Heterogeneities in the solar atmosphere exist on many different length scales ranging from values as large as the solar radius (~106 km) down to features which are identifiable only by interferometry (~102 km). Rather than simply cataloguing the observed parameters of each and every known type of heterogeneity, I would like to concentrate on a few types of heterogeneities, with a view to identifying the information which is currently available concerning the physical mechanisms responsible for creating the in homogeneities. It is only if we can first identify the physics of each type of heterogeneity that we can hope to take even the first step towards predicting how each particular heterogeneity should scale to other stars. Since the present session is a joint discussion among mainly stellar astronomers, I feel that this approach is probably the most favorable method to present some of the large amount of information now available on solar features. Of course we expect that our solar information will be of most use to stellar astronomers in interpreting observations of stars which have similar spectral types to the sun. Nevertheless, we hope that nature will be kind enough to allow us to scale at least some of our information over a non-negligible area in the H.R. diagram.

The observations of spectrum-variability and light-variability of Ap stars are reviewed. It is shown that these variations are interpretable as due to the changing aspect of the spotted surface as the star rotates. It is stressed that we understand fairly well the geometry of the phenomenon but the physics is very far from being understood.

Magnetic Ap stars are probably those where the presence of a spotted surface is very evident. Their spectrum-variability (profiles, line-intensity and radial velocity), light-variability and magnetic field variability, all occurring with the same period, are explained in a simple way if we assume that these variations are due to the changing aspect of the spotted surface as the star rotates. The oblique rotator model was proposed by Babcock in 1949 and by Stibbs in 1950 and was worked out in great detail by Deutsch (1954). This model allows us to explain the magnetic field variation from some + 1000 to some - 1000 gauss in a few days; it explains the crossover effect, the line-width versus period relations, the line - intensity and radial velocity variation, and in part also the light curves. The main objection against the oblique rotator hypothesis was the supposed existence of many irregularly variable magnetic stars. However, the large number of observations accumulated in the last twenty years indicates that probably all magnetic Ap spectrum-variables are regular variables with periods which are generally of a few days, but includes a small group of long period variables (100 days up to 23 years for HD 9996). The light variability, which is the quantity measurable with the highest precision, has often remained undetected, because the amplitude is always small, in many cases few hundreths of magnitude.

The first detection of spots on dwarf M stars was by Kron on YY Geminorum in 1948. Since then phenomena attributable to starspots have been found by numerous authors beginning with Kraft and Krzeminski on the star now known as BY Draconis and on CC Eridani by Evans. There is a close correlation between spottedness, Balmer emission and flaring in dwarf M stars. Evans appears to have been the first to attempt to analyse the small range light variations on spotted stars in terms of modulation caused by axial rotation. In comparison with the solar case one must assume that the spots are very large and long lived. There is also an ambiguity in dealing with very large dark spots as between a star with a dark area or a cool star with a bright area (the zebra effect— is a zebra a black animal with white stripes or a white animal with black stripes?). It is difficult to arrive at a solution for the spot parameters which is unique. In particular several analyses which have appeared have assumed that maximum light corresponds to the immaculate star whereas in fact the maximum light in the variation corresponds to the immaculate star dimmed by the light of any constant star abstraction.

There is no generally accepted definition of AR Lac Stars, and the term RS CVn stars is used interchangeably or to refer to a particular subgroup. For the purposes of this discussion I use the term AR Lac stars to refer to detached close binaries showing Ca II emission in at least the cooler component outside eclipse, the hotter component being a main-sequence or subgiant star of spectral type F or G. Most of the systems show irregularities in their light curves as well as period changes. In order to determine whether a system is detached, one must know both the mass ratio and the relative radii. The determination of minimum masses is a fairly straightforward spectroscopic task, and provisional values are available for 22 of the systems, two or possibly three of them being non-eclipsing. All but 3 (AD Cap, RT Lac, RV Lib) have masses of the two components within 30% of each other. Because of appreciable irregularities in the light curves, the radii are subject to considerable uncertainty even when photometry of good precision is available. Nevertheless the 9 systems with very provisional radii all appear to be detached. These all have mass ratios near unity. We may assume, as a working hypothesis, that the other systems with mass ratios near unity are also detached and hence also belong in the AR Lac group. Most of the data referred to are to be found in IBVS 1083.

Resolved images of the disks of the largest stars observed with the largest telescopes can be constructed using the class of techniques called speckle imaging. The observations must be made with narrow passbands (~ 10 nm), short exposures (~ 20 ms) compensation for atmospheric dispersion, high magnification and good signal-to-noise ratio. One specific technique applied to a Ori (Lynds et al., 1976) shows slight but apparently real differences in the images of the disk corresponding to low and high opacity in the stellar atmosphere which we interpret as due to temperature differences. There are also significant differences in the star’s diameter and/or limb darkening at the two different opacity wavelengths.

There are several observations that suggest the existence of surface heterogeneities on the T Tau-type stars. Firstly Dr. Ismailov from Shemakha has found strong variations of emission line profiles for 5-10 minutes: 2-component profiles become 3 or 1 component and vice-versa. The results were confirmed recently in the Sternberg Institute. Secondly, variations of polarization degree and relative intensities in the IR calcium triplet were found when the stellar brightness were being constant. Then, the relative intensities of the Ca triplet are not equal to what must be expected for both optically thin or thick but homogeneous emission regions. On the basis of the data Petrov and Sheberbakov from Crimean Observatory have proposed that T Tau stars have active regions with strong magnetic fields similar to stellar ones: they can give noticeable heterogeneities for calcium emission, for polarization effects in dust envelopes and local processes for profile variations. The existence of rather strong and local magnetic fields on T Tauri stars may give a key for explaining the mysteries of these variables. Small variations of strong magnetic fields on stars with a convective zone can give a new mechanism for stellar variability. As it is known, only a quarter of the energy flux that reaches the bottom of the convective zone, is emitted from a sunspot in form of light and the main part of the energy escapes in form of hydromagnetic waves. Then, the sunspot exists if the field is not less than 1 kilogauss. Therefore the field acts as a relay: if the field is low we have light, if the field is strong we have mainly hydromagnetic waves. The transition from low to high field may occupy only a small part of the whole field range. The field variations on T Tau stars can give strong brightness variations without additional energy sources but only by magnetic relay operation. I hope that this scheme gives a possibility to explain a set of features of T Tau stars (such as their high ratio of chromospheric to photospheric radiation, IR excesses, the nature of their flares) as well as main features of FU Ori-type flares.