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.

These stars may be differentiated by mass: Massive YSOs, intermediate mass YSOs, and brown dwarfs.

YSOs are usually classified using criteria based on the slope of their spectral energy distribution, introduced by Lada (1987). He proposed three classes (I, II and III), based on the values of intervals of spectral index ${\displaystyle \alpha \,}$:

${\displaystyle \alpha ={\frac {d\log(\lambda F_{\lambda })}{d\log(\lambda )}}}$.

Here ${\displaystyle \lambda \,}$ is wavelength, and ${\displaystyle F_{\lambda }}$ is flux density.

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