Spectrum Number of Double Stars
Bright-line 0
Class B 4
Class A-F 131
Class G-N 28
Class M-N? 1
The message which this table brings is clear. The double stars whose spectra are of the Bright-Line and Class B varieties have their components so close together that only 4, of Class B, are visible. The great majority fall in Classes A to K; 159 out of 164. The component stars in these classes are far enough apart to be visible in the telescopes, and yet are close enough to be revolving in periods reasonably short. In the Class M double stars, this program contains not more than one star, and I believe the explanation is this: double stars of Class M are in general so far apart, and therefore their periods of revolution are so long, that they do not get upon programs of rapidly revolving stars. Also, the fainter components in many red stars must have cooled off so far that they are invisible. The distances between the components of visual double stars are in general the greater as we proceed from the helium stars through the various spectral classes up to Class M. There are reasons for believing that two stars revolving around their center of mass have gradually increased their distance apart, and therefore their revolution period. If this is true, the Classes G and K; double stars are effectively older than Classes A and F double stars, and these in turn are effectively older than Class B double stars.
7. The spectrograph has great advantages over the telescope in discovering and observing double stars whose components are very close together, by virtue of the facts that the spectrograph measures, velocities of approach and recession in absolute units—so many kilometers per second—and that the speeds of rotation in binary systems are higher the closer together the two components are. The observations of the brighter helium stars, especially those made at the Yerkes Observatory by Frost and Adams, have shown that one helium star in every two and one half on the average is a very close double. In beta Cephei, an early Class B star, the components are so close that they revolve around each other in 4 1/2 hours; many systems have periods in the neighborhood of a day, of two days, of three days, and so on. Similar observations made with the D. O. Mills spectrographs in both hemispheres have shown that about one star in every four of the bright stars, on the average, is a double star. In general, the proportion of spectroscopic doubles discovered to date is greatest in Class B and decreases as we proceed toward Class M. The explanation is simple: in the Class B doubles the components are close together, their orbital velocities are very high and change rapidly, and the spectrograph is able to discover the variations with little loss of time. As we pass toward the yellow and red spectroscopic binaries we find the components separated more and more, the orbital velocities are smaller and the periods longer, the variations of velocity are more difficult to discover, and in the wider pairs we must wait many years before the variations become appreciable. There is a very marked progression of the average lengths of periods of the spectrographic double stars as we pass from the Class B to the Class M pairs. Similarly, the eccentricities of the orbits of the binaries increase as we proceed in the same direction. Accumulating evidence is to the effect that the proportion of double stars to single stars may be as great in the Classes A to K as in Class B.
8. Kapteyn believes that he is able to divide the individual stars—those whose proper motions are known—into the two star streams which he has described; and he finds that the first stream is rich in the early blue stars, less rich relatively in yellow stars, and poor in red stars, whereas the second stream is very poor in early blue stars, rich in yellows, and relatively very rich in reds. His interpretation is that the stream-one stars are effectively younger than the stream-two stars, on the whole. Stream one still abounds in youthful stars: they grow older and the yellow and red stars will then predominate. Stream two abounds in stars which were once young, but are now middle-aged and old.
The eight lines of argument outlined are in harmony to the effect that there is a sequence of development from nebulae to red stars.
The extremely red stars are all faint, only a very few being visible to the naked eye, and these near the limit of vision. Our knowledge concerning them is relatively limited. That these, and all stars, will become invisible to our telescopes, and ultimately be dark unshining bodies, is the logical conclusion to which the evolutionary processes will lead. As I have already stated, both Newcomb and Kelvin were inclined to believe that the major part of gravitational matter in the universe is already invisible.
It should be said that a few astronomers doubt whether the order of evolution is so clearly defined as I have outlined it; in fact, whether we know even the main trend of the evolutionary process. We occasionally encounter the opinion that the subject is still so unsettled as not to let us say whether the helium stars are effectively young or the red stars are effectively old. Lockyer and Russell have proposed hypotheses in which the order of evolutionary sequence begins with comparatively cool red stars and proceeds through the yellow stars to the very hot blue stars, and thence back through the yellow stars to cool red stars.
I think the essentially unanimous view of astronomers is to the effect that the great mass of accumulated evidence favors the order of evolution which I have described. We are all ready to admit that there are apparent exceptions to the simple course laid down, but that these exceptions are revolutionary in effect, and not hopeless of removal, has not yet, in my opinion, been established.
PHYSICAL CONDITIONS GOVERN APPEARANCES OF SPECTRA
A question frequently asked is this: if the yellow and red stars have been developed from the blue stars, why do not the thousands of lines in the spectra of the yellow and red stars show in the spectra of the blue stars? Indeed, why do not the elements so conspicuously present in the atmosphere of the red stars show in the spectra of the gaseous nebulae? The answer is that the conditions in the nebulae and in the youngest stars are such that only the SIMPLEST ELEMENTS, like hydrogen and helium, and in the nebulae nebulium, which we think are nearest to the elemental state of matter, seem to be able to form or exist in them; and the temperature must lower, or other conditions change to the conditions existing in the older stars, before what we may call the more complicated elements can construct themselves out of the more elemental forms of matter. The oxides of titanium and of carbon found in the red stars, where the surface temperatures must be relatively low, would dissociate themselves into more elemental components and lose their identity if the temperature and other conditions were changed back to those of the early helium stars. Lockyer's name is closely connected with this phenomenon of dissociation. There is no evidence, to the best of my knowledge, that the elements known in our Earth are not essentially universal in distribution, either in the forms which the elements have in the Earth, or dissociated into simpler forms wherever the temperatures or other conditions make dissociations possible and unavoidable.