We critically review the basic assumptions of the standard model for the synthesis of the X-ray background (XRB) in the light of new data from ultradeep surveys by Chandra and XMM, resolving major parts of it. Important constraints come in particular from the observed redshift distributions of faint hard X-ray sources - showing large excesses at

Generally, chemical peculiarity found for stars on the upper main sequence excludes delta Scuti type pulsation (e.g. Ap and Am stars), but for the group of lambda Bootis stars it is just the opposite. This makes them very interesting for asteroseismological investigations. The group of lambda Bootis type stars comprises late B- to early F-type

We present observations of lunar Leonid impact flashes recorded in 2001 November from Spain. Four impact flashes were detected on November 18. Another flash was also recorded on the same night, which appears to be impact related, and two more on that night are possibly, but not unambiguously, of impact nature. On November 19 another flash was

In this work we investigate the evolution of the X-ray emission of a cluster of single young massive stars with different metallicities. We have considered the X-ray contribution originated by the diffuse gas heated by the mechanical energy released by stellar winds and SN explosions as well as the X-ray contribution from SN remnants. The resulting

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Using CCD observations in the narrow band filters in Hα , Hβ , [O Ii], [O Iii], [S Ii] and [S Iii] lines we have compiled a catalogue of 338 H Ii regions for the inner parts of NGC 5457 and 158 for NGC 4395. The extinction, equivalent widths, distributions, excitations, ionization hardness, ionization parameters and metallicities for these regions