We present evidence that the H II regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escapes from the regions. The key piece of evidence is the presence of glitches, local sharp peaks at an apparently invariant luminosity, in the Hα luminosity

Impacts of meteoroids on the Moon should cause detectable optical flashes, but the population of objects that are big enough is very low, and hitherto no unambiguous impact flashes have been recorded. The flux of meteoroids associated with the Leonid meteor shower of 18 November 1999 was predicted to produce observable flashes on the night side of

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We have obtained intermediate resolution optical spectra of the black-hole candidate Nova Sco 1994 in 1996 June, when the source was in an X-ray/optical active state (R~15.05). We measure the radial velocity curve of the secondary star and obtain a semi-amplitude of 279+/-10kms-1 a value which is 30per cent larger than the value obtained when the

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We have obtained narrowband images and high-resolution spectra of the planetary nebulae NGC 6337, He 2-186, and K4-47 with the aim of investigating the relation between their main morphological components and several low-ionization features present in these nebulae. The data suggest that NGC 6337 is a bipolar PN seen almost pole-on, with polar