ee a in interstellar space, but we shall not discuss them. Interstellar absorption lines can be distinguished from stellar ab- 2.0- . T 4 i t T i Theoretical Spectra from Models t When the first model atmospheres were constructed, the radiation-block- sorption lines by the fact that, in a multiplet, only the component that arises from the ground state is observed. The dilute radiation and low particle density in interstellar space cannot excite atoms and ions even by 0.01 ev. ing effects of the strong stellar lines in the ultraviolet spectral region were ignored. These models, for stars of types O to B2, predicted a rather bright continuous spectrum in the ultraviolet spectral region (Fig. 1). Even so, it may be seen that only stars of “ LOR - = aad In the case of normal dwarf or mainsequence stars, the geometric extent of the layers through which the line spectrum is formed is smail in comparison to the radius of the star. Consequently it is sufficient to consider that the steilar atmosphere consists of plane parallel layers of gas in hydrostatic equilibrium under the local acceleration of gravity. On the surface of a mainsequence star, gravity is about 104 centimeters per square second. The composition of the stellar atmosphere is constant throughout the relevant layers. The theoretical spectra to be presented have been computed for models having a fractional abundance, by weight, of hydrogen equal to 0.68; of helium, 0.32. The fractional abun- dance by weight of all other elements is small (1.41 x 10—-+* for Fe, for in- stance), and it is usually neglected when a model is constructed. When the line spectrum is computed, a representative abundance, relative to hydrogen, for each element is adopted according to the results of abundance analyses of stellar spectra. The condition that hydrostatic equilibrium should exist gives a relation between gas pressure and geometrical depth in the atmosphere. The relation between temperature and depth in the atmosphere is found by requiring that a constant flux of energy flow through the model. As the radiation field in alt frequencies from 0 to « passes through each layer of the model, no energy is lost or gained, but the energy is re- distributed in frequency as a result of the interactions with the atoms and ions in the atmosphere. In principle this redistribution may be followed by solution of an equation of radiative transfer appropriate to the physical situation encountered. In practice these interactions between atoms, ions, and radiation are represented in a somewhat schematic manner that is complex enough to describe the major 1274 spectral types O to B4 will produce a - Models of Stellar Atmospheres =t a 5 ©.0}- E 4 2 a Zz =z = [ |, S000A 2500A o ! 2 i] 4 4 |. 3 250A 1 6 1. 8 { wave number (2!) WwW | TA ! count; only O-type stars will produce f 2 '|4 Fig. 1. The continuous spectrum of OB stars, predicted without attention to the blocking of radiation by the ultraviolet lines. The unit of flux is 10° erg per square centimeter frequency interval. per second per unit simple enough for expeditious handling by a large computer. Preliminary studies have indicated that only stars classified, according to empirically selected spectroscopic criteria, types O or B may be expected to produce an ultraviolet flux of energy comparable to that in the normally observed spectral region; they are among the hottest known, Consequent- ly the results we present have been calculated with equations suitable for atmospheres in which the electron temperature varies between 8000° and 60,000°K. brief, the monochromatic flux emerging from the stellar surface has been calculated by use of the MilneEddington transfer equation. This equation represents the postulates that the radiation is removed from the beam by absorption in the continua of H, H-, He I, and He II and in lines, and as a result of coherent isotropic scattering by electrons; and that energy is returned to the beam by reemission, as though the gas were in thermodynamic equilibrium at the local elec- tron temperature, and as a result of coherent isotropic a significant flux of energy at wavelengths shorter than the Lyman limit. A few models have been constructed that take into account the line blanketing (3). An example of the resultant changes in the overall intensity distribution of the emergent spectrum ap- trends of the processes that occur but In greater flux between 3000 and 911.6 A than they produce in the spectral region observed with ground-based instruments (2). The predicted ultraviolet fluxes below 1500 A will be reduced considerably when the absorption by the ultraviolet lines is taken into ac- scattering by the electrons. The methods of constructing model stellar atmospheres and of computing the line and continuous spectrum have been described (J). pears in Fig. 2; both models shown may be classified type B1.5 according to the size of the Balmer jump. For main-sequence B-type stars a singlevalued empirical relation exists between spectral type and the intensity jump at the Balmer limit of hydrogen; since this relation is rather insensitive to gravity and to the electron pressure in the atmosphere, it may be used to obtain a first approximation of the equivalent spectral type of a model atmosphere. In practice, spectral types are assigned to stars according to the relative intensity of a few empirically selected strong absorption lines. Including the strong lines in the procedure for constructing the model is physically more correct than ignoring them. One result is that the effective temperature is reduced by about 10 percent from its value in an unblanket- ed model having the same Balmer jump and thus nominal spectral type. The temperature-pressure structure is not significantly changed in the deeper layers of a blanketed model from that in an unblanketed model of the same spectral type. However, in the extreme outer layers of a blanketed model, the temperature for a given value of the pressure is lowered from its value in an unblanketed model. This difference between models having the same nomi- nal spectral type will be important only for interpretation of lines so strong that they are formed effectively in the SCIENCE, VOL. 158