Degree of difficulty 2 (of 5)

Minimum aperture 50mm

Designation NGC 4590

Type Globular cluster

Class X

Distance 36,580 ly (R2005) 35,610 ly (RR Lyr, 2001)

Size 120 ly

Constellation Hydra

R.A. 12h 39.5min

Decl. –26° 45′

Magnitude 7.6

Surface brightness –

Apparent diameter 11′

Discoverer Messier, 1780

History Charles Messier observed M 68 on the 9th of April 1780 as a “nebula without star, very faint, difficult to see with telescopes, nearby is a star of 6th magnitude.” In his book, Glyn Jones names Méchain as the discoverer, but there is no mention of this elsewhere. Hence, in agreement with most authors, we give the credit of discovery to Messier.

William Herschel described M 68 as “a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together.” His son John saw it “all clearly resolved into stars of 12th magnitude, very loose and ragged at the borders.”

Smyth, by contrast, was hampered by his smaller telescope and the haze of English midlands. He only saw a “large, round and very pale nebula.” His countryman and contemporary, Reverend Webb, however, described M 68 as “well resolved” and noticed a red star.

Astrophysics M 68 not only appears to be far away from the Milky Way as seen from our viewpoint, but it is also a typical cluster of the outer galactic halo. Its 500-million-year orbit has a considerable eccentricity of 0.5 and leads M 68 up to 100,000 light-years away from the galactic center.

Its present distance to Earth is 36,000 light-years, which yields a moderate physical diameter of 120 light-years for this cluster. Its class X means that M 68 has one of the lowest concentrations of all Messier globular clusters.

M 68 is over 10 thousand million years old and has a low metal abundance (see M 79). 42 variables are known in it, including 2 SX Phoenicis and 40 RR Lyrae stars.