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

This class consists of two groups of objects: protostar
Protostar

A protostar is a large star that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation....
s and pre-main sequence star
Pre-main sequence star

A pre-main sequence star is a star in the stage when it has not yet reached the main sequence. It can be a T Tauri star or FU Orionis star or an Herbig Ae/Be stars ....
s. Sometimes they are divided by mass - massive YSO (MYSO), intermediate mass YSO and brown dwarf
Brown dwarf

Brown dwarfs are sub-star objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth....
s.

YSO are usually classified using criteria based on the slope of their SED
Spectral Energy Distribution

A spectral energy distribution is a plot of flux, brightness, or flux density versus frequency or wavelength of light. It is used in many branches of astronomy to characterize astronomical sources....
, introduced by Lada C.J. and Wilking B.A. in 1984. They proposed three classes (I, II and III), based on the values of intervals of spectral index
Spectral index

In radio astronomy, the spectral index of a radio astronomy can hint at its physical properties. The spectral index is calculated from an assumed power law dependence for flux given by where is flux and is frequency....
 :

.

Here is frequency, is flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 density.

The is calculated in the wavelength interval of 2.2–10 (near infrared region). Later Greene et al.






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

This class consists of two groups of objects: protostar
Protostar

A protostar is a large star that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation....
s and pre-main sequence star
Pre-main sequence star

A pre-main sequence star is a star in the stage when it has not yet reached the main sequence. It can be a T Tauri star or FU Orionis star or an Herbig Ae/Be stars ....
s. Sometimes they are divided by mass - massive YSO (MYSO), intermediate mass YSO and brown dwarf
Brown dwarf

Brown dwarfs are sub-star objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth....
s.

YSO are usually classified using criteria based on the slope of their SED
Spectral Energy Distribution

A spectral energy distribution is a plot of flux, brightness, or flux density versus frequency or wavelength of light. It is used in many branches of astronomy to characterize astronomical sources....
, introduced by Lada C.J. and Wilking B.A. in 1984. They proposed three classes (I, II and III), based on the values of intervals of spectral index
Spectral index

In radio astronomy, the spectral index of a radio astronomy can hint at its physical properties. The spectral index is calculated from an assumed power law dependence for flux given by where is flux and is frequency....
 :

.

Here is frequency, is flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 density.

The is calculated in the wavelength interval of 2.2–10 (near 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 .

  • 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 envelope
Circumstellar envelope

Circumstellar envelope is the part of the star,having roughly spherical shape and not gravitationally bound to the star core.Usually circumstellar envelopes are formed from the dense Stellar_wind or present...
s. Eventually they become optically visible on the stellar birthline
Stellar birthline

The stellar birthline is a predicted line on the Hertzsprung-Russell diagram that relates the B-V colour and luminosity of protostars at the end of the initial phase of accretion onto them....
 as pre-main sequence stars.

YSO are also associated with early star evolution phenomena: circumstellar jets and bipolar outflow
Bipolar outflow

A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars , or with evolved post-AGB stars ....
s, masers
Astrophysical maser

An astrophysical maser is a naturally occurring source of Stimulated emission spectral line emission, typically in the microwave portion of the electromagnetic spectrum....
, Herbig-Haro object
Herbig-Haro object

Herbig-Haro objects are small patches of nebula associated with newly-born stars, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby at speeds of several hundred kilometres per second....
s, protoplanetary disk
Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig Ae/Be stars....
s (circumstellar disks or proplyds).

See also

  • Herbig-Haro object
    Herbig-Haro object

    Herbig-Haro objects are small patches of nebula associated with newly-born stars, and are formed when gas ejected by young stars collides with clouds of gas and dust nearby at speeds of several hundred kilometres per second....
  • Protostar
    Protostar

    A protostar is a large star that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation....
  • Bok globule
    Bok globule

    A Bok globule is a dark cloud of dense dust and gas in which star formation sometimes takes place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across ....