Abstract
Although some evidence had been available earlier, it was the work of Morton (1967) that proved that the OB Supergiants are losing mass. He obtained spectra with a rocket borne spectrograph of the three stars in Orion's belt, δ, ε and ζ Orionis. These stars are all supergiants of spectral type 09.5 or BO. The spectra showed resonance lines of ions such as C III, C IV, Si IV, N V with P-Cygni type profiles. The deepest part of the absorption profile corresponded to a velocity away from the star of 1400 km s−1, and the absorption profile extended out to wavelengths corresponding to velocities of 2000 km s−1 away from the star. Since these velocities are much greater than the escape velocities from the surface of these stars, it is clear that matter is escaping from them. With a number of simplifying assumptions Morton deduced that the rate of mass loss is about 10−6 M⊙ yr−1. This order of magnitude has remained unchanged by the many high quality observations made since that time.
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