Stellar Wind

Old stars in the process of dying can also have very strong stellar winds. When stars like the sun exhaust the hydrogen fueling their nuclear fires, they expand into red giants. A typical red giant is about the size of the earth's orbit around the sun. Because red giants are so large and the gravitational force decreases with distance from the center, the gravitational force at the surface of a red giant is much less than at the surface of the sun. Hence only a gentle push is needed to allow matter to escape and form a stellar wind. This push might come from the light leaving the star. Light or other radiation striking an object will produce a very small but non zero force that is called radiation pressure. The radiation pressure on dust grains might provide the needed push. Similarly, this radiation pressure might also play a role in causing the stellar winds from the hot O stars mentioned in a previous paragraph. In addition, many red giants pulsate. They expand and contract in periods of a few years. These pulsations can also provide the needed push to cause significant stellar winds and to cause the loss of quite a bit of mass.

In some cases red giants form planetary nebulae, glowing shells of gas around the star. According to one model, the pulsations create a gentle wind moving at about 6 mi per second (10 km per second). This wind is gentle but can carry away the mass of the sun in as little as 10,000 years. Removing this much mass from the shell of the star exposes the more violent core and unleashes a wind blowing out thousands of miles per second. The fast wind slams into the slow wind and creates a shock wave that heats up the shell of gas until it glows as a planetary nebula. The remaining core of the star collapses into a white dwarf star about the size of the earth.