What does YSO mean in Unclassified?

Young stellar object

Young stellar object denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre–main sequence stars. Sometimes they are divided by mass – massive YSO, intermediate mass YSO and brown dwarfs. YSO are usually classified using criteria based on the slope of their SED, introduced by Lada C.J. in 1987. He proposed three classes, based on the values of intervals of spectral index : . Here is wavelength, and is flux density. The is calculated in the wavelength interval of 2.2–20 . 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 envelopes. Eventually they become optically visible on the stellar birthline as pre–main sequence stars.

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