noun Astronomy.
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.

1945–50; proto- + star Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
protostar (ˈprəʊtəʊˌstɑː)
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 10th Edition
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Cite This Source's 21st Century Lexicon
Main Entry:  protostar
Part of Speech:  n
Definition:  any early stage in the formation of a star when an interstellar cloud of gas and dust starts to collapse but before nuclear synthesis has begun at its core
Example:  A protostar is a cloud of gas undergoing gravitational contraction as it is developing into a star.
Usage:  protostellar adj's 21st Century Lexicon
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American Heritage
Science Dictionary
protostar   (prō'tə-stär')  Pronunciation Key 
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Eventually, the core collapses into itself, creating a protostar.
But the model stops short of simulating the moment the protostar achieves full nuclear fusion and transforms into a true star.
What emerges from the collapse is much smaller, denser and hotter-a protostar.
They were ancient when the sun was still a protostar.
Image for protostar
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