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M 54

Kroglasta kopica - Globular Cluster M54 (NGC 6715), class III, in sagittarius

Rektascenzija 18 : 55.1 (u:m)
Deklinacija -30 : 29 (sto:m)
Razdalja 82.8 (*1000 sv.l.)
Vizual. magnituda 7.6 (mag)
Zorni kot 9.1 (loc min)

M54 is a quite conspicuous cluster (also the author wonders that Messier found it "very bright" from his location in Paris). It 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" or "Teapot"), namely 0.5 degrees south and 1.5 degrees west. It is bright but small so that it may be overlooked in smaller binoculars (i.e. taken for a star), its bright core being only 2.1' in diameter, while the outlayers reach out to 6' on photos, or even to 9.1' at very long exposures. Herschel could resolve its outer regions in 15th and some 14th magnitude stars. It is not easy to resolve its core, however. M54 has at least 82 known variables, the majority 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 80,000 light years. At this distance, M54 would be one of the most luminous known globular clusters, outshined only by spectacular Omega Centauri in our Milky Way. Also, its diameter would become as large as about 200 light years. 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.

  • Vec posnetkov M54

    Povzeto po:
    Hartmut Frommert (spider@seds.org)
    Christine Kronberg (smil@lrz.uni-muenchen.de)

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