Stellar wind is the term used to describe the flow of gas ejected from the upper atmosphere of a star. This gas may be electrically-charged or neutral. Different types of stars produce different types of stellar winds, and these may be fast or slow and contain large or small amount of mass. Radiation pressure is one driving force for the winds.
Stars give off radiation and emit particles. Stellar wind is considered one way in which stars lose mass. The loss of mass is part of the life cycle and evolution of a star.
The sun emits a stellar wind also known as solar wind. The wind produced by the sun is charged with electrons and protons. The high temperature allows these electrons and protons to escape the gravity of the sun. Pressure and expansion from the corona drives the winds. While the temperature of the sun is high, it is actually categorized as a cool star.
Stars that are in later stages of their life cycle, or evolution, are termed post-main sequence stars. Stellar wind from these stars normally contains a larger amount of matter with a slower wind speed. Red giants and other cool luminous stars are in this category. Radiation pressure on dust in the atmosphere is a driving factor for the wind and the materials it contains.
Large or massive stars typically have high-speed winds that contain less material. These stars may be classified as hot luminous stars. Radiation pressure or flux on heavy elements, such as nitrogen and carbon, is the driving force for these winds. This type of star may also produce stellar wind bubbles. These are glowing shells of gas and can indicate new star formations.
Massive stars can emit as much as 50% of their mass through stellar wind during the main portion of their evolution. The amount of mass lost during this time period can affect the next stages of the life cycle. Stars in middle mass categories, that lose mass quickly, will become white dwarfs. The loss of mass prevents them from exploding as a supernova.
Spiral winds are also a form of stellar winds. Sunspots and other irregularities in the atmosphere of a star can create slow or fast gas streams. Hot stars will produce heavier, lower speed winds in areas with increased brightness. As the star revolves, faster wind speeds collide with the slower wind, forming a spiral wind.
Wind is not isolated only to the atmosphere of our planet. Wind on Earth is produced from pressure differences and temperature changes. Shifts in pressure, temperature, and radiation all also produce the stellar wind found in the atmosphere of stars. Ultraviolet light photos show the outer atmosphere of the sun and can provide an interesting look at the outward-flowing winds. Images are also available of stellar wind bubbles, providing a look into the history of stars.