INTERLUDE 20-1 Mass Loss from Giant Stars
Astronomers now know that stars of all spectral types are active and have stellar winds. Consider the highly luminous, hot, blue O- and B-type stars, which have by far the strongest winds. Satellite and rocket observations have shown that their wind speeds may reach 3000 km/s. The result is a yearly mass loss sometimes exceeding 10-6 solar mass per year. Over the relatively short span of 1 million years, these stars blow a tenth of their total mass—more than an entire solar mass of material—into space. These powerful stellar winds, driven directly by the pressure of the intense ultraviolet radiation emitted by the stars themselves, hollow out vast cavities in the interstellar gas.

The accompanying figure shows a Hubble Space Telescope image of the supergiant star AG Carinae—50 times more massive than the Sun and a million times brighter—shedding its outer atmosphere. It is shown here puffing out vast clouds of gas and dust. (The star, at center, is intentionally obscured to show more clearly the surrounding nebulae; the bright vertical line is also an artifact.)

Observations made with radio, infrared, and optical telescopes have shown that luminous cool

 stars (for example, K- and M-type red giants) also lose mass at rates comparable to those of the luminous hot stars. Red-giant wind velocities, however, are much lower, averaging merely 30 km/s. They carry roughly as much mass into space as do O-star winds because their densities are generally much greater. Because luminous red stars are inherently cool objects (with surface temperatures of only about 3000 K), they emit virtually no ultraviolet radiation, so the mechanism driving the winds must differ from that in luminous hot stars. We can only surmise that gas turbulence or magnetic fields or both in the atmospheres of the red giants are somehow responsible. The surface conditions in red giants are in some ways similar to those in T Tauri protostars, which are also known to exhibit strong winds. Possibly the same basic mechanism—violent surface activity—is responsible for both.

Unlike winds from hot stars, winds from these cool stars are rich in dust particles and molecules. Nearly all stars eventually evolve into red giants, so these winds provide a major source of new gas and dust to interstellar space. These stellar winds provide a vital link in the cycle of star formation and the evolution of the interstellar medium.