We measure the radio luminosity function (RLF) of steep-spectrum radio sources using three redshift surveys of flux-limited samples selected at low (151 and 178MHz) radio frequency, low-frequency source counts and the local RLF. The redshift surveys used are the new 7C Redshift Survey (7CRS) and the brighter 3CRR and 6CE surveys totalling 356

The low excitation properties of the planetary nebula (PN) NGC 6720 are known to be unusual, and to imply large ring/core emission ratios. We point out that such characteristics are by no means confined to this source alone, and that high ratios may occur in a large fraction of elliptical and circular PNe. Such trends may arise because of the

This paper presents the results of a detailed theoretical investigation of the iron line formation NLTE problem in a three-dimensional model of the solar photosphere, which we have obtained from a very recent radiation hydrodynamics simulation of solar surface convection. In this first paper we have neglected the effects of horizontal radiative

A combination of two penumbral models similar to those used by Martínez Pillet (2000) is presented. One matches the observed rms fluctuations perfectly while the other reproduces the observed Net Circular Polarization. No factor three mismatch exists in this case. The rationale of using two different, but strongly coupled, models is explained.

The conservation of magnetic flux demands a magnetic field fine structure as small as 15 km to explain the net circular polarization observed in penumbrae (Sánchez Almeida cite{san98a}). A recent work by Martínez Pillet (cite{mar00}) claims that modeling penumbrae as a collection of fluxtubes 100 km wide suffices. We identify several shortcomings

From optical R-band data of the double quasar QSO 0957+561A,B, we made two new difference light curves (with an overlap of about 330d between the time-shifted light curve for the A image and the magnitude-shifted light curve for the B image). We observed noisy behaviours around the zero line and no short time-scale events (with a duration of months