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.

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 \( \alpha \, \):[1]

\( \alpha ={\frac {d\log(\lambda F_{\lambda })}{d\log(\lambda )}}. \)

Here \( \lambda \, \) is wavelength, and \( F_{\lambda } \) is flux density.

The α {\displaystyle \alpha \,} \alpha \, is calculated in the wavelength interval of 2.2–20 \( {\mu }m \) (near- and mid-infrared region). Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at \( {\lambda }<10{\mu }m \) .[2] Greene et al. (1994) added a fifth class of "flat spectrum" sources.[3]

Class 0 sources – undetectable at \( {\lambda }<20{\mu }m \)
Class I sources have \(} {\alpha }>0.3 \)
Flat spectrum sources have \( 0.3>{\alpha }>-0.3 \)
Class II sources have \( -0.3>{\alpha }>-1.6 \)
Class III sources have \( {\alpha }<-1.6 \)

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 \( {\displaystyle 24{\mu }m}) \) 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

Bok globule


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.
Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (March 1993). "Submillimeter Continuum Observations of \( {\rho } \) 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.

Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (October 1994). "Further Mid-infrared Study of the \( {\rho } \) 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


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

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Accretion Molecular cloud Bok globule Young stellar object
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super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

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Yellow Carbon
S CN CH White dwarf Chemically peculiar
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Shell B[e]


White dwarf
Helium planet Black dwarf Neutron
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Blue dwarf Green Black dwarf Exotic
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Stellar nucleosynthesis

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Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism


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Star systems

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Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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