|Right Ascension||12 : 39.5 (h:m)
|Declination||-26 : 45 (deg:m)
|Visual Brightness||7.8 (mag)
|Apparent Dimension||12.0 (arc min)
Discovered 1780 by Pierre Méchain.
This 8-8.4 mag globular cluster lies at a distance of about 33,000 light years, and its members are spread over a volume of about 140 light years diameter. It has at least 42 known variables. Harlow Shapley had already found of which 28 so-called "cluster Variables" (Cepheids of type II, WW Virginis stars), one of which (No. 27) has later been shown to be not a cluster member. 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.
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 Pierre Méchain on April 9, 1780. Charles Messier mentions a 6th mag star in his description for M68, which is actually a 5.4-mag double star: ADS 8612, 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. 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.
Last Modification: 9 Dec 1999, 22:58 MET