Messier 19

Globular Cluster M19 (NGC 6273), class VIII, in Ophiuchus

Discovered by Charles Messier in 1764.

Messier 19 (M19, NGC 6273) is the most oblate known globular cluster, being about ellipticity E3-E4. Shapley found it at ellipticity 6, corresponding E4, elongated at position angle 15 deg. He estimated that there could be counted twice as many stars along the major axis as along the minor. This deformation of the cluster from the globular shape may have to do with its proximity to the Galactic Center: While about 28,000 light years away from our Solar System, it is quite near to the Galactic Center, only about 5,200 light years away. It is located nine degrees above the galactic plane (i.e., at a galactic latitude of 9 deg North) and slightly west og the Galactic Center, as seen from Earth; it is perhaps very slightly more remote from us than the center of the Milky Way. M19 is receding from us at 146 km/sec.

M19 is fairly rich and dense, and considerably concentrated (of Shapley's class VIII). At its distance, its diameter of 17 arc minutes corresponds to a linear one of about 140 light years along the major axis, and its absolute magnitude is about -9 Mag. In amateur instruments it appears smaller, perhaps about 6' visually and 13.5' photographically - still corresponding to a linear diameter of 110 light years.

The brightest stars of M19 are about 14th magnitude, its horizontal branch level - the brightness of developed giant stars on the horizontal branch in the HRD - is near 15.3 (Deep Sky Filed Guide to Uranometria 2000.0). Helen Sawyer Hogg gives the average magnitude of its 25 brightest member stars as 14.8 mag and its overall spectral type as F5 (in Handbuch der Physik, according to Kenneth Glyn Jones). Only four RR Lyrae variable stars have been found in M19.

M19 was one of Charles Messier's original discoveries, detected on June 5, 1764. In 1784, William Herschel was the first to resolve it into "countless stars of mag 14, 15, 16" (John Herschel). In his more colorful language, Admiral Smyth saw M19 as 'a fine, insulated globular cluster of small and very compressed stars of creamy and white tinge and slightly lustrous to the center.'

M19 is easily found about 8 deg east of Antares in the Milky Way, and is visible as a small globular glow, with its ellipticity easily notable. It is quite easily resolved.

A further globular cluster, NGC 6293, of mag 8.4 and 1.9' diameter is located 1.5 deg to the ESE, and another one, NGC 6284, of mag 9.5 and 1.5' diameter, 1.6 deg to the NNE.

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