A plot showing the energy emitted by a source as a function of the radiation
wavelength or frequency. It is used in many branches of astronomy to characterize
astronomical sources, in particular mainly in → near infrared
and → middle infrared to study
→ protostars or
→ young stellar objects. The SED of these objects is
divided in four classes.
Class 0 in which the SED
represents a very embedded protostar, where the mass of the central core is small
in comparison to the mass of the → accreting envelope. The SED
is characterized by the → blackbody radiation of the
envelope and peaks at → submillimeter wavelengths.
Class I objects possess a SED that peaks in the → far infrared
and is characterized by a weak contribution of the blackbody of the central protostar (detected
in near infrared)
and the emission of a thick disk and dense envelope. These objects
have less mass in the envelope and more massive central cores with respect to
Class II objects are the → classical T Tauri stars
with a SED due to the emission of a thin disk and the central star.
They have accumulated most of their final mass and have dispersed almost completely their
Finally, Class III objects have pure photospheric spectra. Their SED is peaked in the optical
and is well approximated by a blackbody emission with a faint
→ infrared excess
due to the
presence of a residual optically thin disk that may be the origin of
This classification scheme can be made more quantitative by defining a
→ spectral index.