With regard to the 3rd-type stars, such as Betelgeuse and Mira Ceti, Schuster says, “It has been already mentioned that observers differ as to whether their position is anterior to the hydrogen or posterior to the solar stars, and there are valid arguments on both sides.”
Scheiner, however, shows, from the behaviour of the lines of magnesium, that stars of type I. (Sirian) are the hottest, and type III. the coolest, and he says, we have “for the first time a direct proof of the correctness of the physical interpretation of Vogel’s spectral classes, according to which class II. is developed by cooling from I., and III. by a further process of cooling from II.”[296]
Prof. Hale says that “the resemblance between the spectra of sun-spots and of 3rd-type stars is so close as to indicate that the same cause is controlling the relative intensities of many lines in both instances. This cause, as the laboratory work indicates, is to be regarded as reduced temperature.”[297]
According to Prof. Schuster, “a spectrum of bright lines may be given by a mass of luminous gas, even if the gas is of great thickness. There is, therefore, no difficulty in explaining the existence of stars giving bright lines.” He thinks that the difference between “bright line” stars and those showing dark lines depends upon the rate of increase of the temperature from the surface towards the centre. If this rate is slow, bright lines will be seen. If the rate of increase is rapid, the dark-line spectrum shown by the majority of the stars will appear. This rate, he thinks, is regulated by the gravitational force. So that in the early stages of condensation bright lines are more likely to occur. “If the light is not fully absorbed,” both bright and dark lines of the same element may be visible in the same star. Schuster considers it quite possible that if we could remove the outer layers of the Sun’s atmosphere, we should obtain a spectrum of bright lines.[298]
M. Stratonoff finds that stars having spectra of the Orion and Sirian types—supposed to represent an early stage in stellar evolution—tend to congregate in or near the Milky Way. Star clusters in general show a similar tendency, “but to this law the globular clusters form an exception.”[299] We may add that the spiral nebulæ—which seem to be scattered indifferently over all parts of the sky—also seem to form an exception; for the spectra of these wonderful objects seem to show that they are really star clusters, in which the components are probably relatively small; that is, small in comparison with our sun.
If we accept the hypothesis that suns and systems were evolved from nebulæ, and if we consider the comparatively small number of nebulæ hitherto discovered in the largest telescopes—about half a million; and if we further consider the very small number of red stars, or those having spectra of the third and fourth types—usually considered to be dying-out suns—we seem led to the conclusion that our sidereal system is now at about the zenith of its life-history; comparatively few nebulæ being left to consolidate into stars, and comparatively few stars having gone far on the road to the final extinction of their light.
Prof. Boss of Albany (U.S.A.) finds that about forty stars of magnitudes from 3½ to 7 in the constellation Taurus are apparently drifting together towards one point. These stars are included between about R.A. 3h 47m to 5h 4m, and Declination + 5° to + 23° (that is, in the region surrounding the Hyades). These motions apparently converge to a point near R.A. 6h, Declination + 7° (near Betelgeuse). Prof. Boss has computed the velocity of the stars in this group to be 45·6 kilometres (about 28 miles) a second towards the “vanishing point,” and he estimated the average parallax of the group to be 0″·025—about 130 years’ journey for light. Although the motions are apparently converging to a point, it does not follow that the stars in question will, in the course of ages, meet at the “vanishing point.” On the contrary, the observed motions show that the stars are moving in parallel lines through space. About 15 kilometres of the observed speed is due to the sun’s motion through space in the opposite direction. Prof. Campbell finds from spectroscopic measures that of these forty stars, nine are receding from the earth with velocities varying from 12 to 60 kilometres a second, and twenty-three others with less velocities than 38 kilometres.[300] It will be obvious that, as there is a “vanishing point,” the motion in the line of sight must be one of recession from the earth.
It has been found that on an average the parallax of a star is about one-seventh of its “proper motion.”[301]
Adopting Prof. Newcomb’s parallax of 0″·14 for the famous star 1830 Groombridge, the velocity perpendicular to the line of sight is about 150 miles a second. The velocity in the line of sight—as shown by the spectroscope—is 59 miles a second approaching the earth. Compounding these two velocities we find a velocity through space of about 161 miles a second!
An eminent American writer puts into the mouth of one of his characters, a young astronomer, the following:—