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

This class consists of two groups of objects: protostar

Protostar

A protostar is a large mass 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. For a one solar-mass star it lasts about 100,000 years...

s and pre–main sequence stars. Sometimes they are divided by mass - massive YSO (MYSO), intermediate mass YSO and brown dwarf

Brown dwarf

Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs 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 brightness or flux density versus frequency or wavelength of light. It is used in many branches of astronomy to characterize astronomical sources. For example, in radio astronomy, an SED with a negative spectral index around −0.7 would indicate a...

, introduced by Lada C.J. in 1987. He proposed three classes (I, II and III), based on the values of intervals of spectral index

Spectral index

In astronomy, the spectral index of a source is a measure of the dependence of radiative flux density on frequency. Given frequency \nu and radiative flux S, the spectral index \alpha is given implicitly byS\propto\nu^\alpha....

 :

.

Here is wavelength, and 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...

 density.

The is calculated in the wavelength interval of 2.2–20 (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 .

• 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 presentbefore formation of the star...

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 colour and luminosity of protostars at the end of the initial phase of accretion onto them. Prior to this point, the protostar is deeply embedded in the cloud of dust and gas from which it formed, and so...

 as pre–main sequence stars.

YSO are also associated with early star evolution phenomena: polar jet

Polar jet

A polar jet is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc. When matter is emitted at speeds approaching the speed of light, these jets are called...

s 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 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 nebulosity 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 star...

s (circumstellar disks or proplyds).

See also

• Bok globule
  Bok globule
  Bok globules are dark clouds of dense cosmic 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...
  
  
• Herbig-Haro object
  Herbig-Haro object
  Herbig–Haro objects are small patches of nebulosity 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...
  
  
• Pre–main sequence star
• Protostar
  Protostar
  A protostar is a large mass 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. For a one solar-mass star it lasts about 100,000 years...