Young Stellar Object

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. Sometimes they are divided by mass - massive YSO (MYSO), intermediate mass YSO and brown dwarfs.YSO are usually classified using criteria based on the slope of their SED, 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 : .
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The generic name for a star prior to its main sequence phase