Messier 68

Right Ascension	12 : 39.5 (h:m)
Declination	-26 : 45 (deg:m)
Distance	33.3 (kly)
Visual Brightness	7.8 (mag)
Apparent Dimension	11.0 (arc min)

Discovered 1780 by Charles Messier.

Messier 68 (M68, NGC 4590) is a beautiful globular cluster situated in an unusual place for such objects, in the hemisphere opposite to the Galactic Center.

This 7.8th magnitude globular cluster lies at a distance of about 33,000 light years, and its members are spread over a volume of about 106 light years diameter. It has at least 42 known variables. Harlow Shapley had already found of which 28 so-called "cluster Variables" (RR Lyrae stars), one of which (No. 27) has later been shown to be not a cluster member (Greenstein, Bidelman and Popper, 1947). Shapley also gave the ellipticity of this globular as 9 in 1930, while in 1949, he described it as round when accounting for its 2000 brightest stars. In amateur telescopes it actually appears round, although some observers (including John Mallas) perceived it as oval.

Former catalogs systematically give fainter visual magnitudes, probably because this southern cluster was estimated from northern observers: Helen Sawyer Hogg lists it at 9.12 mag, Mallas/Kreimer at 8th mag, Becvar, Kenneth Glyn Jones and the Sky Catalogue 2000.0 at mag 8.2. The newer Deep Sky Field Guide to Uranometria 2000.0 gives mag 7.7, and in its second edition, a total apparent visual brightness of mag 7.3.

According to Kenneth Glyn Jones, M68 contains about 250 giant stars of absolute mag greater than zero, about half as much as M3 or M13. Its brightest star is of magnitude 12.6, while the horizontal branch level of this cluster is at mag 15.6, according to the Deep Sky Field Guide to Uranometria 2000.0. Helen Sawyer Hogg has found 25 stars being brighter than mag 14.8, and lists its overall spectral type as A6.

Past distance measurements for M68 have varied: Shapley's early determination had been 50,000 light years (15.5 kpc), while Becvar gives 37,500 ly (11.5 kpc), T.D. Kinman's average is 39,000 ly (12.0kpc), and McCluere et.al (1937) obtained 36,000 ly (11.2 kpc). Our modern value of 33,300 ly is from William E. Harris' Galactic Globular Clusters Database.

M68 is approaching us at 112 km/sec.

The nearby mark in the lower right shows the non-member Mira-type variable FI Hydrae, which has a period of about 324 days and can become as bright as 9th magnitude, and thus the appearence of the field varies considerably.

M68 was discovered by Charles Messier on April 9, 1780. Because of some dubious error, Admiral Smyth has assigned this discovery to Pierre Méchain, and in the 1960s, Kenneth Glyn Jones adopted this view, despite the fact that this is not acknowledged by Messier in his Catalog description, as he did for all of Méchain's true discoveries. The discovery is correctly assigned to Messier e.g. by Dreyer's NGC, Helen B. Sawyer [Hogg] (1947) and Burnham. As most of Messier's globular clusters, it was first resolved into stars by William Herschel, in 1786.

Messier mentions a 6th mag star in his description for M68, which is actually a 5.4-mag double star: ADS 8612 (also cataloged as B320), A: 5.4 mag, B: 12.2 mag at PA 152 deg and separation 1.6" (in 1926).

M68 is quite difficult to observe for Northern observers because of its southern declination. They may best find it by following a line from the stars Delta to Beta Corvi (mag 3), which points toward 5.4-mag ADS 8612 mentioned above. M68 is then easily located about 45' NE of this star.

A faint patch in binoculars, the brightest stars of M68 are resolved by telescopes starting from 4-inch aperture under good conditions; these instruments show a mottled round nebulous patch with a bright center, gradually fading to its edges. A 6-inch resolves the outer parts of this cluster, a halo of 12' diameter. Larger telescopes show its nature as a rich cluster well to the core.

NGC Online data for M68

References

• J.L. Greenstein, W.P. Bidelman, and D.M. Popper, 1947. Variable 27 in the globular cluster Messier 68. Publications of the Astronomical Society of the Pacific, Vol. 59, p. 143 [ADS: 1947PASP...59..143G].
• Helen B. Sawyer [Hogg], 1947. A Bibliography of Individual Globular Clusters. Publications of the David Dunlop Observatory, University of Toronto, Vol. 1, No. 20, pp. 381-469 [ADS: 1947PDDO....1..383S]. Here p. 392 and 401.

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Hartmut Frommert
Christine Kronberg
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Last Modification: August 30, 2007