Stars form when an interstellar cloud collapses under its own gravity and breaks up into pieces comparable in mass to our Sun. Heat, rotation, and magnetism all compete with gravity to influence the cloud’s evolution. The evolution of the contracting cloud—the changes in its temperature and luminosity—can be conveniently represented as an evolutionary track on the Hertzsprung–Russell diagram. A cold interstellar cloud containing a few thousand solar masses of gas can fragment into tens or hundreds of smaller clumps of matter, from which stars eventually form.
As a collapsing prestellar fragment heats up and becomes denser it eventually becomes a protostar—a warm, very luminous object that emits radiation mainly in the infrared portion of the electromagnetic spectrum. At this stage of its evolution, the protostar is also known as a T Tauri star, after the first object of this type discovered. Eventually, a protostar’s central temperature becomes high enough for hydrogen fusion to begin, and the protostar becomes a star. For a star like the Sun, the whole formation process takes about 50 million years. Protostellar winds encounter less resistance in the directions perpendicular to a protostar’s disk. Thus they expel two jets of matter in the directions of the protostar’s poles. As the protostellar wind gradually destroys the disk, the jets widen until, with the disk gone, the wind flows away from the star equally in all directions.
More massive stars pass through similar stages, but much more rapidly. Stars less massive than the Sun take much longer to form. The zero-age main sequence is the region on the H–R diagram where stars lie when the formation process is over. Mass is the key property for determining a star’s characteristics and life span. The most massive stars have the shortest formation times and main-sequence lifetimes. At the other extreme, some low-mass fragments never reach the point of nuclear ignition. The universe may be populated with a vast number of brown dwarfs—objects that are not massive enough to fuse hydrogen to helium in their interiors.
Many of the objects predicted by the theory of star formation have been observed in real astronomical objects. The dark interstellar regions near emission nebulae often provide evidence of cloud fragmentation and protostars. Radio telescopes are used for studying the early phases of cloud contraction and fragmentation; infrared observations allow us to see later stages of the process. Many well-known emission nebulae, lit by several O-type stars, are partially engulfed by molecular clouds, parts of which are probably fragmenting and contracting, with smaller sites forming protostars. Shock waves can compress other interstellar clouds and trigger star formation. Star birth and the production of shock waves are thought to produce a chain reaction of star formation in molecular cloud complexes.
A single collapsing and fragmenting cloud can give rise to hundreds or thousands of stars—a star cluster. The formation of the most massive stars may play an important role in suppressing further formation of lower-mass cluster members. Open clusters, with a few hundred to a few thousand stars, are found mostly in the plane of the Milky Way. They typically contain many bright blue stars, indicating that they formed relatively recently. Globular clusters are found mainly away from the Milky Way plane and may contain millions of stars. They include no main-sequence stars much more massive than the Sun, indicating that they formed long ago. Globular clusters are believed to date from the formation of our Galaxy. Loosely bound groups of newborn stars are called stellar associations. Infrared observations have revealed young star clusters or associations in several emission nebulae. Eventually, clusters break up into individual stars, although the process may take billions of years to complete.
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