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 mainl
y 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 0. 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 circumstellar envelope. 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 planetesimals. This classification scheme can be made more quantitative by defining a spectral index.
By convention spectral analysts designate the level of ionization with a roman numeral that is one unit larger that the actual number of missing electrons. For example HeII is singly ionized Helium (o
ne electron missing).