File talk:Ysobyclassoni4.png

http://coolwiki.ipac.caltech.edu/index.php/Color-Magnitude_and_Color-Color_plots [1]--

Class 0: these are very young stars, still buried deep inside their cocoon of gas.

--Class I: these are protostars surrounded by an infalling envelope of gas.

--Class II: these protostars, now in the T-Tauri stage, have developed protoplanetary disks.

--Class III: these are nearly fully developed stars, with just a remnant disk (and possibly planets) surrounding them.