Astrophysics (index)

Young Stellar Object

(early stage of a star)

A Young Stellar Object (YSO) is a star in the early stages of development, i.e., a Protostar or a Pre-Main-Sequence Star. They can be hard to see because their normal environment, a Star-Forming Region usually suffers from Extinction.

Classes of YSOs:

  • Class O - undetectable at λ < 20um
  • 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


α = —————————

Where λ is the Wavelength and Fλ is the Flux density.

The classes roughly follow the evolution. They also often show Jets, disks, etc.

(star type,evolution)

Referenced by:
Herschel Orion Protostar Survey (HOPS)
I Band
Spitzer Space Telescope (SST)