ORION star formation complex poster
Our current classification scheme for young stellar objects (YSOs) was possible thanks to the advent of the first space-based infrared sky surveys more than 30 years ago. As illustrated in Figure 1, the Classes 0 to III – proposed by Lada (1987, IAUS, 115, 1) and André et al. (1993 ApJ, 206, 122).
While these classes can be interpreted as an evolutionary sequence reflecting the different phases of star formation, the specific duration of each phase can only be inferred statistically from population studies based on the relative fractions of YSOs observed in each of the classes (For example, Figure 1 shows timescales estimated by Evans et al. 2009, ApJS, 181, 321). Moreover, studies of YSOs based solely on their SEDs are prone to uncertainties due to unknowns such as YSOs’ inclination, geometry, and reddening.