The position
of a star on the main sequence basically depends on its mass.
For most of its life, both its luminosity and temperature are
determined by
its mass. We can see from the formula 
that the
radius is determined by L and T, which are given by
its mass. Thus every intrinsic property of the
star--its luminosity, radius, and surface temperature--are
determined
by its mass. Let's take this idea even further. The amount of
time a
star can live by burning hydrogen depends on its mass (the
available fuel)
and its luminosity (how quickly it uses that fuel). And as we'll
see, the
mass of a star also determines what other elements it can use as
nuclear
fuel, and what kind of stellar remnant it will end up as. This
is one
of the great achievements of astrophysics: given the initial mass
of a
star, we can write it's entire life story in amazing detail.
To summarize, here's what the mass of a star can tell us:
Start:Stars
