This relatively bright deep-sky object is well-placed for viewing now.
Globular cluster M15 lies some 36,000 light-years away, but glows well enough to make it a must-see object through your telescope.
Credit: Don Goldman
- M15, a magnitude 6.2 globular cluster within the constellation Pegasus, possesses a diameter of 12.3′ and is discernible to the naked eye from dark observing sites.
- Located by extending a line 4° from Theta through Epsilon Pegasi, M15 exhibits resolved stars and star chains through a 4-inch telescope, leading to descriptions of an oval morphology.
- Advanced observation with an 11-inch or larger telescope enables the detection of Pease 1, the inaugural planetary nebula identified within a globular cluster, positioned approximately 1′ northeast of M15’s core.
- Successful observation of Pease 1 requires magnification of 200x or greater, the application of a nebula filter, and favorable atmospheric seeing conditions to discern the object from the surrounding stellar population.
If you live in the Northern Hemisphere and are looking for a great object to observe on a fall night, M15 is a showpiece globular cluster that resides within the boundaries of the constellation Pegasus the Winged Horse. It’s fairly sizeable, having a diameter of 12.3′.
From a dark observing site, sharp-eyed observers can spot this magnitude 6.2 cluster with their naked eyes. Don’t be confused by the magnitude 6.1 star only 17′ to the east. Confirm your sighting through binoculars or a telescope.
And speaking of telescopes, a 4-inch scope will resolve dozens of stars around M15’s strikingly bright core. Look for the chains of stars that wind out from its central region. Through a small telescope, these star patterns cause some observers to describe M15 as slightly oval.
Finding M15 is pretty easy. Use Theta and Epsilon Pegasi as pointers. Just draw a line from Theta through Epsilon, and continue another 4°. By the way, 4° is the width of two fingers held at arm’s length.
Look deeper
With an 11-inch or larger telescope, try for the challenge object inside M15 — Pease 1, the first planetary nebula found within a globular cluster. In 1928, American astronomer Francis Gladheim Pease discovered this object when he noticed an unusually bright ‘‘star’’ on a photographic plate taken with the 100-inch Hooker Telescope on Mount Wilson.
Look for this planetary nebula through an eyepiece that will give you a magnification of 200x or more. Use a nebula filter. The filter will suppress the brightness of the myriad stars surrounding Pease 1. Be aware of the sky conditions. You’ll need good seeing to spot the tiny planetary, which lies about 1′ northeast of M15’s core. If your seeing is bad, try for it on another night. But, hey, even in mediocre seeing, M15 will delight you.
