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Recent Progress in the Interpretation of Stellar Spectra

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Recent Progress in the Interpretation of Stellar Spectra REVIEWS OF MODERN PHYSICS VOLUME 16. NUMBERS 3 AND 4 JULY-OCTOBER, 1944 Recent Progress in the Interpretation of Stellar Spectra OTTO STRUVE Yerkes Observatory, University of Chicago, Williams Bay, Wisconsin, and McDonald Observatory, University of Texas, Fort Davis, rexas I. PECULIAR STELLAR SPECTRA Their spectra are unusual, and their atmospheres HE modern interpretation of stellar spectra do not resemble the reversing layer of the sun. dates from the appearance, in 1920, of The discovery that departures from thermo- ex- M. N. Saha's classical paper "Ionization in the dynamic equilibrium may be important in Solar Chromosphere. " ' Since that time the plaining the spectra of the "peculiar" stars has theory of ionization and excitation of stellar revived interest in the question of the mecha- atmospheres has been developed in considerable nisms which produce excitation and ionization. detail by H. N. Russell, E.'A. Milne, R. H. This is a field with which physicists —practical Fowler, and many others. This theory, which can and theoretical —are much more familiar than be designated as the "classical theory" of stellar are astronomers. Hence, I propose to discuss spectra, is dominated throughout b'y the concept several specific problems from this point of view, of thermodynamic equilibrium and the two in the hope that the information now lacking for fundamental equilibrium formulae based upon their complete solution may in time be supplied it: the Boltzmann formula which determines the by the physicists. ratio of the number of atoms in an excited state In order to understand the peculiar character to the number of atoms in the ground state, and of the spectra I shall discuss, it is necessary to the Saha ionization formula which determines describe briefly the appearance of a normal stellar the ratios of the number of ionized atoms to the spectrum. If we choose all stars included in a number of unionized atoms. The particular typical unit of volume of galactic space, we shall mechanisms of excitation and ionization —photo- encounter an enormous preponderance —perhaps electric ionization, collisions, etc.—play no con- of the order of ninety-nine percent —of stars, spicuous part in this theory and ar'e rarely whose spectra consist of absorption lines on the mentioned in that part of the astrophysical background of a continuous emission spectrum, literature of the past quarter of a century which whose distribution resembles that of a blackbody is devoted to stellar spectroscopy. The applica- having a temperature somewhere within the tion of the concept of thermodynamic equilibrium range of 3000'K and 100,000'K. The absorption with its corollary, the concept of detailed lines are of atomic, or molecular origin, and have, balancing, is, however, only an approximation. with few exceptions, been satisfactorily identi- The very fact that spectral lines are formed in a fied. All of these normal stellar spectra can be stellar atmosphere proves that the state of the classified in a two-dimensional scheme, the rela- atmosphere cannot be accurately described by tive intensities of the absorption lines changing means of these concepts. Hence, astrophysicists gradually as functions of two physical param- are led to ask the question how accurate is the eters: the temperature and the pressure. It is of "classical theory&" It has been shown that for fundamental importance in astrophysics that the ordinary stellar atmospheres of . stars which dependence upon temperature is much more resemble the sun, the departures from thermo- pronounced than the dependence upon pressure. dynamic equilibrium are usually insignificant, Hence, most empirical classification schemes are and the application of the Boltzmann and Saha essentially temperature sequences. The pressure formulae is therefore justified. But in recent dependence is introduced as a small correction years, various phenomena have been found which to this principal sequence. For example, among cannot be reconciled with this simplifying idea. the stars of intermediate temperature, the in one reason Most of them occur stars which, for gradual increase in the intensity of the line Cal "peculiar. " or another, have been classified as 4226 corresponds to a sequence of decreasing ' M. N. Saba, Phil. Mag. 40, 472 (1920). temperature. But two stars having identical lines 86 I NTERPOLATION GF STELLAR SPECTRA of CaI 4226 may show different intensities for mechanism with frequencies of transitions not SrII 4215. Because of the pressure dependence favored by the primary mechanism, Ruorescence of Saba's ionization equation, the star of lower plays an important role. 8owen has called atmospheric pressure shows the stronger line of attention especially to the fact that certain SrII 