As is well known, the word “chromosphere” was coined to denote the bright, thin, colored ring seen as the solar limb was obscured by the Moon at the time of a total eclipse. This region of the Sun's atmosphere was found to be the source of many strong emission lines — the flash spectrum — some persisting to such heights as to leave no doubt that their cores originated quite high in the chromosphere. The presence of such an emission line region is not unexpected; however, what gives the solar chromosphere special interest is the fact that its observed spectroscopic properties cannot be explained on the basis that it is a simple extension of a “classical” atmosphere for which radiative, hydrostatic, and local thermodynamic equilibrium all apply Thus, the height above the limb to which most eclipse lines persist is inconsistent with the predicted density scale height.

In “Types of Theoretical Models” we describe two basic types of theoretical models — radiative equilibrium and empirical — that are used to represent stellar chromospheres. The next Section is a summary of recent work on the construction of radiative-equilibrium model atmospheres that show an outward temperature increase in the surface layers. Also, we discuss the chromospheric cooling due to spectral lines. In “Solar Empirical Models” we describe the empirical determination of solar-type chromospheric models that, in order to match observations, imply a temperature rise substantially greater than that predicted by radiative equilibrium. Such a temperature rise must be largely due to mechanical heating. An attempt is made in the concluding Section to apply a scaled solar chromospheric model to a star with a different surface gravity. The results suggest that the chromospheric optical thickness is sensitive to gravity and that the width of chromospheric line emission increases with stellar luminosity, in qualitative agreement with the width-luminosity relationship observed by Wilson and Bappu.

Before starting to survey recent observations related to stellar chromospheres, an operational definition of a chromosphere is needed; such definition must satisfy two requirements: (1) it must be bound to a set of observables which we agree indicate the presence of a chromosphere; (2) it must be reasonable in terms of the physical effects which we say characterize a chromosphere. Indeed one does not want to be a priori confined to call chromospheric indicators only those spectral features which, in the Sun, have been attributed to the chromospheric regions of formation of the spectrum, and which, by analogy, can be said to be a sign of a chromosphere in stars similar enough to the Sun.

I would like to describe observations of emission lines in stellar sources, in the ultraviolet region of the spectrum not accessible to ground observation. As we have heard, the interpretation of emission lines may involve both geometrical and temperature effects, so that the occurrence of emission lines does not constitute prima facie evidence for chromospheres. On the other hand, we have not yet, at this conference, formulated a definition of a chromosphere that excludes any particular category of stellar emission-line objects.

In principle, information on chromospheric structure is also contained in the continuum. However, the measurement of accurate spectral energy distributions depends on the very difficult process of ultraviolet photometric calibration. This work is continuing both at Goddard and the University of Wisconsin.

The remarks in this talk will apply only to chromospheres of comparatively late type stars which have significant convective envelopes. This is not to imply that mechanical heating does not occur in other stars, but only that, to the best of my knowledge, little or no satisfactory progress in applying mechanical heating theories to the outer atmospheres of non-solar type stars (without convective envelopes) has been made. Indeed, practically all of the progress that has occurred has been in solar work, so most of my remarks will pertain to the Sun.

The serious work on solar atmospheric heating began in the late 1940's and, since then, has included treatments of wave modes which might be involved and the development of observational techniques to detect them.

To the spatial inhomogeneity, Linsky and Avrett could have added the variations with time which are also well known, well observed characteristics of the solar chromosphere. Let me quote also Praderie (1969): large asymmetries are observed in stellar K2 components which vary with time, “so that it seems difficult to think of any interpretation of the K line profile that would ignore motions and inhomogeneities in the atmosphere of those stars” And let me borrow a conclusion from Thomas (1969): “So what we need are ingenious ideas for empirical inference; or theoretical generalization from experience with the solar case” I wonder if the solar experience is sufficient at the present time to permit any theoretical generalization, as has been the case for the solar wind. In order to simplify, I shall restrict the scope of this contribution to the quiet solar chromosphere, and focus only on spicules.

Stellar evolution carries a star through the Hertzsprung-Russell-Diagram. For a given mass, M. one obtains both luminosity, L, and radius, R, as functions of time, t. From these parameters we determine the surface gravity and the effective temperature as functions of t:

It is these two parameters which determine the properties of the outermost layers of a star, the atmosphere and the top of the hydrogen convective zone.