A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution.[1] For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years [2] The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main sequence star at the onset of Hydrogen fusion.


The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966.[3] In the first models, the size of protostars was greatly overestimated. Subsequent numerical calculations[4][5][6] clarified the issue, and showed that protostars are only modestly larger than main-sequence stars of the same mass. This basic theoretical result has been confirmed by observations, which find that the largest pre-main-sequence stars are also of modest size.

Protostellar evolution

Infant Star's First Steps
Infant star CARMA-7 and its jets are located approximately 1400 light-years from Earth within the Serpens South star cluster.[7]

Star formation begins in relatively small molecular clouds called dense cores.[8] Each dense core is initially in balance between self-gravity, which tends to compress the object, and both gas pressure and magnetic pressure, which tend to inflate it. As the dense core accrues mass from its larger, surrounding cloud, self-gravity begins to overwhelm pressure, and collapse begins. Theoretical modeling of an idealized spherical cloud initially supported only by gas pressure indicates that the collapse process spreads from the inside toward the outside.[9] Spectroscopic observations of dense cores that do not yet contain stars indicate that contraction indeed occurs. So far, however, the predicted outward spread of the collapse region has not been observed.[10]

The gas that collapses toward the center of the dense core first builds up a low-mass protostar, and then a protoplanetary disk orbiting the object. As the collapse continues, an increasing amount of gas impacts the disk rather than the star, a consequence of angular momentum conservation. Exactly how material in the disk spirals inward onto the protostar is not yet understood, despite a great deal of theoretical effort. This problem is illustrative of the larger issue of accretion disk theory, which plays a role in much of astrophysics.

A diamond in the dust
HBC 1 is a young pre-main-sequence star.[11]

Regardless of the details, the outer surface of a protostar consists at least partially of shocked gas that has fallen from the inner edge of the disk. The surface is thus very different from the relatively quiescent photosphere of a pre-main sequence or main-sequence star. Within its deep interior, the protostar has lower temperature than an ordinary star. At its center, hydrogen-1 is not yet fusing with itself. Theory predicts, however, that the hydrogen isotope deuterium fuses with hydrogen-1, creating helium-3. The heat from this fusion reaction tends to inflate the protostar, and thereby helps determine the size of the youngest observed pre-main-sequence stars.[12]

The energy generated from ordinary stars comes from the nuclear fusion occurring at their centers. Protostars also generate energy, but it comes from the radiation liberated at the shocks on its surface and on the surface of its surrounding disk. The radiation thus created must traverse the interstellar dust in the surrounding dense core. The dust absorbs all impinging photons and reradiates them at longer wavelengths. Consequently, a protostar is not detectable at optical wavelengths, and cannot be placed in the Hertzsprung-Russell diagram, unlike the more evolved pre-main-sequence stars.

The actual radiation emanating from a protostar is predicted to be in the infrared and millimeter regimes. Point-like sources of such long-wavelength radiation are commonly seen in regions that are obscured by molecular clouds. It is commonly believed that those conventionally labeled as Class 0 or Class I sources are protostars.[13][14] However, there is still no definitive evidence for this identification.

Observed classes of young stars

Class peak emission duration (Years)
0 submillimeter 104
I far-infrared 105
II near-infrared 106
III visible 107[15]


Video about the protostar V1647 Orionis and its X-ray emission (2004).
Protostar outburst - HOPS 383 (2015).
Witness the Birth of a Star
A protostar inside a Bok globule (Artist's image).
Stellar cluster RCW 38, around the young star IRS2, a system of two massive stars and protostars.

