Stellar Winds
- Evidence of episodic stellar mass loss in the form of novae or supernovae has been known since antiquity. But the realization that stars could also have a continuous wind dates from the 1960's, largely from analogy with the solar wind.
- Low-density, optically thin coronal winds from solar-like, low mass, main-sequence stars can only be inferred indirectly, e.g. by X-ray observations suggesting stellar coronae.
- But for some stars -- e.g. during the Red Giant phase of a solar-mass star, or from hot, luminous, high-mass stars -- the stellar winds are dense enough to be optically thick in spectral lines.
- Lines formed by scattering of the stellar radiation within the expanding wind develop a characteristic shape -- a P-Cygni profile -- whose features provide a direct diagnostic of key wind parameters, like the wind speed and mass loss rate.