A mathematical description of the relative frequency with which stars of various masses are formed; its high-mass end is known as the Salpeter function after the American astrophysicist Edwin Ernest Salpeter (1924–2008). Massive stars are less numerous than lighter ones. The frequency of stars more massive than the Sun decreases slightly more steeply than the inverse square of the mass. However, the frequency of stars up to the Sun’s mass falls off less steeply than the inverse square of their mass. At masses less than around half a solar mass the initial mass function decreases again, so that there are fewer stars with masses less than half that of the Sun. The relative shape of the initial mass function appears to be constant throughout the Galaxy.