Messier
15, or just M15, is a
globular cluster located in the direction of the
constellation
Pegasus, the winged horse with magical powers born of
Medusa’s headless blood. As with many of the objects in Charles
Messier’s Catalog, M15 was discovered by another astronomer
Giovanni Domenico Maraldi, who did not get the globular cluster named after him,
but did posthumously receive the honor of the named lunar crater
Maraldi.
Globular clusters are spherical collections of
stars bound together by gravity with a higher concentration of
stars at their centers. Globular clusters are in contrast to
open
clusters, which are formed out of the same giant molecular
cloud, less dense, and usually do not stay together as long as
globular clusters. Some have suggested that globular clusters
are early galaxies that have not merged with others to form an
actual galaxy, however, the fact that most large galaxies have
many globular clusters orbiting about them may give doubt to
this theory. In contrast, globular clusters have tens to hundreds
of thousands of stars, galaxies have millions, billions, and
even a trillion or more stars. Why so many globular clusters
exist around galaxies and are not swallowed up (yet?) by them
remains a mystery.
While globular clusters are
relatively dense, their stars are on average half a trillion
miles apart or more. At the core of this cluster, however, you
might see up to 1000 stars as bright as Venus in the night sky,
but unlikely any planets like Venus since their relatively close
proximity to other stars makes
protoplanetary disks unlikely to maintain stable enough to
form planets.
At only 35,700 light years from Earth, M15
is around 175 light years in diameter. M15 is is between 11.5
and 13.5 billion years old making it one of the oldest known
globular clusters. It is also one of our galaxy’s most densely
populated globular clusters. Its center is especially dense as
its core has collapsed (see center of image above) with an
enormous number of stars surrounding what may indeed be a
massive black hole. M15 is home to around 100,000 stars and
contains 112
variable
stars, 8
pulsars, 2
X-ray sources, 1 double
neutron
star, and even a
planetary nebula, called
Pease 1
(visible in the image above). Only three other globular clusters
have been found to contain planetary nebula.
At apparent
magnitude 6.2, M15 would be a very challenging object to view
with the unaided eye even at a very dark site, however, it is
easily visible through binoculars or small telescopes. The
brightest stars in M15 are around 12.5 magnitude or 400 times
fainter than the faintest star visible to the naked eye.
*Using a mouse, hover over the image above for annotations,
insets, and enlargements. This hover feature may be unavailable
on smart phones.
.
