Star Formation

When you look up on a clear night, you see stars—thousands of them—glittering against the seemingly empty backdrop of space. But there is something else out there; vast clouds of cold, dark gas and dust, visible only by the dimming effect they have on starlight shining through them. This is the interstellar medium, and it is the birthplace of the stars.

In most places the interstellar medium is almost a vacuum, a million trillion times less dense than air. In other places, however, there are much greater concentrations of clouds, sometimes so thick and dense that we cannot see through them at all. Such a cloud is the famous Horsehead Nebula in the constellation Orion. Often these clouds are enormous, thousands of times as massive as the Sun.

Unlike the Sun, however, these interstellar clouds have relatively weak gravity. The gravitational attraction between two particles decreases as the separation between them increases, and even in a huge cloud like the Horsehead An infrared image of the molecular cloud and region of star formation NGC 7538. The bright areas (just below center, and left of bottom center) are sites where stellar formation is taking place. Giant clouds of dust are fragmenting and forming new protostar systems; the ultraviolet radiation of the newborn stars is lighting up the surrounding dust. The larger hazy glow (right of top center) is radiation from the protostars pouring out from a gap in the dust clouds. Photograph by Ian Gatley. Photo Researchers, Inc. Reproduced by permission.
Nebula, the matter is much more thinly distributed than in the Sun. Therefore, the matter in the cloud tends not to condense. It remains roughly the same size, slowly changing its shape over the course of millennia.