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THE MAGELLANIC CLOUDS. XVI. INFRARED STARS AND STELLAR EVOLUTION BY HARLOW SHAPLEY HARVARD COLLEGE OBSERVATORY Communicated July 15, 1955 1. Everywhere we see clear evidence that throughout a long past the stars have evolved. Time has left marks that can be read by photometer and spectrograph. This evolution, we assume, will continue indefinitely toward the various future ends that are prescribed by the stellar masses and compositions and by the involvements with gas, dust, and neighboring stellar bodies. That the stars evolve is most simply shown by the fact that they shine. Since radiation and mass are equivalent, the masses and related characteristics all must continuously change. But can we in the limited time at our disposal actually witness and measure their evolutionary progress? We record a few progressive changes over the years, but are they, perhaps, cyclic rather than irreversible? Do they actually mark one-way variations in the bright- ness, or size, or vibratory period of a celestial body? (We are here considering, of course, evolution of the stars themselves, not merely changes of position that may indicate the evolution of star associations and galaxies.) 2. For half a century we have recognized changing periods among the variable stars-changes large enough to be numerically evaluated; but often the change has reversed direction, and what promised to be an example of progressive evolution proves, instead, to be a slow, regular or semiregular cyclic change. This frequent failure to progress measurably and unidirectionally in our time is true of luminosi- ties as well as of periods of vibration or periods of revolution in double-star systems. In some eclipsing binaries, for instance, the apparently progressive variation in period length can be charged to slow apsidal rotations in the elongated orbits; sometimes the variations can be attributed to the presence in the stellar system of a third unseen body. The actual running-downi of an eclipsing binary has not yet been recorded, and there is little prospect that it soon will be. With respect to the cepheid variables, we have long thought that such stars, as a consequence of their high output of radiation, must change, irreversibly in mean density, and therefore in period length. A number of cepheid periods are known to change progressively over the years, as found, for example, by Martin' and verified by Miss Wright2 for the cluster cepheids in co Centauri; but in a few decades the direction of the changes may be reversed. (An apparent shortening or lengthening of a period that is actually constant could arise as the result of the "light-time" across the orbit in an unsuspected, widely separated, double-star system; the period appears shorter as the variable component in its orbital motion approaches the ob- server, longer as the variable recedes.) Recently Dr. 0. Struve called attention to the Beta Canis Majoris type of pulsing star (spectral class B) as possibly being subject to evolution sufficiently rapid to reveal changes in magnitude and mean density (i.e., period length) in a short time.- The eruption of an ordinary nova might be taken to represent one phase of the one-directional transition of a star along its evolutionary path, but already, in the relatively brief time covered by our records, a few of the novae have recurred,4 824 Downloaded by guest on September 25, 2021 VOL. 41, 1955 ASTRONOMY: H. SHAPLEY8825 and they must therefore be considered a special kind of cyclic variable star. The supernova, however, is so disastrous that a repetition of its violent explosion is not likely. In fact, the supernova phenomenon, which in some galaxies has affected two or three different stars in less than two decades,5 may be so commoil, that eventually a large proportion of all stars will have been affected by such explosive transforma- tions. 3. The search for measurable evidence of current stellar evolution would have little chance of success if we gauged the speed of evolution of all stars by that of the sun. The terrestrial fossil plants of half a billion years ago prove the sun's long- term stability. But the recent theoretical studies of stellar structure and behavior show that many stars, especially the bright supergiants, are young compared with the earth's crust. They emit light, and therefore mass, at a furious rate. A million years ago many of these giants did not exist as stars. If we take the minutest of Bok's globules6 to be proto-stars in nebulous regions, detectable against a background of bright nebulosity, we must, by any theory of stellar evolution, assume the continuing development of these lightless globules through decreasing volumes and increasing temperatures and luminosities. It