Stars emit not only radiation but also particles. The emission of particles is called the stellar wind.

The two most important parameters regarding a stellar wind that can be derived from the observations are the mass loss rate Ṁ, which is the amount of mass lost by the star per unit time, and the terminal velocity v∞, which is the velocity of the stellar wind at a large distance from the star. By convention, the mass loss rate Ṁ is always positive and it is expressed in units of solar masses per year, with 1 M⊙ yr-1 = 6.303 × 1025 g s-1. A star with Ṁ = 1--6M⊙ yr-1, which is not an unusual value, loses an amount of mass equal to the total mass of the earth in three years. The terminal velocity v∞ of a stellar wind ranges typically from about 10 km s-1 for a cool supergiant star to 3000 km s-1 for a luminous hot star.

The values of Ṁ and v∞ are important because

(1) Ṁ describes how much material is lost by the star per unit of time. This is important for the evolution of the stars, because stars with high mass loss rates will evolve differently from those with low mass loss rates.

(2) Different stellar wind theories predict different mass loss rates and different terminal velocities for a star. So by comparing the observed values with the predictions we can learn which mechanism is responsible for the mass loss from a star.