Messier 15 or M15 (also designated NGC 7078)
is a globular cluster in the constellation Pegasus. At an
estimated 12.0 billion years old, it is one of the oldest known
globular clusters. M15 is about 33,600 light-years from Earth,
and 175 light years in diameter. It has an absolute magnitude of
-9.2, which translates to a total luminosity of 360,000 times
that of the Sun. Messier 15 is one of the most densely packed
globulars known in the Milky Way galaxy. Its core has undergone
a contraction known as 'core collapse' and it has a central
density cusp with an enormous number of stars surrounding what
may be a central black hole. Home to over 100,000 stars, the
cluster is notable for containing a large number of variable
stars (112) and pulsars (8), including one double neutron star
system, M15 C. M15 also contains Pease 1, the first planetary
nebula discovered within a globular cluster in 1928. Just three
others have been found in globular clusters since then.
