A star forms by accumulation of material that falls in to a protostar from a
circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission. As material in the disk is depleted, the infrared excess decreases. Thus, YSOs are usually classified into evolutionary stages based on the slope of their
spectral energy distribution in the mid-
infrared, using a scheme introduced by Lada (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). Andre
et al. (1993) discovered a class 0: objects with strong submillimeter emission, but very faint at {\lambda}. Greene
et al. (1994) added a fifth class of "flat spectrum" sources. •
Class 0 sources – undetectable at {\lambda} •
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} 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. Class II objects have circumstellar disks and correspond roughly to classical
T Tauri stars, while Class III stars have lost their disks and correspond approximately to weak-line T Tauri stars. An intermediate stage where disks can only be detected at longer wavelengths (e.g., at 24{\mu}m) are known as transition-disk objects. ==Characteristics==