What does this figure imply about the luminosity of pre-main-sequence stars? Check out Figure 1 in http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1955PASP...67..154H&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf .



It seems to show that the luminosity of a pre-main-sequence star depends primarily on the mass of the star, and that this basic relation continues to hold, to reasonable accuracy, into the main sequence.  Since there is no fusion in a pre-main-sequence star, is it correct to conclude that the relationship between the mass and luminosity of a pre-main-sequence star does not hinge on fusion physics?
It shows that the mass of the (proto)star is the major factor determining bolometric magnitude. 

The brightness of the proto-star increases a little as it contracts and becomes significantly hotter (and the radius decreases), The star moves toward the left of the diagram. As fusion in the core becomes significant the star reaches the maximum point on the curve, this stabilises the core and the star settles into the main sequence, it cools slightly and becomes slightly less bright.