4215. This results from the fact that the members of high level multiplets of OIII are ionization potential of Sr+ is higher than that of strong, while other members are absent. This Ca, Fe, and most of the other atoms represented he attributes to the fact that the line HeII 303.78 in the spectra of these stars. (1s'g —2p'P) is undoubtedly exceedingly strong An entirely diferent aspect is presented by the in all nebulae (although atmospheric absorption spectra of the nebulae (Fig. 1). Here we see a renders it unobservable) and agrees very closely series of strong emission lines superposed over a with the line OIII 303.80 (2P'P2 —3d'P2). The ' continuous spectrum which presents a fairly emission of HeII causes the OIII level 3d'P2 to conspicuous break at X3647, the limit of the be overpopulated, and all lines originating from Balmer series. The continuum consists of a it are therefore greatly enhanced. superposition of a true recombination spectrum (3) Because high level transitions of certain of H, and what appears to be a blackbody con- ions, OIII, OII, XII, etc. , are weak in the tinuum produced by the non-selective scattering nebulae, Bowen has suggested that the excitation of star light by many small particles in the of low metastable levels occurs predominately by nebula. the mechanism of collisions with free electrons. The emission lines in the nebulae are excited It can be shown that the forbidden lines origi- by three mechanisms, as was first clearly recog- nating from these levels are too strong to be nized by Bowen: attributed wholly to the mechanism of recom- (1) The continuous radiation of stars located bination. in, or near, the nebulae causes photoelectric There exist a number of stars whose spectra ionization of H, He, etc. , and also produces combine the normal features of a stellar spectrum excitation of atoms through line absorption. with features usually associated with gaseous (2) Because of certain chance coincidences in nebulae. These stars are usually designated as the frequencies of lines excited by the primary emission-line stars, and the spectroscopic symbols "?,"';w~::"; ';"''; "»g', '??? ', '. ", ' ' ', j' '»&~:;;:, . .'~;;p',. .q~~'" -',",. ~;». ~', , '„;i.? . »,', , , ~ ~, '", . „""., ',»a., ~, , »a, N~A /pe? g, „' -. ,, , », ;. .„.. ; „. ~ &II%I I I II ~ .?'.. ~'/? ?: »» a I $8RI» II 1»»= g~l I ~ sim 5 l Igy II I ' ~»» ss»»» ~ -- . — — . I ? Fro. 1. Nebulae: (a) Orion, (b) IC 418, (c) IC 2165. OTTO STRUVE TABLE I. Physical characteristics of stars. +Group White dwarf Dwarf Giant Super-giant Nebula Property+ Example Sirius 8 Sun= 8 Capella Betelgeuse Orion Mass 18 18 58 158 Radius 0.018 18 =700 000 km 108 3008 Mean density 10' g/cm' 1 g/cm' 10 'g/cm' 10-6 g!cm3 Height of abs. atm. 100 km) 100 km 1000 km 100000 km 10"km Mean electron pressure of atm. 104—10' d/cm' 100 d/cm' 1—10 d/cm' 10 ' d/cm2 No. of electr. per cm' of atm. 10"—10'~ 1014 1013 1012 10'—104 T ~10,000' 3,000 to 50,000' same same 50,000'K which define their place in the classification bidden transitions of LFe] X, XI, XIII, XIV, scheme are followed by the letter e. These stars XV; LNi] XII, XIII, XV, XVI; LCa] XII, are quite rare, and most of them are faint. They XIII, XV; LA] X, XIV. The ionization poten- have been known for many years from the ob- tials of these ions are as high as 814 volts (for ' jective prism work of the Harvard Observatory, Ca XIV) and represent the highest stages of but in recent years they have been investigated ionization thus far observed in any astronomical mostly at Mount Wilson by P. W. Merrill, at source. The ionization level of the ordinary solar the University of Michigan by D. B.McLaughlin, reversing layer is, by contrast, only about 8 at the Victoria Observatory by C. S. Beals, and volts. The ionization equation shows that ordi- at the McDonald Observatory by P. Swings, and nary thermal ionization, with the solar tem- the writer. perature, cannot account for this high ionization To understand the significance of the emission- level in the corona. ' Hence, the mechanism of line stars, it may be useful to review the principal ionization must be something different. More- physical quantities which serve to describe dif- over, the existence of a layer showing strong ferent kinds of stars and nebulae. This has been forbidden lines in the vicinity of the sun shows done in Table I. The data given are enly rough that the concept of thermodynamic equilibrium approximations for each group and are not the can certainly not be used in describing its accurate values derived for the stars listed in the spectrum. first line. It is of interest to note that Adams and Joy The "classical theory" of stellar atmospheres' at Mount Wilson have found the strongest is concerned with the reversing layers whose coronal lings XX5303, 6375 of LFe XIV] and properties are described in the last four lines of LFe X] in the spectrum of the peculiar star RS Table I.
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