# Young stellar object

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V1331 Cyg, a YSO surrounded by a reflection nebula

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 mass

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

## Classification by spectral energy distribution

YSOs are usually classified using criteria based on the slope of their spectral energy distribution, introduced by Lada (1987). He proposed three classes (I, II and III), based on the values of intervals of spectral index ${\displaystyle \alpha \,}$:[1]

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

Here ${\displaystyle \lambda \,}$ is wavelength, and ${\displaystyle F_{\lambda }}$ is flux density.

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

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

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.

## Characteristics

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

2. ^ Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (March 1993). "Submillimeter Continuum Observations of ${\displaystyle {\rho }}$ 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.
3. ^ Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (October 1994). "Further Mid-infrared Study of the ${\displaystyle {\rho }}$ 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.