Observational evidence of mass loss

It is an acknowledged fact that stars lose mass. In addition to the outflow of photons, there usually is an outflow of material particles. But unlike the flow of radiation, which is supplied by energy generation in the interior, the flow of mass is not replenished. As a result, the stellar mass decreases at a rate that is usually measured in solar masses per year and denoted by Ṁ, where the negative sign is omitted. Shedding of mass may take two forms: a sudden ejection of a mass shell, usually following an explosion, or a continuous flow, usually referred to as a wind. We shall deal with explosive mass ejection in Chapter 10, and devote the present discussion to stellar winds.

Indirect evidence for mass loss was brought in the previous chapter and theoretical indication for its probable occurrence was mentioned in Chapter 5. There is, however, direct observational evidence for continuous rapid expansion of the outer layers of stars beyond the stellar photosphere that marks the outer edge, and into the interstellar medium. The most common is exhibited by a characteristic shape of spectral lines, known as P-Cygni lines, named after the star P Cygni – one of the brightest in our Galaxy, discovered in 1600 as a new star (see upcoming Chapters 10 and 11) – where they are prominent. A P-Cygni line profile consists of a blue-shifted absorption component and a red-shifted emission component.