; else where two streams are seen, and in other places several 