The Protostellar Family
Like planets, stars are born, but they do so in considerably more spectacular fashion and at rates highly dependent on their environment. A protostar, sometimes called an embryonic star, is a molecular cloud (S 25) undergoing gravitational contraction and accretion of gas and dust prior to initiating fusion reactions. The end of accretion and the onset of a stellar wind (S 29) is often considered the dividing line between a collapsing protostar and the next stage in stellar evolution, the so-called pre-main sequence stars (S 2 and S 3) such as T Tauri or higher mass Herbig Ae/Be objects, discussed in the next two entries. But the transition is not sharp: some pre-main sequence stars are still accreting matter. Other classes of objects associated with newborn stars are jets (S 20) and Herbig-Haro objects (S 21). Protostars are also called “young stellar objects” (YSOs), but that term sometimes also encompasses pre-main sequence stars. Surprisingly, our Galaxy births only about one star per year, but “starburst” galaxies such as M82 may form stars at a thousand times that rate.