QUESTION:
How can you know what's really happening on a star when you can't
really see what's happening up close?

ANSWER from Jesse Bregman on May 23:
There are many ways we can tell what is probably happening on and around
a star which is far away.  The way we can tell what the star is made of
is by looking at the light from the star with an instrument called a
spectrograph.  It takes the starlight and spreads it out into its different
colors just as a prism does.  In addition to the rainbow of colors you are
used to seeing, if you look very carefully you will see dark lines in the
rainbow (or spectrum) and occasionally some very bright lines of single
colors.  These dark and bright lines are made by atoms of individual
elements.  Each element produces a different set of lines which form a
sort of fingerprint for that element.  We can measure the elements in
laboratories here to find out what their fingerprints are, and then we
can tell what the star is made of.  The element Helium was discovered in
a spectrum taken of the sun before it was found on earth (that is why it
is called Helium, from helios which means sun in Greek).  From specta,
we know that stars are made of mostly hydrogen (75-80%) and helium
(about 20%), with all the other elements making up about 2%.  The most
common of the other elements are oxygen, nitrogen, and carbon.