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Star formation is the process by which gas in molecular clouds gets transformed into stars.
In the current paradigm of star formation, cores of molecular clouds (regions of especially high density) become
gravitationally unstable, fragment and begin to collapse. 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 from the cloud continues to "rain" onto the protostar. In this stage bipolar flows are produced,
probably a effect of the angular momentum of the falling 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 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.
Observations
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 are best observed in rotational transitions of CO and other molecules; these 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.
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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