Because star formation takes a long time by human standards, the astronomer interested in this subject must take on the role of a cosmic archaeologist. He finds varied examples of collapsing clouds and young stars and attempts to put them in the correct evolutionary sequence. From measurements of the size, density, and temperature of an interstellar cloud one may calculate whether or not the cloud is likely to be expanding or contracting. From data such as these, we know that some of the cold, dark clouds are almost certainly in a state of gravitational collapse, and it has been conjectured that some of the small globules such as those in are in fact protostars of around one solar mass. Some of the warm molecular clouds, such as those in the Orion Nebula region also appear to be collapsing.

The stages of star formation involving the creation and accretion of the hot nucleus are more difficult to observe. There is a star-called FU Orionis which in 193(5 flared up in a few months in what some astronomers believe is an example of this phenomenon. A similar behavior was seen in the star V 1057 Cygni in I969, though other interpretations of these events, not involving the formation of new stars, are possible. The most likely candidates for protostars are certain infrared sources, such as the Becklin-Neugebauer object in the Orion Nebula and W3-IRS5 in the constellation of Cassiopeia. These sources have temperatures of a few hundred degrees and luminosity at least a thousand times that of the Sun. They both lie at the centers of molecular clouds, show H20 maser emission, and are very close to compact ionized hydrogen regions. Neither is visible optically, and both were unknown until the advent of infra¬red astonomy. As a protostar evolves further and the light from its nucleus begins to shine through and reflect off the outer layers of dust, a small nebula may be seen. This process is thought to be the origin of the so called Herbig-Haro objects which are sometimes found associated with young stars.

The pre-main-sequence stage of star formation . For low-mass stars ,a comparative slow process. A lot of stars are therefore visible in the region of the HR diagram just above the main sequence. Some of these stars belong to the T Tauri class of irregular variables, with emission linen and strong ultra¬violet and infrared excesses. Such stars often occur hi very young clusters which also contain signs of gas and dust . The be and Ae emission-line stars are probably similar to the T Tauris except that they have a larger mass. The most massive stare evolve so quickly that the outer layers of the cloud out of which the star formed remain as an opaque cocoon for some of the main-sequence lifetime. Since O stars give out mainly ultraviolet radiation, a com¬pact ionized hydrogen region is formed in the cocoon, and the star become visible (indirectly) through its radio and infrared radiation rather than by the light it emits. Finally, forces such as radiation pressure, and supernova shocks will blow away the remaining gas and dust and will leave only the cluster of new stars.

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