Now, let's look at the 'y' axis. At the left we see
the value of the luminosity is compared with that of the Sun.
Corresponding to that at the right is
the value of the stars absolute magnitude which
is defined as the magnitude that the star would have At a distance of 10 parsecs.
A moment's thought will show you that in
order to know that number, the absolute magnitude,
you must know the star's distance, so you can convert the observed
apparent magnitude To the value it would have at 10 parsecs.
But now look what you can do.
You can stand the problem on its head. Imagine you observed the spectrum of a
star that looks identical to, say, one whose B minus V color
is 1.5 and whose luminosity is 0.01 times that of the sun.
You know then, that your star has an absolute magnitude of plus ten.
Now, by comparing your observed apparent magnitude, to the
value of plus ten, you immediately know the distance.
You have derived what is called a spectroscopic parallax.
It is not really a parallax at all, but a way to get the distance to stars
for which the parallax might be unmeasurable because it is too far away.
So you can see how powerful the HR diagram can be.
But now we can go beyond this and
using some theoretical results of stellar structure understand how
stars evolve in time. This, in turn, will lead us to a
basic understanding Of how some of our x ray sources come into being.
The first thing that guides us is the
rather surprising fact, that stars have a fairly restricted
range of masses, again the name of Henry
Norris Russell pops up, and it turns out that
by the middle of the 20th century.
We had a fairly accurate appraisal that the very low luminosity stars
had masses of about 1/10th of a solar mass, while the upper end of
the scale was populated with stars of about 50 solar masses.
That range, a factor of about 500, stands in stark
contrast to the range in luminosity, which you can see is a factor
of 10 billion. How is that possible?
Well, clearly, one way out is if the high mass stars
exhaust their fuel much more rapidly than the low mass objects.
In other words, they burn a tremendous amount of fuel,
hence providing a high luminosity, but they must burn out very
rapidly since they aren't billions of times more massive than there are
low luminosity counter parts and in deed our model show that
stars at the very low end of the main sequence have enough fuel to
last for over 10 billion years, whereas at the high end
objects exhaust their supply in a mere 1 million years or so
So we ought to be able to use observations of various clusters of stars
to see this effect if indeed some are older than others.