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.

Globular Star Cluster M15
Globular star cluster M15 is easily visible with only binoculars and lies about 33,000 light years away toward the constellation Pegasus. ESA/HST/NASA

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.