We present high resolution spectroscopic data of the two giant extragalactic HII regions NGC 5461 and NGC 5471 in M 101. We calculate the physical conditions in the two nebulae with a large number of diagnostics, and determine their chemical abundances by applying ionization correction factors (ICFs) to the observed ionic abundances. The comparison

Microvariations (magnitude changes of several hundredths in timescales ranging from minutes to hours) may arise from three sources: jet and accretion instabilities and gravitational microlensing events. Each may produce different changes on the quasars continuous spectra. For this reason, BVR observations can yield a high degree of discrimination

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The ISO mission has shown the importance of establishing a reliable spectrophotometric calibration over the range of wavelengths covered by its multiple observing modes. We examine the needs for infrared calibration with the new generation of 8-10 metre class telescopes in the light of this experience, in particular the 10-m Gran Telescopio

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Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure that continues from 10 kpc down to 100 pc. Deep inside the bar, two massive CO spirals with streaming motions of 20--40 km/s cover nearly 180 deg in azimuth and connect to two narrow K-band spirals which delineate the