View source for Pfister form

{{Star formation}}

'''Young stellar object''' (YSO) denotes a star in its early stage of evolution.

This class consists of two groups of objects: [[protostar]]s and [[pre–main sequence star]]s. Sometimes they are divided by mass – massive YSO (MYSO), intermediate mass YSO and [[brown dwarf]]s.

YSO are usually classified using criteria based on the slope of their [[Spectral energy distribution|SED]], introduced by Lada C.J. in 1987. He proposed three classes (I, II and III), based on the values of intervals of [[spectral index]] <math>\alpha \,</math>:

<math>\alpha=\frac{d\log(\lambda F_\lambda)}{d\log(\lambda)}</math>.

Here <math>\lambda \,</math> is wavelength, and <math>F_\lambda</math> is [[Luminous flux|flux]] density.

The <math>\alpha \,</math> is calculated in the wavelength interval of 2.2–20 <math>{\mu}m</math> (near- and mid- infrared region).
Later Greene et al. in 1994 added a fourth class of "flat spectrum" class sources. In 1993 Andre et al. discovered class 0—objects with strong submillimeter emission, but very faint at <math>{\lambda}<10{\mu}m</math>.

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

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 envelope]]s. Eventually they become optically visible on the [[stellar birthline]] as pre–main sequence stars.

YSO are also associated with early star evolution phenomena: [[polar jet]]s and [[bipolar outflow]]s, [[Astrophysical maser|masers]], [[Herbig-Haro object]]s, [[protoplanetary disk]]s (circumstellar disks or proplyds).

==References==
*Lada, C. J. (1987), IAU Symposium 115: Star Forming Regions, 115, 1
*Greene, Thomas P.; Wilking, Bruce A.; Andre, Philippe; Young, Erick T.; Lada, Charles J. (1994), ''Further mid-infrared study of the rho Ophiuchi cloud young stellar population: Luminosities and masses of pre-main-sequence stars'', The Astrophysical Journal, vol. 434, pp.&nbsp;614–626
*Andre, Philippe; Ward-Thompson, Derek; Barsony, Mary (1993), ''Submillimeter continuum observations of Rho Ophiuchi A – The candidate protostar VLA 1623 and prestellar clumps'', The Astrophysical Journal, vol. 406, pp.&nbsp;122–141

==See also==
{{Portal|Star}}
* [[Bok globule]]
* [[Herbig-Haro object]]
* [[Pre–main sequence star]]
* [[Protostar]]

{{Star}}

{{DEFAULTSORT:Young Stellar Object}}
[[Category:Star formation]]
[[Category:Star types]]

{{astronomy-stub}}
{{star-stub}}