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We discuss the impact of HST on our understanding of physics of hot massive stars. Since these stars are very luminous, they can be studied as individual stars in several parent galaxies. In order to take full advantage of the high spectral resolution and high signal-to-noise ratio offered by the HST spectra, new highly accurate and reliable methods of spectroscopic analysis have to be developed. We show that thanks to the recent dramatic advances in fast numerical methods it is now possible to construct model atmospheres of an unprecedented degree or realism and accuracy.
We concentrate on two basic issues of the modern stellar atmospheres models of hot stars. First, we discuss the effects of departures from local thermodynamic equilibrium (referred to as non-LTE or NLTE). We show that model atmospheres are now becoming avaliable that include the opacity of tens to hundred thousand metal lines in full NLTE (the so-called NLTE line-blanketed model atmospheres). It is now possible to derive the basic stellar parameters (effective temperature, luminosity, chemical composition) to a high degree of accuracy. Second, we discuss models which do not employ the traditional core-halo approximation, i.e. an arbitrary separation of the static photosphere and the dynamic stellar wind -- the so-called unified model atmospheres. We review the recent research effort in this direction, and show several interpretational consequences of this methodological improvement.
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