QUESTION:
Why are there so many stars and only one of them explodes every four
hundred years?

ANSWER from Mike Haas on October 20, 1995:
There are lots of stars in the Milky Way galaxy - about
100,000,000,000 - and we haven't seen one explode since Kepler's
supernova in 1604 - almost 400 years ago - so what gives?  The answer
to your question has three parts:  First, we don't see all the
supernovae that occur because our galaxy is a dirty, dusty place.  This
dust obscures our view of the more remote parts, just like fog, snow,
or dust prevents us from seeing very far down the road on some stormy
days.  Astronomers estimate that a supernova actually explodes
somewhere in our galaxy about once every 30 years and that we miss out
on most of these events because we simply can't see through the
intervening material very well.  That is one of the reasons for
developing and flying a facility like the Kuiper Airborne Observatory -
infrared light is better able to penetrate the dust and allows us to
see further than does visible light.

Second, stars are born with many different masses (or sizes).  We
compare the mass of each star to that of our own Sun.  If the star is
as large as the Sun, we say it has 1 solar mass.  If it is 10 times
larger, we say it has 10 solar masses, etc.  Most of the stars in our
galaxy are smaller than our Sun.  These stars are not large enough to
ever supernova - they may become planetary nebulae for a while, but
eventually they become white dwarfs and die a slow and rather
unspectacular death.  Astronomers think that most stars with masses
less than about three or four solar masses end their life in this
rather uneventful way.  Stars larger than three or four Suns may
supernova or they may loose mass gradually by ejecting matter in a
stellar wind, usually during their red giant phase.  Hence, only a
small fraction of the stars (about 1% or less) will eventually become
supernovae.

Third, stars live for a very long time.  The brightest, hottest, most
massive stars live for about 10 million years; fainter, less massive
stars like the Sun live for over 10 billion years.  This is just like
people - because we typically live 70 years or longer, not many of us
die in any given one hour period.  If we each lived 10 million years,
not many of us would die in any given thirty year period!

So, in short, the answer to your question is (a) we don't see all the
supernova that do explode, (b) only the largest stars form supernova,
and (c) these stars live a very long time as normal stars before
running out of fuel, collapsing, and then exploding as supernovae.
(This is kind of like asking why don't more 450 lb men die between 5
and 6 pm on the Friday after Christmas? - well we don't hear about every
one that does, there aren't very many of them in the first place, and
they each live a relatively long time).  For astronomers, this is one
of the charms - maybe, just maybe, a supernova went off in our galaxy
10,000 years ago and the light will reach our telescopes for the first
time tonight.