M 69
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M 69

February 19, 2005
Globular Cluster M69 (NGC 6637), class V, in Sagittarius

Right Ascension: 18 : 31.4 (h:m)
Declination: -32 : 21 (deg:m)
Distance: 29.7 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 9.8 (arc min)

Discovered 1751-52 by Abbe Nicholas Louis de la Caille.

M69, similar as its neighbor M70, is one of the smaller and fainter globular clusters in Messier's catalog. It can just be seen in a dark night with a 7x50 or 10x50 pair of binoculars, if the observing location is not too much north. However from Paris, Messier's observing place, it is a difficult object.

M69 was discovered by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects as Lacaille I.11. Charles Messier missed this southern cluster when he first looked for it in 1764, but found it with the better scope he had in 1780, and cataloged it on August 31, 1780.

M69 is only 7.1 arc minutes in diameter in long exposure photos, corresponding to roughly 61 light years at its 29,700 light years distance. Very deep photos show that it is somewhat more extended: 9.8 arc minutes, or linearly, about 85 light years. The visually bright compact core is less than half, only about 3'. As M69 is quite close to the Galactic Center (only about 6,200 light-years distant), its tidal gravitational radius is comparatively small, 8.35' or 72 light years. Its stellar concentration is about average for a globular cluster, as it is of concentration class V. Its central core has a diameter of 0.68' or roughly 6 light years, while its half-mass radius is 0.83' or about 7.2 light years.

M69's spectral type has been determined as G2 or G3, and its color index is B-V = 1.01. It is one of the metal-richest globulars, meaning that its stars show a relatively high abundance of elements heavier than Helium. Nevertheless, this value is still significantly lower than that for the younger (Population I) stars like our Sun, indicating that even this globular was formed at early cosmic times when the universe contained less heavier elements, as these elements still had to be formed in the stars.

The distance of M69, about 29,700 light years, is roughly the same as that of its apparent neighbor, M70 which is at about 29,400 light years. This indicates that these two globulars happen to be physically neighbored; their mutual distance can be calculated to be as small as about 1,800 light years. In contrast, the also apparently nearby situated globular M54 is about three times as distant.

M69 is poor in variable stars: Shapley found not a single one at all, and the number of known variables is now still as low as 8, 2 of them being Mira-type variable stars with periods of about 200 days.

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