Sky Object of the Month – October 2017

Sky Object of the Month – October 2017

(Courtesy LVAS Observer’s Challenge*)

Messier 15 (NGC 7078) – Globular Cluster in Pegasus (Mag. 6.2; Diam. 18’)

Pease 1 – Planetary Nebula in M15 (Mag. 14.9[p]; Diam. 1”)

            As difficult as last month’s LVAS Challenge (NGC 6905) was to locate, this month’s target, the globular cluster Messier 15, is a breeze to find. It lies 4o NW of the 2nd magnitude star Enif (epsilon [ε] Pegasi) and, at magnitude 6.2, can be glimpsed with the unaided eye from dark-sky locations. It’s visible in binoculars as an out-of-focus star and in small-aperture scopes as a small roundish haze.                                          Telescopes in the 4 to 6-inch aperture range will resolve the outer portions of M15, but even much larger instruments will have difficulty resolving the core. That’s because Messier 15 is quite possibly the densest globular cluster in the Milky Way. Half of its estimated 200,000 stars are concentrated within a 10 light year radius from the core. The jury is still out on whether this high concentration is due to the gravitational pull of a massive centrally-located black hole or merely the cumulative gravitation of the stars themselves.

If you own a large-aperture scope, try your luck with the embedded planetary nebula Pease 1. In his book Cosmic Challenge, author Phil Harrington includes this planetary in a chapter devoted to “monster-scope” challenges. Discovered in 1928, it’s one of just four planetary nebulae inhabiting a globular cluster and the ‘easiest” to capture visually. Those fortunate enough to have notched this 15th magnitude object have used scopes typically with apertures of 14 inches and up, although Pease 1 has reportedly been sighted in 8-inch instruments. With a diameter of just 1 arc-second, Pease 1 mandates near-perfect seeing conditions and a magnifying power in excess of 300X. An accurate finder chart like the one found on the messier.seds website (www.messier.seds.org/more/m015_ps1fc.html) is a must, as is an OIII filter to help you confirm the sighting. As you flicker the OIII filter back and forth between eye and eyepiece, Pease 1 will retain its brightness while surrounding stars fade noticeably.

M15 was discovered by the Italian astronomer Jean-Dominique Maraldi on the night of September 7, 1746 during observations of Comet de Chéssaux and independently by Messier about 18 years later. It lies about 34,000 light years away and is some 175 light years in diameter. Spectroscopic analysis shows that Messier 15 is approaching us at a rate of 66 mi (107km)/sec.

Glenn Chaple for the LVAS

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www.universetoday.com, IAU, and Sky and Telescope                                 

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Mario Motta, MD

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M15 and Pease 1 (pinkish object near top left)   Hubble image

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