How many 0.75 MŽ main sequence stars would it take to equal the
luminosity of one 15 M;) star?

"In order to remain stable via hydrostatic equilibrium, a star's
luminosity increases with mass as (the star's mass)p. The value of the
exponent p varies between 3 and 4. For the rare massive stars (M* > 30
Msun), p = 3 and for the more common low-mass stars (M* < 10 Msun), p
= 4."

http://www.astronomynotes.com/evolutn/s2.htm "Stellar Evolution" by
Nick Strobel

So, assuming p=4, Q = 15^4/0.75^4 = 160000 0.75 M main sequence stars.

Sincerely,

Wonko

Terrific! Thanks!!