A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
Different types of stars have different types of stellar winds.
Post-main-sequence stars nearing the ends of their lives often eject large quantities of mass in massive ( solar masses per year), slow (v = 10 km/s) winds. These include red giants and supergiants, and asymptotic giant branch stars. These winds are understood to be driven by radiation pressure on dust condensing in the upper atmosphere of the stars.
Massive stars of types O and B have stellar winds with lower mass loss rates ( solar masses per year) but very high velocities (v > 1–2000 km/s). Such winds are driven by radiation pressure on the resonance absorption lines of heavy elements such as carbon and nitrogen. These high-energy stellar winds blow stellar wind bubbles.