We study the effect of microturbulence in the line formation calculations of H and He lines, in the parameter range typical for O and early B stars. We are specially interested in its effect on the determination of stellar parameters: Teff, log g and specially on the He abundance. We first analyze the behaviour of H and He model lines between 4 000

We present a mechanism to produce X-ray transient periodic variations in AGN. This mechanism is triggered off by Compton scattering in the infalling plasma into a black hole, which can induce an electric charge in the singularity. The charging and discharging processes can affect the X-ray luminosity of AGN.

The dynamo model of solar magnetic field generation assumes that magnetic flux is retained at the dynamo site for times of the order of the solar-cycle period. However, flux tubes in the solar convection zone are expected to be buoyant, rising to the surface on timescales much shorter than the solar cycle. Since the initial 1955 paper by Parker on

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We present a study of the diffuse ionized gas (DIG) for a sample of nearby spiral galaxies using Hα images, after constructing their H II region catalogues. The integrated Hα emission of the DIG accounts for between 25% to 60% of the total Hα of the galaxy and a high ionizing photon flux is necessary to keep this gas ionized. We suggest that Lyman

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