The ultraviolet radiation from normal stars shown here for selected stars with effective temperatures ranging from 20000 K down to 4500 K (spectral tyes B0 through K0) as observed by the 80 cm space telescope ASTRON shows a strong increase towards shorter wavelengths for hot stars while for cool stars the ultraviolet flux drastically drops down.
The best way to study the physical conditions and the chemical composition of atmospheres and envelopes of hot stars is to observe their ultraviolet spectra. The insert on the figure shows a Rowland spectrograph simulation of the ultraviolet radiative flux at 90-120 nm for a hot normal star with an effective temperature of 40000 K.
This spectral range covers the very interesting Lyman region, which is extraordinarily rich in spectral features. Most of them are attributed to the strongest spectral lines of a number of important transitions of different atomic and molecular species in different stages of ionisation.