What is Protostar?
[[File:Witness the Birth of a Star.jpg|thumb|right|Artist's drawing of a protostar]] '''Protostar''' is an early stage in the evolution of a star that usually grows to the point of beginning [[nuclear fusion]] and becoming a star by gathering mass.<ref>{{Cite book|title = The Formation of Stars|author1=Stahler, S. W. |author2=Palla, F. |publisher = Wiley-VCH|year = 2004|isbn = 3-527-40559-3|location = Weinheim}}</ref> It is made of a contracting cloud of cold and dark [[interstellar medium]] (mostly hydrogen gas). Protostars has lower temperature than an ordinary star. For a sun-like star, [[deuterium]] fusion occurs in the protostar stage, while proton-proton fusion, which sustains stars for billions of years, starts gradually millions of years later. The modern picture of protostars was first suggested by [[Chushiro Hayashi]] in 1966.<ref>{{Cite journal|title = The Evolution of Protostars|last = Hayashi, C.|date = 1966|journal = Annual Review of Astronomy and Astrophysics |volume=4 |pages=171–192 |doi = 10.1146/annurev.aa.04.090166.001131|bibcode = 1966ARA&A...4..171H }}</ref> ==Evolution of Protostars== [[File:Baby Stars in the Witch Head Nebula.jpg|thumb|New stars are forming in Witch Head nebula.]] [[File:PIA18928-Protostar-HOPS383-20150323.jpg|thumb|'''HOPS 383'''; a young protostar]] Although [[outer space]] may seem empty, it is actually full of thinly spread gas and dust called the [[interstellar medium]]. Higher density regions form clouds, or nebulas, where [[star formation]] takes place.<ref>{{cite web | first=C. R. | last=O'Dell | title=Nebula | work=World Book at NASA | url=http://www.nasa.gov/worldbook/nebula_worldbook.html | date=2005-04-29 | publisher=World Book, Inc | access-date=2020-10-16 | archive-date=2005-04-29 | archive-url=https://web.archive.org/web/20050429002503/http://www.nasa.gov/worldbook/nebula_worldbook.html | url-status=dead }}</ref><ref name=prialnik>{{cite book | first=Dina | last=Prialnik | title=An Introduction to the Theory of Stellar Structure and Evolution | pages=