See also


  1. ^ Stahler, S. W. & Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. ISBN 3-527-40559-3.
  2. ^ Dunham, M. M.; et al. (2014). The Evolution of Protostars in Protostars and Planets VI. University of Arizona Press. ISBN 9780816598762.
  3. ^ Hayashi, C. (1966). "The Evolution of Protostars". Annual Review of Astronomy and Astrophysics. 4: 171. Bibcode:1966ARA&A...4..171H. doi:10.1146/annurev.aa.04.090166.001131.
  4. ^ Larson, R. B. (1969). "Numerical Calculations of the Dynamics of a Collapsing Protostar". Monthly Notices of the Royal Astronomical Society. 145: 271. Bibcode:1969MNRAS.145..271L. doi:10.1093/mnras/145.3.271.
  5. ^ Winkler, K.-H. A. & Newman, M. J. (1980). "Formation of Solar-Type Stars in Spherical Symmetry: I. The Key Role of the Accretion Shock". Astrophysical Journal. 236: 201. Bibcode:1980ApJ...236..201W. doi:10.1086/157734.
  6. ^ Stahler, S. W., Shu, F. H., and Taam, R. E. (1980). "The Evolution of Protostars: I. Global Formulation and Results". Astrophysical Journal. 241: 637. Bibcode:1980ApJ...241..637S. doi:10.1086/158377.CS1 maint: Multiple names: authors list (link)
  7. ^ "Infant Star's First Steps". Retrieved 10 November 2015.
  8. ^ Myers, P. C. & Benson, P. J. (1983). "Dense Cores in Dark Clouds: II. NH3 Observations and Star Formation". Astrophysical Journal. 266: 309. Bibcode:1983ApJ...266..309M. doi:10.1086/160780.
  9. ^ Shu, F. H. (1977). "Self-Similar Collapse of Isothermal Spheres and Star Formation". Astrophysical Journal. 214: 488. Bibcode:1977ApJ...214..488S. doi:10.1086/155274.
  10. ^ Evans, N. J., Lee, J.-E., Rawlings, J. M. C., and Choi, M. (2005). "B335 - A Laboratory for Astrochemistry in a Collapsing Cloud". Astrophysical Journal. 626: 919. arXiv:astro-ph/0503459. Bibcode:2005ApJ...626..919E. doi:10.1086/430295.CS1 maint: Multiple names: authors list (link)
  11. ^ "A diamond in the dust". Retrieved 16 February 2016.
  12. ^ Stahler, S. W. (1988). "Deuterium and the Stellar Birthline". Astrophysical Journal. 332: 804. Bibcode:1988ApJ...332..804S. doi:10.1086/166694.
  13. ^ Adams, F. C., Lada, C. J., and Shu, F. H. (1987). "The Spectral Evolution of Young Stellar Objects". Astrophysical Journal. 312: 788. Bibcode:1987ApJ...312..788A. doi:10.1086/164924.CS1 maint: Multiple names: authors list (link)
  14. ^ Andre, P, Ward-Thompson, D. and Barsony, M. (1993). "Submillimeter Continuum Observations of rho Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps". Astrophysical Journal. 406: 122. Bibcode:1993ApJ...406..122A. doi:10.1086/172425.CS1 maint: Multiple names: authors list (link)
  15. ^ "IMPRS" (PDF).

External links

Becklin–Neugebauer Object

The Becklin–Neugebauer Object (BN) is an object visible only in the infrared in the Orion Molecular Cloud. It was discovered in 1967 by Eric Becklin and Gerry Neugebauer during their near-infrared survey of the Orion Nebula.

The BN Object is thought to be an intermediate-mass protostar. It was the first star detected using infrared methods and is deeply embedded within the Orion star-forming nebula, where it is invisible at optical wavelengths because the light is completely scattered or absorbed due to the high density of dusty material.

Evaporating gaseous globule

An evaporating gas globule or EGG is a region of hydrogen gas in outer space approximately 100 astronomical units in size, such that gases shaded by it are shielded from ionizing UV rays. Dense areas of gas shielded by an evaporating gas globule can be conducive to the birth of stars. Evaporating gas globules were first conclusively identified via photographs taken by the Hubble Space Telescope in 1995.EGG's are the likely predecessors of new protostars. Inside an EGG the gas and dust are denser than in the surrounding dust cloud. Gravity pulls the cloud even more tightly together as the EGG continues to draw in material from its surroundings. As the cloud density builds up the globule becomes hotter under the weight of the outer layers, a protostar is formed inside the EGG.

A protostar may have too little mass to become a star. If so it becomes a brown dwarf. If the protostar has sufficient mass, the density reaches a critical level where the temperature exceeds 10 million kelvin at its center. At this point, a nuclear reaction starts converting hydrogen to helium and releasing large amounts of energy. The protostar then becomes a star and joins the main sequence on the HR diagram.

FU Orionis star

In stellar evolution, an FU Orionis star (also FU Orionis object, or FUor) is a pre–main-sequence star which displays an extreme change in magnitude and spectral type. One example is the star V1057 Cyg, which became 6 magnitudes brighter and went from spectral type dKe to F-type supergiant. These stars are named after their type-star, FU Orionis.

The current model developed primarily by Lee Hartmann and Scott Jay Kenyon associates the FU Orionis flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri star. Mass accretion rates for these objects are estimated to be around 10−4 solar masses per year. The rise time of these eruptions is typically on the order of 1 year, but can be much longer. The lifetime of this high-accretion, high-luminosity phase is on the order of decades. However, even with such a relatively short timespan, as of 2015 no FU Orionis object had been observed shutting off. By comparing the number of FUor outbursts to the rate of star formation in the solar neighborhood, it is estimated that the average young star undergoes approximately 10–20 FUor eruptions over its lifetime.

