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. These do not appear on the diagram at left, because they are so buried, they do not appear even at 3.6 or 4.5 microns. --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.