Pre-main - sequence star - wikipedia Pre-main - sequence star Star formation Object classes Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Pre-main - sequence star Herbig Ae / Be star Herbig -- Haro object Theoretical concepts Accretion Initial mass function Jeans instability Kelvin -- Helmholtz mechanism Nebular hypothesis Planetary migration Star portal This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( August 2014 ) A pre-main - sequence star ( also known as a PMS star and PMS object ) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the protostar blows away this envelope, it is optically visible, and appears on the stellar birthline in the Hertzsprung - Russell diagram. At this point, the star has acquired nearly all of its mass but has not yet started hydrogen burning ( i.e. nuclear fusion of hydrogen ). The star then contracts, its internal temperature rising until it begins hydrogen burning on the zero age main sequence. This period of contraction is the pre-main sequence stage. An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses ( M ), or else a Herbig Ae / Be star, if it has 2 to 8 M. Yet more massive stars have no pre-main - sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are main - sequence objects. The energy source of PMS objects is gravitational contraction, as opposed to hydrogen burning in main - sequence stars. In the Hertzsprung -- Russell diagram, pre-main - sequence stars with more than 0.5 M first move vertically downward along Hayashi tracks, then leftward and horizontally along Henyey tracks, until they finally halt at the main sequence. Pre-main - sequence stars with less than 0.5 M contract vertically along the Hayashi track for their entire evolution. PMS stars can be differentiated empirically from main - sequence stars by using stellar spectra to measure their surface gravity. A PMS object has a larger radius than a main - sequence star with the same stellar mass and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the main sequence, because their contraction lasts for only 1 percent of the time required for hydrogen fusion. During the early portion of the PMS stage, most stars have circumstellar disks, which are the sites of planet formation. See also Star portal Protoplanetary disk Protostar Stellar birthline Stellar evolution Young stellar object References Jump up ^ Richard B. Larson ( 10 September 2003 ). `` The physics of star formation '' ( PDF ). Reports on Progress in Physics. 66 ( 10 ) : 1669 -- 73. arXiv : astro - ph / 0306595. Bibcode : 2003RPPh... 66.1651 L. doi : 10.1088 / 0034 - 4885 / 66 / 10 / r03. Jump up ^ Neil F. Comins ; William J. Kaufmann III ( 2011 ). Discovering the Universe. p. 350. ISBN 978 - 1429255202. Jump up ^ Derek Ward - Thompson ; Anthony P. Whitworth ( 2011 ). An Introduction to Star Formation. Cambridge University Press. p. 119. ISBN 978 - 1107627468. Jump up ^ Stahler, S.W. & Palla, F. ( 2004 ). The Formation of Stars. Weinheim : Wiley - VCH. ISBN 3 - 527 - 40559 - 3. Star Formation Accretion Molecular cloud Bok globule Young stellar object Protostar Pre-main - sequence star Herbig Ae / Be Orion T Tauri FU Orionis Herbig -- Haro object Hayashi track Henyey track Evolution Main sequence Red - giant branch Horizontal branch Red clump Asymptotic giant branch Protoplanetary nebula Planetary nebula PG1159 star Dredge - up Instability strip Luminous blue variable Blue straggler Stellar population Supernova Supernova impostor Hypernova Hertzsprung -- Russell diagram Color -- color diagram Luminosity class Subdwarf Dwarf Blue Red White Yellow Brown Subgiant Giant Blue Red Yellow Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Spectral classification WR Be OB Subdwarf O Subdwarf B Late - type Chemically peculiar Am Ap / Bp Oscillating Barium Carbon CH CN Extreme helium Lambda Boötis Lead HgMn Technetium Shell Remnants White dwarf Helium planet Neutron star Radio - quiet Pulsar Binary X-ray Magnetar Stellar black hole X-ray binary Burster Hypothetical stars Boson star Black dwarf Exotic Electroweak star Quark Strange Preon Planck Dark matter star Dark - energy star Black star Gravastar Frozen star Q star Quasi-star Thorne -- Żytkow object Iron star Blitzar Nucleosynthesis Deuterium burning Lithium burning Proton -- proton chain CNO cycle Helium flash Triple - alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S - process R - process Fusor Nova Symbiotic Remnant Luminous red nova Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Bipolar outflow Accretion disk Asteroseismology Helioseismology Eddington luminosity Kelvin -- Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation UBV color Variable star Mira variable Star systems Binary Contact Common envelope Eclipsing Symbiotic Multiple Star cluster Open cluster Globular cluster Super star cluster Planetary system Earth 's Solar System Earth - centric observation of Pole star Circumpolar star Constellation Asterism Magnitude Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric - standard star Lists Star names Arabic Chinese Extremes Most massive Highest temperature Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Guest star Gravity Icarus ( most distant individual star ) Intergalactic star Infrared dark cloud Starfield Category : Stars Star portal Star formation Object classes Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Pre-main - sequence star Herbig Ae / Be star Herbig -- Haro object Theoretical concepts Initial mass function Jeans instability Kelvin -- Helmholtz mechanism Nebular hypothesis Planetary migration Star portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Pre-main-sequence_star&oldid=840195891 '' Categories : Star types Star formation Hertzsprung -- Russell classifications Pre-main - sequence stars Hidden categories : Articles needing additional references from August 2014 All articles needing additional references Talk About Wikipedia Bosanski Català Čeština Español Euskara Français 한국어 हिन्दी Italiano ქართული 日本 語 Português Română Русский Slovenčina ไทย Türkçe Українська 中文 11 more Edit links This page was last edited on 8 May 2018, at 09 : 51 ( UTC ). 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Question: in the pre-main sequence stage protostars shine by
Answer: gravitational contraction