star Birth of stars and evolution to the main sequenceastronomy

Stars form from the gas and dust of the interstellar medium. The matter between stars is not uniformly distributed in space but is spread in a patchy fashion. Occasionally, a massive cloud will accumulate sufficient matter for its own gravitational attraction to draw it still further together. Understanding of the details of this process is still incomplete.

As the core of the cloud begins pulling itself together, its internal temperature and density rise until the protostar within reaches incandescence with a faint red glow. At this stage, the protostar is not yet shining by nuclear processes but rather shines by the energy released via its gravitational contraction. As the internal temperature rises to a few million kelvins, deuterium (heavy hydrogen) is first destroyed. Then lithium, beryllium, and boron are broken down into helium as their nuclei are bombarded by protons moving at increasingly high speeds.

Dwarf stars of the T Tauri type have been observed in the clouds of dark obscuring matter in Taurus as well as in numerous other regions of the Milky Way. These objects might represent stars in the actual process of formation. In fact, several galactic regions in which stars were actually “turned on” may have been observationally identified. Other evidence is provided by colour-magnitude diagrams such as that obtained for the cluster NGC 2264 associated with the star S Monocerotis. In this cluster the brighter part of the main sequence is well defined by stars somewhat more luminous than the Sun. The fainter redder stars whose colours correspond to spectral types G, K, and M all fall above the main sequence defined by the normal dwarf stars. Presumably, they are contracting toward the main sequence, shining mostly by liberation of gravitational energy; however, they might be burning the light helium isotope helium-3 before they reach the main sequence, when the proton-proton reaction will be ignited.

Many additional sites of possible star formation have been identified in the Milky Way and are being investigated carefully for possible changes. Radio and infrared observations have provided, for example, some sketchy evidence for more advanced prestellar objects. The Orion complex is one such region. Illuminated by several O-type stars, the bright Orion Nebula is partly engulfed by a vast molecular cloud. This dark cloud extends well beyond the few light-years encompassed within the usual telescopic images taken in visible light and has been studied by means of the radio-frequency radiation emitted and absorbed by carbon monoxide and formaldehyde.

The Orion molecular cloud also houses several smaller sites of intense radiation emitted by molecules under very special conditions. Molecules such as hydroxyl (OH) and water vapour (H₂O) have been found by radio techniques to be buried within the core of the cloud fragment. Their extent measures about 10 billion km, or roughly a thousandth of a light-year, which is approximately the full diameter of the solar system. While astronomers cannot currently determine whether these regions will eventually form stars more or less like the Sun, it does seem certain that such intensely emitting regions are on the threshold of becoming protostars.

Detailed calculations show that a protostar first appears on the Hertzsprung-Russell diagram well above the main sequence because it is too bright for its colour. As it continues to contract, it moves downward and to the left toward the main sequence.