an early stage in the evolution of a star, after the beginning of the collapse of the gas cloud from which it is formed, but before sufficient contraction has occurred to permit initiation of nuclear reactions at its core.
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British Dictionary definitions for protostar
a cloud of interstellar gas and dust that gradually collapses, forming a hot dense core, and evolves into a star once nuclear fusion can occur in the core
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Scientific definitions for protostar
A celestial object made of a contracting cloud of interstellar medium (mostly hydrogen gas) that eventually becomes a main-sequence star. Disturbances in some region of interstellar medium can cause fluctuations of density through that region, and the denser areas, having more mass, begin to attract more and more of the medium through the force of gravity (a process known as accretion). Ever increasing densities of such protostar regions lead to ever higher temperatures within the accreting body, until the point is reached when thermal energy is sufficient to promote the fusion reactions typical of main-sequence stars. Less massive protostars may take hundreds of millions of years to evolve into stars; massive ones contract more quickly and may take only a few hundred thousand years.
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