Messier 68, NGC 4590

Messier 68 (NGC 4590) is a globular cluster in the constellation Hydra.

This globular was discovered by Charles Messier in 1780. Admiral Smyth erroneously assigned its discovery to Pierre Mechain, but it was correctly assigned in Dreyer's NGC. Messier found M 68 unresolvable, but William Herschel first resolved into stars in 1786.

This magnitude 7.8 globular cluster is quite difficult for Northern observers to see because of its southern declination. Messier mentioned a 6th-magnitude star in his description for M 68; this is actually the double star ADS 8612 (magnitude 5.4 and 12.2, separation 1.6"). M 68 is found about 45' SW of this star. Its brightest star is of magnitude 12.6.

Appearing as faint patch in binoculars, the brightest stars of M 68 are resolved by telescopes starting from 4" aperture under good conditions. These instruments show a mottled, round nebulous patch with a bright center, gradually fading to its edges. A good 8-inch telescope can resolve many stars in the cluster; its bright 3' diameter core is poorly resolved at 125x but rich at 175x. The core is embedded in a 11' diameter halo of faint stars scattered in streamers.

M 68 lies at a distance of about 33,000 light years, and its members are spread over a volume about 106 light years in diameter. M 68 is a rich cluster, with at least 100,000 stars. It contains 42 known variables, of which 28 are RR Lyrae stars. M 68 is approaching us at 112 km/sec.