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(proh -toh-star) A stage in the evolution of a young star after it has fragmented from a gas cloud but before it has collapsed sufficiently for nuclear reactions to begin. This phase may take from 105 to 107 years, depending on the mass of the star. Simple theoretical models of protostars are probably inaccurate, according to recent observations of star-formation regions. The satellite IRAS has probably detected many protostars in its infrared survey, but their identification requires clear criteria for distinguishing true protostars from young stars cocooned in dust. See also star formation.



a provisional name given to the body from which a star is formed. The idea of a protostar emerged in connection with the 1948–49 theory of stellar associations conducted by the Soviet astronomers V. A. Ambartsumian and B. E. Markarian. Stellar associations are characterized by the irregular distribution of stars. Large O associations, as a rule, possess several nuclei in the form of compact stellar groups, for example, open clusters, multiple stars, such as those in the Trapezium in Orion, and stellar sequences, usually containing hot, high-luminosity stars.

Studies have shown that these stellar groups are unstable and rapidly break up. Such groups consist of young stars and possibly are centers of star formation. Several such centers usually exist simultaneously in stellar associations. Single stars in stellar associations probably result from the breakup of previously existing stellar groups. These compact stellar groups, with masses several hundred times that of the sun, often have relatively small linear dimensions (approximately 1 parsec). These and a number of other data lead to the conclusion that the groups of stars in stellar associations basically arise from bodies with comparatively low volume—not more than 0.1 parsec in diameter—and relatively high density. It is these bodies that have been called protostars. The term “protostar” has not yet been applied to any observed celestial body. However, it is reasonable to assume that the physical nature of protostars differs sharply from that of known celestial bodies.


Ambartsumian, V. A., and B. E. Markarian. Zvezdnaia assotsiatsiia vok-rug P Lebedia. (Soobshchenie Biurakanskoi observatorii, issue 2.) Yerevan, 1949.
Ambartsumian, V. A. “O protozvezdakh.” Dokl. AN Armianskoi SSR, 1953, vol. 16, no. 4.
Markarian, B. E. “Peresmotrennyi spisok zvezdnykh assotsiatsii tipa O.” Dokl. AN Armianskoi SSR, 1952, vol. 15, no. 1.


A dense condensation of material that is still in the process of accreting matter to form a star.