The emphasis at the present time is upon problems connected with the origin and evolu- tion of double stars. G. P. Kuiper's discussion in three articles in the Publications of the Astro- nomical Society of the Pacific2 in 1935 and 1955 forms a sound basis for future work. He attrib- utes the origin of double stars to the formation of separate condensations in a primordial inter- stellar cloud which possessed too much angular momentum, or too much mass, for the forma- tion of a single star. In this respect the origin of a multiple system does not differ significantly from that of a star cluster. But the later evo- lution of a binary differs from the evolution of a loose cluster. A visual double star is a very stable formation and is not likely to change very greatly during many hundreds of millions of years, while a galactic star cluster may be dispersed in the Milky Way in an interval of ten or a hundred million years (though at least one cluster, NGC 67, is believed by H. Johnson and A. Sandage to have an age of 5X10* years).
Since we have good reasons for believing that some galactic clusters are very young (the asso- ciations of Orion and f Persei, the double cluster in Perseus, NGC 6231, etc.) while others are much older (Pleiades, Hyades, Praesepe, NGC 725 and NGC 67), it would be of great interest to know whether any binaries in these clusters show differences in their properties that might be related to the differences in their ages. As yet, our information concerning all types of binaries in galactic clusters is too fragmentary for us to draw any conclusions.
Of special interest is the question of the evo- lution of close binary systems. From Kuiper's work on /3 Lyrae, we know that two types of instability may occur, with matter escaping from one or both components either through the
1 Department of Astronomy, University of California at Berkeley, Calif.
» Vol. 47, pp. 15, 121, 1935; vol. 67, p. 887. 1955
bottleneck near the inner Lagrangian point, Llt of the critical zero-velocity surface (instability of type A), or through an opening at the first outer Lagrangian point, Zj (instability of type B). Kuiper thought that the instability of fi Lyrae is a combination of types A and B, but more recent work seems to indicate that both components are unstable and lose matter in the form of two streams moving in opposite direc- tions through the bottleneck near L\. There can be little doubt that this phenomenon pro- duces the secular change in the period of 0 Lyrae. The loss of matter in /3 Lyrae, if the streams are ultimately expelled into space, must amount to about 5 X 1 0 " gr/sec in order to ex- plain the increase of the period. Since the mass of the star is of the order of 103* grams, the life expectancy of /3 Lyrae should be of the order of 100,000 years.
No other star is known that closely resembles /3 Lyrae (in itself a good indicator of the short duration of its particular stage of evolution).
However, many other systems have gaseous rings or streams and many of them, according to F. B. Wood, have changing periods, especially when at least one component is a large subgiant whose volume fills its loop of the critical zero- velocity surface.
One of the most pressing needs in this field is a systematic, prolonged study of the periods of all suitable eclipsing and spectroscopic binaries.
The photometric observations would be rela- tively simple, but should be made with photo- electric means. Results of great value would become available in a few years; but even more important would be a list of accurate periods during the next 10 years which could later be compared with similar lists at intervals of about 25 years. We cannot start too soon with such a program!
The spectroscopic observations could, for many systems, be carried out with telescopes
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of moderate size. We need periodic deter- minations of the velocity curves of all the brighter spectroscopic binaries. Here again results may be expected immediately, not only with regard to the periods but also with regard to changes in the orbital elements. Algol, Spica, 5 Orionis, and many more should be reobserved at least once every 10 years.
One of the most puzzling questions in the field of double-lined spectroscopic binaries is the interpretation of the tendency of many systems to show a strengthening of the violet- displaced absorption lines relative to the red- displaced components, irrespective of which of the two stars happens to be approaching.
This phenomenon was discovered by Miss Maury at Harvard in the spectra of n Scorpii and V Puppis. But later observations at McDonald Observatory failed to show this variation. Yet, independently, we found that it was conspicuous in Spica and in several other short-period spectroscopic binaries of early class B or O. A recent study of v Scorpii by S. J. Inglis shows that it is present in the helium lines but that it may be reversed in sign in the hydrogen lines. I had previously assumed that there is more gas in front of the advancing hemispheres of each component than in front of the receding hemispheres.
However Inglis' result suggests that it may be caused by a difference in ionization and excitation. As far as I know, there are no photoelectric measurements that would help us in explaining this phenomenon. Since it occurs only in close systems (it is also con- spicuous in many W Ursae Majoris systems) I suspect that it is in some way related to the existence of gaseous streams.
S. S. Huang has recently asked whether there might not be accumulations of gaseous material at those two Lagrangian points which correspond to the equilateral triangle solutions of the restricted three-body problem. Unless the mass ratio of the binary is very different from one, these solutions are not stable, but there could still be regions of increased density in the gaseous ring at L% and L4. It is possible that a more careful study of the eclipses of the emission features in the hydrogen lines of RW Tauri, RW Persei, SX Cassiopeiae, U Sagittae,
and many more would enable us to locate the regions in which these emission lines are produced. Joy's original interpretation of RW Tauri in terms of a ring around the brighter component of the binary, or my later interpre- tation in terms of a ring of gas surrounding the entire system, may have to be modified in the light of Huang's idea.
