THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Founded January 1970
Julian date: 2457675.16
"No,we didn't forget!"
*THE DAILY ASTRONOMER*
*Wednesday, October 12, 2016*
*The Pegasus Cluster*
Our year long Messier tour continues with one of the oldest and most
magnificent of all globulars: The Pegasus Cluster.
Deep in the darkness of intergalactic space one will find a sparse
population of immense star clusters called "globulars," so named as they
resemble large globes. They are largely confined to the galactic halo: a
large, spherical region of space centered on the galactic nucleus. These
globulars describe wide orbits around the galactic center, much like comets
follow elongated orbits around the Sun. However, whereas comets can
complete an orbit in tens or hundreds of years, a globular cluster's
orbital period is millions of years in duration. Throughout the aeons,
they alternately ascend above and then descend below the dense galactic
plane. Despite the tidal stresses of each passage, the globulars tend to
retain their shapes,but will often relinquish many of their members to
their cannibalistic parent galaxy. One of the densest and oldest of
these stellar behemoths is the magnificently beautiful, but curiously under
celebrated "Pegasus Cluster"
*The location of M15: The Great Pegasus Cluster is tucked away just
"above" Pegasus's head star "Enif." This chart helps to guide telescope
observers to the location of this well hidden, but magnificent globular
cluster. Image by Roberto Mura. *
The Pegasus Cluster is designated M15, as it was the 15th object to be
included in Charles Messier's famous catalog. As is true with many
Messier objects, M15 was discovered by someone else. The Italian
astronomer Giovanni Domenico Maraldi (1709-1788) first observed the Pegasus
Cluster in 1764. Messier promptly added this object to his collection
because it so closely resembles a comet's nucleus. (Messier compiled his
catalog for the benefit of other astronomers who might misconstrue the
catalog's objects for comets.)
* M15: The Great Pegasus Cluster *
*Located about 33,600 light years from Earth, the Great Pegasus Cluster
contains more than 100,000 stars within a sphere 175 light years in
diameter. This yields an average stellar density of one star for every
1.42 cubic light years, making this cluster one of the densest globulars in
the Milky Way Galaxy. Image: The Hubble Space Telescope*
Even though the Great Pegasus Cluster is about 33,600 light years away, it
is about 300,000 times more luminous than the Sun and can therefore be
observed even with binoculars. ( This cluster could even be barely
observed with the unaided eye in a perfectly dark sky. ) This cluster's
stellar population exceeds 100,000: all packed within a sphere 175 light
years in diameter. The average stellar density is therefore a star for
every 1.4 cubic light years: a Tokyo subway by galactic standards.
However, a globular cluster is not uniformly dense, but instead, the
density diminishes with the cube of the distance from the center.
Therefore, the core must be so dense as to have experienced a "core
collapse," and might well be a black hole.
Even though Omega Centauri and the Hercules Cluster (M13) receive most of
the attention, the Great Pegasus Cluster is among the richest and loveliest
of all globular clusters. Moreover, this time of year, one can easily
pinpoint it, for it is located at the western edge of Pegasus, a horse that
is nearly up in the sky all night long.
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FROM THE CATACOMBS OF INFINITE KNOWLEDGE
*"The first planetary nebula discovered in the globular cluster."*
This title reads like an obscure baseball statistic such as "The first
batter to hit for the cycle against a left handed pitcher with a prime
number jersey in a prime number year during an intermittent rain."
American astronomer Francis Pease (1881-1938) is not exactly a household
name. However, he was one of the astronomers to first measure the angular
diameter of Betelgeuse (0.047"). He also discovered the first planetary
nebula in a globular cluster. He found this nebula in M15, the Pegasus
Cluster. It is well hidden just to the upper left of the core in the
Hubble image we featured in this article. This nebula has been named
Pease 1 in his honor. Only three other planetary nebulae have been found
in globular clusters thus far.
*Dr. Strangelove*
For the life of us, we couldn't find a photo of American astronomer Francis
G, Pease,
so, instead, here's an image of Dr. Strangelove, as portrayed by Peter
Sellers.
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