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.

A young star takes centre stage
V1331 Cyg, a YSO surrounded by a reflection nebula

Classification by spectral energy distribution

A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission. As material in the disk is depleted, the infrared excess decreases. Thus, YSOs are usually classified into evolutionary stages based on the slope of their spectral energy distribution in the mid-infrared, using a scheme introduced by Lada (1987). He proposed three classes (I, II and III), based on the values of intervals of spectral index :[1]


Here is wavelength, and is flux density.

The is calculated in the wavelength interval of 2.2–20 (near- and mid-infrared region). Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at .[2] Greene et al. (1994) added a fifth class of "flat spectrum" sources.[3]

  • Class 0 sources – undetectable at
  • Class I sources have
  • Flat spectrum sources have
  • Class II sources have
  • Class III sources have

This classification schema roughly reflects evolutionary sequence. It is believed that most deeply embedded Class 0 sources evolve towards Class I stage, dissipating their circumstellar envelopes. Eventually they become optically visible on the stellar birthline as pre-main-sequence stars.

Class II objects have circumstellar disks and correspond roughly to classical T Tauri stars, while Class III stars have lost their disks and correspond approximately to weak-line T Tauri stars. An intermediate stage where disks can only be detected at longer wavelengths (e.g., at ) are known as transition-disk objects.


YSOs are also associated with early star evolution phenomena: jets and bipolar outflows, masers, Herbig–Haro objects, and protoplanetary disks (circumstellar disks or proplyds).

Classification of YSOs by mass

These stars may be differentiated by mass: Massive YSOs, intermediate-mass YSOs, and brown dwarfs.

See also


  1. ^ Lada, Charles J. (1987). "Star Formation: From OB Associations to Protostars". In Peimbert, Manuel; Jugaku, Jun (eds.). Star Forming Regions: Proceedings of the 115th Symposium of the International Astronomical Union Held in Tokyo, Japan, November 11–15, 1985. Dordrecht: D. Reidel. pp. 1–17. Bibcode:1987IAUS..115....1L. ISBN 978-90-277-2388-8.
  2. ^ Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (March 1993). "Submillimeter Continuum Observations of Ophiuchi A: The Candidate Protostar VLA 1623 and Prestellar Clumps". The Astrophysical Journal, Part 1. 406 (1): 122–141. Bibcode:1993ApJ...406..122A. doi:10.1086/172425.
  3. ^ Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (October 1994). "Further Mid-infrared Study of the Ophiuchi Cloud Young Stellar Population: Luminosities and Masses of Pre-main-sequence Stars". The Astrophysical Journal, Part 1. 434 (2): 614–626. Bibcode:1994ApJ...434..614G. doi:10.1086/174763.

External links

Media related to Young stellar objects at Wikimedia Commons

AFGL 2591

AFGL 2591 is a star forming region in the constellation Cygnus. Its dense cloud of gas and dust make its interior invisible to optical telescopes. Images in the infrared show a bright young stellar object, with an associated reflection nebula seen as a glowing cone projecting from the young star. A cluster of stars is forming within the molecular cloud, but most of the infrared radiation is coming from this star, AFGL 2591-VLA3.Initially AFGL 2591 was thought to be a single young, massive star expelling clouds of gas and dust in multiple events. It was estimated to be about 10 times the mass of the sun and at a distance of only 1,000 parsecs (3,300 light-years).

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.

DG Tauri B

DG Tauri B, near the T Tauri star DG Tauri, is a young stellar object located 450 light-years (140 parsecs) from Earth, within the Taurus constellation. Observations of DG Tauri B were first made in October, and later December 1995 at the 6 element Owens Valley millimeter wave array. Its most notable characteristics are its bipolar jets of molecular gas and dust emanating from either side of the object. Red-shifted carbon monoxide emissions extend out 6,000 au to the northwest of the object from the undetermined source, and are symmetrically distributed about the jet, while blue-shifted CO emissions are confined to a region with a roughly 500 au radius.

HD 259431

HD 259431 (MWC 147 or V700 Monocerotis) is a young stellar object in the constellation of Monoceros.

Helium-weak star

Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (ie. cooler) spectral type then their hydrogen lines.

Kelvin–Helmholtz mechanism

The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.

However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was shown by Hans Bethe to be nuclear fusion.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

PV Cephei

PV Cep is variable star of Orion type located in the constellation of Cepheus at a distance of over 1600 light-years from Earth. Although the terms 'Orion variable/Orion type' are now no longer used by most astronomers. The term 'Young Stellar Object' or YSO is preferred, since 'Orion Variable' is a term which was given at a time when these objects were thought to be more homogeneous than is now known to be the case. It has been used by the CGVS compilers but astronomers generally do not use these terms any more.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

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 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. For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years 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.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.


Sh2-297 (also known as Sharpless 297) is an emission nebula in the constellation Canis Major.

The region was catalogued in 1959 in the extended seconded edition of the Sharpless catalogue. This area is part of the Canis Major OB1 Association, and is a very active area of new star formation.Studies in 1988 found that the bright star illuminating the nebula was 8th magnitude HD 53623 / HIP 34178 with spectral class B1II/III. Later in 2004 it was shown that there was embedded a cold but massive Young Stellar Object or YSO within Sh2-297 near the edge of one of the dark rifts. This object has been observed in the far-infrared, but it is so deeply embedded in an interstellar cloud that it is undetectable in shorter wavelength observations such as the Two Micron All Sky Survey (2MASS), leading it to be originally named "Unidentified young stellar object 1" or UYSO-1. It was further revealed that this unseen stellar source produces a carbon monoxide (CO) bipolar outflow with a total mass of 5.4 M☉ solar masses, while the surrounding extended envelope weighs 30 M☉–40 M☉. Some 96 other YSO's have been discovered to be part of Sh-297, having a mean age of one million years and range in masses between 0.3 M☉ and 2.0 M☉. Many variable stars are also assigned with this nebula complex, including the three brightest: MW Ori, TT Ori and V559 Ori.

Distance is estimated between 1.0 and 1.4 kpc. (3,300–4,600 ly.), averaging 1.2 kpc. or 3,900 ly.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Theta Orionis

The Bayer designation θ Orionis (Theta Orionis) is shared by several astronomical objects, located near RA 05h 35m DEC −05° 24′:

θ1 Orionis (41 Orionis), the Trapezium Cluster, an open star cluster, the Orion OB Association 1d

θ1 Orionis A (41 Orionis A, HD 37020, V1016 Orionis), a trinary star system

θ1 Orionis B (41 Orionis B, HD 37021), a quintet star system

θ1 Orionis B West (COUP 766, MAX 97), an astronomical X-ray source

θ1 Orionis B East (COUP 778, MAX 101), an astronomical X-ray source

θ1 Orionis C (41 Orionis C, HD 37022), a binary star system

θ1 Orionis D (41 Orionis D, HD 37023), a B0.5Vp variable star

θ1 Orionis E (COUP 732), a spectroscopic binary star system

θ1 Orionis F, a B8 variable star

θ1 Orionis G (COUP 826, MAX 116), a young stellar object

θ1 Orionis H (COUP 746, MAX 87), a young stellar object

θ2 Orionis (43 Orionis)

θ2 Orionis A (43 Orionis, HD 37041), a spectroscopic trinary

θ2 Orionis B (HD 37042), a B1V variable star

θ2 Orionis C (HD 37062), a binary star systemThe various components are spread over several arc-minutes in and near the Orion Nebula.

V1094 Scorpii

V1094 Scorpii is a young stellar object in the constellation of Scorpius, located in the young Lupus Star Forming Region.

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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