The possibility that radiative instabilities in stellar atmospheres are driven by transfer of energy processes between stellar gas and the radiation field is considered. Harmonic temperature fluctuations in gray radiative equilibrium atmospheres are introduced, and the linear response of the radiation field to the ensuing Planck-function and

Temperatures and angular diameters are derived for 114 F-M stars using the infrared flux (IRFM) method and initially both the Vega flux calibrations of Dreiling and Bell (DB), and of Mountain et al. (1985). The DB calibration gives a significantly smaller variation of temperature with wavelength (0.10%) than the Mountain one (1.59%), and we adopt

Optical and radio observations of the BL Lacertae object OJ 287 made early in 1989 show a sudden fade in both wavelength ranges. From January to early April the V magnitude was near 16. However, in late April and early May B = 18.4, V = 17.4, R = 16.7 and I = 15.8 were observed. At the same time, the flux at 22 GHz and 37 GHz fell to 1.9 Jy and 1.7

The variability of the broad emission lines in Seyfert galaxies is naturally explained in terms of photo-ionization models of the broad line region (BLR) as the response of the emitting gas to changes in the central ionizing continuum. An analysis is made of the response to a single continuum event of a simple model of the BLR consisting of an

New long-slit optical spectroscopy and optical and NIR images of NGC 520 are presented. Velocity-dispersion and radial-velocity profiles are obtained from the spectra with the Fourier quotient and cross-correlation techniques. At the NW optical emission peak, a sharp rise in the velocity dispersions implies a concentration of mass. The NW optical

Observations of interstellar absorptions superposed on the chromospheric Mg II emissions of nearby cool stars have been used to explore the structure of the local interstellar medium along many lines of sight. The interstellar cloud in which the sun is immersed (the 'local cloud', hereafter LC) has not previously been detected using this technique