# Young stellar object

### Young stellar object

Small young stellar object known as SSTC2D J033038.2+303212, located in the constellation of Perseus.[1]

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.

## Contents

• Classification by mass 1
• Classification by spectral energy distribution 2
• Characteristics 3
• References 5

## Classification by mass

Sometimes they are divided by mass – massive YSO (MYSO), intermediate mass YSO and brown dwarfs.

## Classification by spectral energy distribution

YSO are usually classified using criteria based on the slope of their SED, introduced by Lada C.J. in 1987. He proposed three classes (I, II and III), based on the values of intervals of spectral index \alpha \,:

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

Here \lambda \, is wavelength, and F_\lambda is flux density.

The \alpha \, is calculated in the wavelength interval of 2.2–20 {\mu}m (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 {\lambda}<10{\mu}m.

• Class 0 sources – undetectable at {\lambda}<20{\mu}m
• Class I sources have {\alpha}>0.3
• Flat spectrum sources have 0.3>{\alpha}>-0.3
• Class II sources have -0.3>{\alpha}>-1.6
• Class III sources have {\alpha}<-1.6

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.

## Characteristics

YSO are also associated with early star evolution phenomena: jets and bipolar outflows, masers, Herbig–Haro objects, and protoplanetary disks (circumstellar disks or proplyds).