young stellar object

A young stellar object (YSO) is a general term for a developing star, i.e., a protostar or pre-main-sequence star. They can be challenging to observe because their normal environment, star-forming regions often suffer from extinction, so, for example, YSOs are often not recorded by Gaia. Fortunately, some have significant magnetic fields and are radio sources (radio YSOs). They often have jets and/or circumstellar disks. A classification of YSOs:

• Class 0 - undetectable at λ < 20 μm
• Class I - α > 0.3
• Flat spectrum sources: 0.3 > α > -0.3
• Class II - -0.3 > α > -1.6
• Class III - α < -1.6

where α is its spectral index based on mid-infrared wavelengths:

    d log(λFλ)
α = ——————————
    d log(λ)

where λ is wavelength and F_λ is the wavelength's associated spectral flux density. The classes roughly follow their evolution, from a lower-temperature black-body spectrum of the YSO's surrounding dust, with subsequent classes associated with the growing temperature of the forming star.

The abbreviation MYSO is used for massive young stellar objects, the presumed precursors of massive stars such as O-type stars and B-type stars.