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.

A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope.

Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission.

Thus, YSOs are usually classified into evolutionary stages based on the slope of their spectral energy distribution in the mid-infrared, using a scheme introduced by Lada (1987).

He proposed three classes (I, II and III), based on the values of intervals of spectral index

Andre et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at

[2] Greene et al. (1994) added a fifth class of "flat spectrum" sources.

Eventually they become optically visible on the stellar birthline as pre-main-sequence stars.

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