Messier 54

Discovered 1778 by Charles Messier.

Messier 54 (M54, NGC 6715) is a quite conspicuous globular cluster, although Charles Messier, who discovered it on July 24, 1778, describes it as "very faint" from his location in Paris (Kenneth Glyn Jones erroneously mis-translated Messier's description as "very bright nebula," and this error found its way to Kepple and Sanner's Night Sky Ovserver's Guide). William Herschel could resolve its outer regions in 15th and a few 14th magnitude stars. It is not easy to resolve, however.

As its concentration class III indicates, this cluster is comparatively concentrated. Its bright core is only 2.1' in diameter, with an intense nucleus of about 1', while the outlayers reach out to 6' on photos, or even to 9.1' at very long exposures. The second edition of Uranometria 2000.0 even gives a diameter of 12.0 arc minutes. The apparent magnitude of the brightest cluster stars is about 15.5, the horizontal branch level magnitude is 17.7. The overall spectral type was given by Helen Sawyer Hogg as F7 and the color index as +0.01. It is receding from us at about 142 km/s.

M54 has at least 82 known variables, the majority of 55 being of RR Lyrae type, but there are also two semi-regular red variables with periods of 77 and 101 days.

Its distance, for years, was estimated to be about 50-65,000 light years. However, in 1994, the exciting discovery was made that M54 was probably not a member of our Milky Way at all, but of a newly discovered dwarf galaxy ! This galaxy is now called SagDEG, for Sagittarius Dwarf Elliptical Galaxy, and one of the most recently discovered Local Group galaxies.

M54 coincides with one of two major concentrations of the SagDEG galaxy, and is receding from us at a very similar velocity (about 130 km/sec). This makes it probable that M54 is within this galaxy, which was estimated at a distance of 80-90,000 light years; a recent estimate for M54 was given at 87,400 light years. At this distance, M54 would be one of the most luminous known globular clusters with an absolute visual magnitude M_v of -10.01, a brilliance of about 850,000 suns like ours, and outshined only by spectacular Omega Centauri in our Milky Way. Also, its diameter would become as large as about 300 light years. It is about three times as distant as its two apparently close neighbors, M69 and M70. And perhaps most interesting, it would make M54 the first extragalactic globular cluster ever discovered, by Charles Messier on July 24, 1778, and thus add an extra first to Messier's list of fame.

M54 is easy to find as it is close to Zeta Sagittarii, the southernmost star of Sagittarius' "dipper" asterism of 4 or 5 stars (also called the "Milky Dipper", and part of the "Teapot"), namely 0.5 degrees south and 1.5 degrees west.

This globular cluster is bright but small so that it may be overlooked in smaller binoculars or finder scopes (i.e. taken for a star). Because of its large distance, this globular cluster is difficult to resolve. Binoculars and small telescopes show it as a round nebulous object gradually fading toward the edges. A 4-inch shows a mottled texture, under good conditions with some starlike knots, which John Mallas reported as suggestions of stars. It stays unresolved even in large amateur telescopes, which still show only mottled texture; Kenneth Glyn Jones describes it as looking more like a planetary nebula at first sight.

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