Star Formation

The scenario described above leads to a situation like that shown in the Great Orion Nebula. Brilliant, newly born stars blaze in the foreground, while the great cloud surrounding them glows in the background. This nebula glows because the intense radiation from the massive young stars near it is heating it. Contrast this with the Horsehead Nebula, which has no such sources of heat and therefore is dark.

These newly formed stars can themselves trigger star formation. Radiation—that is, light—exerts pressure on surrounding matter. The young stars in the Orion Nebula are huge by stellar standards, and their radiation is intense. Many of them lose mass continuously in a stellar wind that streams out into the cloud. After a few million years, the most massive of them will explode as supernovae. These effects can cause other parts of the neighboring cloud to begin contracting. Therefore, star formation might be able to bootstrap its way through an entire cloud, even if only part of the cloud is disrupted by a shock wave.

An interstellar cloud does not always have to be disrupted by a shock wave to form stars, however. Sometimes a cloud may collapse spontaneously, and the process describing this phenomenon was discovered by the astronomer James Jeans (1877-1947). Above the so-called "Jeans mass," which depends on the temperature and density of the cloud, a cloud will break up and contract spontaneously under its own gravity. Large clouds can break up into numerous cloudlets this way, and this process leads to the formation of star clusters such as the Pleiades. Often, two stars will form very close to one another, sometimes separated by a distance less than that from the Earth to the Sun. These binary systems, as well as multiple systems containing three to six stars, are quite common. They are more common, in fact, than single stars: most of the stars you see at night are actually binaries.