The prototypes of this class are: FU Orionis, V1057 Cygni, V1515 Cygni, and the embedded protostar V1647 Orionis, which erupted in January 2004.

IRAS 18162−2048

IRAS 18162-2048 is a far-infrared source discovered by IRAS spacecraft in 1983. It is associated with a massive (~10 solar masses) protostar, which accretes gas from a disk that surrounds it. IRAS 18162-2048 emits two collimated radio jets along its axis of rotation. The jets are made of chains of radio sources aligned in a southwest-northeast direction. The northern jet terminates in Herbig–Haro object HH 81N, while the southern one terminates in Herbig–Haro objects HH 80 and HH 81. The total luminosity of IRAS 18162-2048 is about 17,000 solar luminosities. The total extent of this system of jets and radio sources is about 5 pc.In 2010 HH 80–81 jet of IRAS 18162-2048 were found to emit polarized radio waves, which indicated that they were produced by relativistic electrons moving along the magnetic field estimated at 20 nT. This observation was the first of kind demonstrating that a protostar can have a magnetized jet.


L1488-IRS2E is an object located in the Perseus star forming region. A clump of dense gas and dust, L1488-IRS2E is one-tenth as luminous as our sun and thus is unlikely to be a true protostar at this time. However, its density is high enough that it is ejecting streams of matter from itself, and so it is a likely candidate for the first discovered core in hydrostatic quasi-equilibrium. This would mean that L1488-IRS2E represents an early phase in stellar development which has so far remained unobserved due to the short time that a star spends in this phase and the low luminosity which comes from a star not yet developed past it.

L1551 IRS 5

L1551 IRS 5 is a protostellar envelope surrounding a binary protostar system in the constellation of Taurus 450 light-years from Earth. The binary system itself is known as L1551 NE. It is one of Jim Kaler's The 100 greatest stars.

LRLL 54361

LRLL 54361 also known as L54361 is thought to be a binary protostar producing strobe-like flashes, located in the constellation Perseus in the star-forming region IC 348 and 950 light-years away.

This newly discovered object may offer insight into a star's early stages of formation, when large masses of gas and dust are falling into a newly forming binary star - called a pulsed accretion model. This object emits a burst of light at regular intervals of 25.34 days, possibly caused by repeated close approaches between the two component stars which are gravitationally linked in an eccentric orbit - the flashes may be the result of large amounts of matter falling into the growing protostars. Since the stars are obscured by the dense disk and envelope of dust surrounding them, direct observation is difficult. This process of star birth has been witnessed in its later stages, but has to date not been seen in such a young system, nor with such intensity and regularity. These new stars are thought to be only a few hundred thousand years old.

LRLL 54361 was first detected by the Spitzer Space Telescope as a variable object inside the star-forming region IC 348. The Hubble Space Telescope confirmed the Spitzer observations and revealed the detailed structure around the protostar. Hubble images show two large, clear-swept regions in the disk around the stars. The monitoring of LRLL 54361 continues using other instruments, including the Herschel Space Telescope, and astronomers hope to obtain more direct measurements of the binary star and its orbit.


Photo-erosion is the dispersion of the outer layers of a prestellar core by the ionizing radiation of a nearby star.

This erosion prevents the accretion of these outer layers around the protostar at the centre of the core; and this, in turn, prevents the protostar from becoming a fully fledged star. The protostar instead becomes a brown dwarf or planetary-mass object.

Pre-main-sequence star

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.


A quasi-star (also called black hole star) is a hypothetical type of extremely massive star that may have existed very early in the history of the Universe. Unlike modern stars, which are powered by nuclear fusion in their cores, a quasi-star's energy would come from material falling into a central black hole.

A quasi-star is predicted to have formed when the core of a large protostar collapses into a black hole during its formation and the outer layers of the star are massive enough to absorb the resulting burst of energy without being blown away (as they are with modern supernovae). Such a star would have to be at least 1,000 solar masses (2.0×1033 kg). These stars may have also been formed by dark matter halos drawing in enormous amounts of gas via gravity, in the early universe, which can produce supermassive stars with tens of thousands of solar masses. Stars this large could only form early in the history of the Universe before the hydrogen and helium were contaminated by heavier elements; thus, they may have been very massive Population III stars.

