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.   

_______________________________________________________________
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.
_________________________________________________________________