Globular clusters are huge symmetrical systems containing up to a million stars, which lie round the edge of our Milky Way Galaxy. Of the 150 globular star clusters confirmed to date (with perhaps 10 to 20 more still undiscovered), a high proportion are visible in the summer sky, towards the Sagittarius region of the galactic core. However, autumn evenings offer a bright globular east of the Milky Way, well placed for viewing from both the Northern and Southern Hemispheres.
M15 is easily found about 4° northwest of Enif (Epsilon Pegasi), the star at the tip of Pegasus’ nose. It was discovered by French-Italian astronomer Giacomo Filippo Maraldi in 1746 and rediscovered by Messier in 1764. Shining at magnitude +6.2, M15 is generally considered to rank among the dozen finest objects of its type in the northern sky.
From a dark site, keen-eyed observers can spot M15 with their naked eyes. Most binoculars reveal it as a nebulous patch with no visible stars embedded within, although some stellar resolution is barely discernible in larger glasses at 20x if the observer uses averted vision. A 4-inch scope will resolve dozens of stars around M15’s strikingly bright core, with star chains winding out from the central region.
The cluster has an unusual resident, a 14th-magnitude planetary nebula, Pease 1, on its northeast side. In fact, M15 is one of the two globulars known to contain a planetary nebula; the other is GJJC1, only 3 arcseconds across, located in M22. Look for Pease 1 with a 12-inch or larger telescope, at a magnification of at least 200x. Make sure to use a nebula filter and be aware of the sky conditions; you will need good seeing to spot the tiny planetary among the myriad stars near it.
M15 has a diameter of 120 light years and revolves once around the Galaxy every quarter billion years or so in a prograde orbit, meaning it moves about the Galaxy in the same direction as the Galaxy’s own rotation. In 1974, M15 was discovered to be a source of x-ray energy, which may suggest that one or more supernova remnants are buried deep within the cluster.
Finder map – field width 15°, stars to magnitude +8.5.