Messier 15 (NGC 7078) in Pegasus is a showpiece object of the early autumn sky and one of the finest globular clusters to grace the northern sky. Globular clusters are a truly spectacular class of deep-sky object, being densely-packed and near-spherical collections of ancient stars.
Under a pristine dark sky, eagle-eyed observers may spot it with the naked eye; through 10 x 50 binoculars it appears as a bright, fuzzy ball. Messier 15 lies well above the celestial equator, so observers at mid-northern latitude won’t face any low-altitude issues. Indeed, its observable throughout September at a decent altitude from nightfall through to near dawn.
How to observe:
Messier 15 shines at magnitude +6.0 and spans 16’ to 18’ at its fullest extent (sources differ on M15’s apparent diameter). It’s located four arcminutes north-west of magnitude +2.4 Enif (epsilon [ε] Pegasi), a position that from London sees it culminate shortly after 10.30pm BST at an altitude of around 50 degrees.
A telescope in the 80mm (~three-inch) class shows Messier 15 as a bright unresolved haze some 4’ in apparent diameter. It is a well-concentrated globular that’s rated IV on the I to XII concentration scale. A 150mm (six-inch) telescope, working a power of around 200x, intensifies the core’s appearance, though you’ll need the resolving power of at least a 200mm (eight-inch) ’scope to begin to pick out the cluster’s outlying stars. On a steady and transparent night, a 300mm (12-inch) telescope can resolve Messier 15 to its core.
Can you spot planetary Pease 1?
Messier 15 is one of a small, select group of globular clusters that can boast the presence of a planetary nebula among their teeming stars. Pease 1 was the first planetary to be detected within a globular cluster when, in 1927, American astronomer Francis Gladheim Pease spotted the nebula on a photographic plate taken with the 100-inch (~2.5-metre) Hooker Telescope at Mount Wilson Observatory in California, USA.
That object, today known as Pease 1 and catalogued additionally as PK 65-27.1, shines faintly at magnitude +14.9 and lies just 1.5’ north-east of Messier 15’s core. It shows up well in the best amateur images but is a tough visual challenge, owing partly to its diminutive 3” apparent diameter and its position close to Messier 15’s core, where it’s embedded deeply among many stars of similar brightness.
To have any chance of finding it, a magnification of around 300x will need to be brought to bear on a very transparent and still night. If stars are twinkling fiercely overhead, forget it! A 250–300mm (ten- to twelve-inch) telescope may be sufficient to track it down; once you’re sure that you’re looking in the right place, slip an O-III filter into the light train.