During the interval 1940-1950 several McDonald astronomers made a superficial sur- vey of the velocity curves and spectroscopic features of about 100 eclipsing variables. Most of the systems presented no peculiarities and the results were sufficient for statistical studies, but a considerable number yielded results of exceptional interest. A few of these stars have since been investigated more exhaustively (U Cephei, U Sagittae, SX Cassiopeiae, RX Cassiopeiae, RZ Scuti), but quite a number have not yet been scrutinized. I can mention here only a few of them. WX Cephei (J.
Sahade and C. U. Cesco) shows amazing changes in the intensities of the two Ca II absorption components. They are unlike the changes discussed above. DN Orionis has an exceptionally small range of velocity and, therefore, a very small mass function. It is one of the most peculiar binaries in this respect, resembling XZ Sagittarii (J. Sahade). Both must possess subgiant components of very small mass (about 0.1 or 0.2©) but of large di- ameter. These components violate the con- ventional mass-luminosity relation. AW Pegasi and V 377 Centauri (Sahade) are even more peculiar, and we have been unable to interpret their spectra. Details about these and other systems may be found in the Astroph3rsical Journal or in the Contributions from the W. J. McDonald Observatory.
The famous visual double star 7 Virginis has been suspected by Lick astronomers to vary in light. Since both stars can be observed spectroscopically with high dispersion, a test of each component (or of the integrated light of both) for variability would be of interest.
Any visual binary whose orbit is known should be investigated at least in integrated light for variability. The discovery of an RR Lyrae or another pulsating component in a double star, whose masses are known, would be a major
NEW HORIZONS IN ASTRONOMY
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triumph. It might even be possible from the colors (or spectra) and magnitudes to pick out promising pairs, but the chances for success can hardly be estimated. I recommend this important search only to those who have other problems that are certain to give sig- nificant results.
A few eclipsing binaries must be observed as nearly continuously as is possible, both photo- e l e c t r i c a l l y and spectroscopically. Among these I should list e Aurigae, W Cephei, /S Lyrae, UX Monocerotis (R. Lynds suspects that the small, bright, and hot component of this system is a short-period pulsating variable, while the other component is a large and cool subgiant). No spectrographic equipment is now available for such continuing observations.
I therefore endorse Th. Dunham's project for the construction of a new spectrographic telescope.
I hope that someone will have observed the light curve of « Aurigae over a wide range of wavelengths. It is important to know whether the depth of the eclipse (a totality began Dec.
15, 1955, ending 330 days later) is really the same in all wavelengths.
What is the dispersion of the mass-luminosity relation, expecially at the upper end? If nuclear evolution proceeds without appreciable change in mass, we should expect some scatter among the O- and B-type stars. Most of our present information comes from the velocity curves of double-lined binaries, and this prob- lem is looked after by the astronomers at Vic- toria. Special attention should, however, be devoted to such stars as J. S. Plaskett's massive binary, in which the more massive component has the weaker absorption lines; the profiles of the lines of this component show irregular changes.
It is not yet clear whether the yellow and red giants in Population I obey the conventional mass-luminosity relation. K. O. Wright's work on Capella seems to indicate that it does, and my own earlier measurements suggested only a relatively small departure. D. M. Popper's work on f Aurigae and other similar systems may soon solve this problem. It may help us in constructing the evolutionary tracks of massive stars, but here again we must be
prepared to find that the giants lose mass by expansion. A. J. Deutsch's work on the ex- panding shell of the system of a Herculis sup- ports this hypothesis.
What is the nature of the so-called Trumpler stars? Some of them occur in binary systems but we have as yet no firm knowledge of their masses. As Trumpler has shown, their lines have large red shifts which, if interpreted by the theory of general relativity, would give very large masses (or very small radii). There are some reasons for believing that their masses are not more than about 75o> But how do we then explain the red shifts?
Are all post-novae and all SS Cygni stars close binaries? And why are there so many Wolf-Rayet stars which are components of close binary systems? Are all of these objects the products of the evolution of binaries (as Huang suggests in a recent article in the Astro- nomical Journal8) and are the RR Lyrae stars the products of nuclear evolution of single stars (or of wide visual binaries) uncomplicated by the loss of mass resulting from the instability of a close binary? It is not yet clear why loss of mass into space (as distinguished from the formation of streams within a binary, or the exchange of mass between the components of a binary) should be significantly different.
We must assume that once a stream of gas is generated in a binary it is somehow expelled from the entire system, and that the absence of such streams in single, slowly rotating stars impedes their loss of mass by "corpuscular radiation."
It would be unreasonable to conclude this report without stressing the importance of continuing routine observations of all classes of double and multiple stars. In preparing his program, an investigator should not be concerned solely with the solution of immediate problems but should devote a part of his time to the accumulation of accurate observations which will bear fruit long after his death.
The most important astronomical problems—
those of evolution of stars, of perturbations in double star orbits, of slow periodic or irregular variations, and many more—require tens or hundreds of years of persistent effort. It is
•VoLftl, p. 40, 1956.
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especially difficult to sustain an enthusiastic every astronomer has profited from the observa- attitude on the part of a scientist if he does tions of his predecessors; he should therefore not expect to gather the harvest resulting be willing to pass on to his successors a store from his labor. Nevertheless, we must all of good observations to make up for what he follow the dictum of Ejnar Hertzsprung: has inherited from former generations.