is generally accepted, as Whipple and Spitzer and others have supposed, that in its early stages a star formed out of interstellar dust will be such an object of low but rising temperature. It will be distinctly infrared (although hydrogen emission might appear at an early stage). Not until it has contracted gravitationally far toward the point where atomic transformations begin to play a part in the main- tenance of radiation will the star's light enter the visual section of the spectrum. It will at that time be very faint, even though its mass be such that later it becomes a supergiant blue star. The question before us is: How rapidly does a proto-star progress from the zero of luminosity to an absolute brightness thousands of times that of the sun? 4. Current astrophysical theory not only indicates the extreme youth of some of the supergiant stars but suggests that certain galaxies also may be of recent origin- not youthful, of course, with respect to the lives of the atoms of which they are com- posed, but youthful in their present display of millions of superluminous stars. The Large Cloud of Magellan is suggested as possibly being such a young aggregation, and, if it were not for the presence of a number of what appear to be typical globular clusters (of very old stars) among the supergiant stars of that chaotic galaxy, we might surmise that it is all "new." Both the dust, which dims or totally obscures the remoter galaxies, and the gaseous nebulosity are widely distributed throughout both Clouds and are especially abundant in the Large Cloud. It seems reasonable to surmise that stars may currently be coming into existence in the Clouds of Magellan; a number of exceedingly red objects, photographically faint because of low surface brightness, may now be in the course of their swift early contraction from globules of lightless dust to relatively stable stars. With this thought in mind, we have made a preliminary survey of the colors of stars in various sections of the Large Cloud. Thousands of reddish objects were examined. The material available is not ideal for such a research. We should have accurate measures in various colors to much fainter limits than we have attained, and we should have sets of photographs (or photoelectric measures) spaced at in- tervals of ten years for half a century. Downloaded by guest on September 25, 2021 22B ASTRONOMY: H. SHAPLEY PROC.. N. A. S. 5. .The e.trexmely red. stars in the Large Magellanic Cloud, among which we should loqk-forr early stages in the transformation from lightless proto-stars to ra- diant bJlu giants,. are illustrated dramatically by the two photographs (Fig. 1) of the egion-of the Tarantula Nebula (30 Doradus). Thei upper photograph was made with the MF 10-inch triplet at the Boyden Sta- tion, with an exposure of one hour. The emuslion is Eastman 103a-O, and the filter, Corning No. 5840 (UV). The resulting ultraviolet picture emphasizes the very blue stars, of which there are many in this part of the Cloud, and emphasizes also the diffuse nebulosity, which reveals the strength in emission of the higher-frequency Balmer lines of-hydrogen. Very red low-temperature stars- do not appear on this photograph. We are indebted to T. E. Houck for making the plate. The lower- photograph of Figure 1 was made by W. D. Victor -with the ADH Baker-Schmidt reflector at the Boyden Station with an exposure of-two hours. The emulsion s Eastman I--N, and the filter Wratten No. 89A. The extremely blue stars areming from this photograph, and most of the hydrogen emission makes no impressibn. Red'stars of spectral class-MO anid later are conspicuous. The two photographs are.azingly different. The 30 Doradus Nebula and its involve"oluster-of supergiant starsare recognizable near the centers, and the images of stapqwiifspectral type G appear nearly- equal on the two photographs.. A few -of the brighAesars classified as (3 by Miss Cannon in the Henry Draper Catalogue7 are marile on-the upper photograph- of Figure 1. Except for them, it would be dif- ficult -fi entify fields in common. Tj rs 6fmedmcolor-,y pe 5 robably are largely-suosed mem- ber e gal0ctic system. The extremely blue stars must be Cloud members (the galA tius -33 0), and -most- of -the extremely red objects -are probably youii~t~A hsin the` Clud'sgrowing membership. Of the twenty-ne stars that we havesqieted inTable I as being extraordinarily red, only two are in-the vicinity of 30 1O4 0s; 6. ¶'i otbii rough~.color indices for some of the stars that may be in the rapid- trantoiU st4%, we have used plate made with the ADH -reflector. The emu-lonsar-e 103a-O (blue) with no filter, 103a-E (red) with Corning No. 2403 filter, and 4-N,~(infrared) with Wratten No.

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