Once the black hole had formed at the core of the protostar, it would continue generating a large amount of radiant energy from the infall of additional stellar material. This energy would counteract the force of the gravity, creating an equilibrium similar to the one that supports modern fusion-based stars. A quasi-star is predicted to have had a maximum lifespan of about 7 million years, after which the core black hole would have grown to about 1,000–10,000 solar masses (2×1033–2×1034 kg). These intermediate-mass black holes have been suggested as the origin of the modern era's supermassive black holes. Quasi-stars are predicted to have surface temperatures limited to about 4,000 K (3,730 °C), but, with diameters of approximately 10 billion kilometres (66.85 au) or 7,187 times that of the Sun, each one would produce as much light as a small galaxy.


SES-7 (formerly known as Indostar 2 / ProtoStar 2) is a Dutch commercial communication satellite operated by SES World Skies. Originally launched on 16 May 2009 by Boeing for ProtoStar Ltd and later purchased through auction by SES S.A. for SES World Skies unit for $180 million. SES-7 operates in geostationary orbit and intended to be located at a longitude of 108.2° East covering South Asia and Asia-Pacific region. SES-7 is built for optimized HD DTH (direct-to-home) satellite television service and broadband Internet access across the Asia-Pacific region.

The spacecraft was originally built for PanAmSat (now Intelsat) to be used as Galaxy-8iR, but that contract was terminated on November 15, 2002. The satellite was renamed SES-7 in May 2010.

Star formation

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

T Tauri

T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri; but it is actually 420 light years behind it and was not formed with the rest of them. Faint nebulosity around T Tauri is a Herbig–Haro object called Burnham's Nebula or HH 255.

Like all T Tauri stars, it is very young, being only a million years old. Its distance from Earth is about 460 light years, and its apparent magnitude varies unpredictably from about 9.3 to 14.The T Tauri system consists of at least three stars, only one of which is visible at optical wavelengths; the other two shine in the infrared and one of them also emits radio waves. Through VLA radio observations, it was found that the young star (the "T Tauri star" itself) dramatically changed its orbit after a close encounter with one of its companions and may have been ejected from the system.

Physically nearby is NGC 1555, a reflection nebula known as Hind's Nebula or Hind's Variable Nebula. It is illuminated by T Tauri, and thus also varies in brightness. The nebula NGC 1554 was likewise associated with T Tauri and was observed in 1868 by Otto Wilhelm von Struve, but soon disappeared or perhaps never existed, and is known as "Struve's Lost Nebula".

The T Tauri wind, so named because this young star is currently in this stage, is a phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the hydrogen that has agglomerated into the protostar. A protostar is the denser parts of a cloud core, typically with a mass around 104 solar masses in the form of gas and dust, that collapses under its own weight/gravity, and continues to attract matter.

The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After a few million years, thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed, and its future evolution is now set.

V883 Orionis

V883 Orionis is a protostar in the constellation of Orion. It is associated with IC 430 (Haro 13A), a peculiar Hα object surveyed by Guillermo Haro in 1952. It is located about 1350 light years (414 parsecs) away, and is associated with the Orion Nebula.V883 Orionis, like most protostars, is surrounded by a circumstellar disc of dust. The dust has a water snow line, a certain distance where the stellar irradiance from the star is low enough that water can freeze to snow. The water snow line was directly imaged by ALMA, when a stellar outburst increased the amount of insolation and pushed the line out farther.


W33A is a protostar located approximately 12,000 light-years away from Earth, in the constellation Sagittarius. As a star in the early stages of formation, so has attracted the interest of astronomers, who observed that while the protostar is accumulating material from surrounding clouds of gas and dust, it is simultaneously ejecting fast moving jets of particles from its north and south poles.


W75N(B)-VLA2 is a massive protostar some 4,200 light-years from Earth, about 8 times more massive and 300 times brighter than our Sun, observed in 1996 and 2014 by the Karl G. Jansky Very Large Array (VLA). In 2014 its stellar wind had changed from a compact spherical form to a larger thermal, ionized elliptical one outlining collimated motion, giving critical insight into the very early stages of the formation of a massive star. Being able to observe its rapid growth as it happens (in real time in an astronomical context) is unique, according to Huib van Langevelde of Leiden University, one of the authors of a study of the object.

The authors of the study believe W75N(B)-VLA2 is forming in a dense, gaseous environment, surrounded by a dusty torus. The star intermittently ejects a hot, ionized wind for several years. Initially the wind can expand in all directions and forms a spherical shell; later it hits the dusty torus, which slows it. There is less resistance along the poles of the torus, so the wind moves more quickly there, giving rise to an elongated shape.

Young stellar object

Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars.

Luminosity class
Hypothetical stars
Star systems
Related articles
Object classes
Theoretical concepts

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