Star formation
Star formation is the process by which hydrogen and helium in
molecular clouds change into the ball of
plasma we call a
star.
According to current theories of star formation, cores of molecular clouds (regions of especially high density) become gravitationally unstable, fragment, and begin to
collapse (sometimes, shockwaves from
supernovae will trigger star formation in nearby gas clouds). Part of the gravitational energy lost in this collapse is radiated in the
infrared, with the remainder increasing the temperature of the core of the object. The accretion of material happens partially through a
circumstellar disc. When the density and temperature are high enough,
deuterium fusion ignition occurs, and the outward
pressure of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto the
protostar. In this stage bipolar flows are produced, probably an effect of the
angular momentum of the infalling material. Finally,
hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away.
The stages of the process are well defined in stars with masses around one
solar mass or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars are studied in
stellar evolution.
Key elements of star formation are only available by observing in
wavelengths other than the
optical. The structure of the molecular cloud and the effects of the protostar can be observed in near-IR extinction maps(where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky),continuum dust emission and
rotational transitions of
CO and other molecules; these last two are observed in the millimeter and
submillimeter range. The radiation from the protostar and early star has to be observed in
infrared astronomy wavelengths, the
extinction caused by the rest of the cloud where it is being formed is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the atmosphere is almost entirely opaque from 20um to 850um, with narrow windows at 200 and 450um. Even outside this range atmospheric subtraction techniques must be used.
The formation of individual stars can only be directly observed in
our Galaxy, but in distant galaxies star formation has been detected through its unique
spectral signature.
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The theory of collapse of gas under its own gravity was developed by
Jeans and although the theory does not treat many phenomena known to be important it is a very useful and widely used first approximation. The original paper is available free of charge (to participating institutions):
* J. H. Jeans. Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character, Vol. 199. (1902), pp. 1-53. Stable URL: [
1]The theory of solar mass star formation is detailed in:
*Shu, Frank H.; Adams, Fred C.; Lizano, Susana. Star formation in molecular clouds - Observation and theory. Annual review of astronomy and astrophysics. Volume 25 (A88-13240 03-90). Palo Alto, CA, Annual Reviews, Inc., 1987, p. 23-81.Stable URL: [
2]
