Messier 69

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)

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 1780 by Charles Messier.

Globular cluster Messier 69 (M69, NGC 6637), similar to 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 Charles Messier and added to his catalog on August 31, 1780, the same night he found M70. The discovery occurred when Messier was looking for a nebulous object cataloged by Lacaille in 1751-52 as Lac I.11; he had already looked for that object in vain in 1764. Messier thought he had recovered Lacaille's object and identified M69 with Lac I.11 (NGC 6634), but this is probably a misidentification, as Glen Cozens of Australia has pointed out:

M69 is considerably fainter than any other Lacaille object and could probably not be seen with Lacaille's 0.5-inch aperture telescope.
Lacaille's position deviates from M69 by an untypically large about 1.2 degrees, which cannot be easily assigned to a copy or grid error.
There is a grouping of three stars of mag 8.3, 7.8, 8.7 well at Lacaille's position, which might have looked like a nebula in Lacaille's modest instrument.

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.

Historical Observations and Descriptions of M69

More images of M69

Amateur images of M69

Marco Castellani's data for M 69

Christine Clement's Catalog of Variable Stars in M69

Holger Baumgardt's Fundamental Data for M69 (NGC 6637)

SIMBAD Data of M69

NED Data of M69

Publications on M69 (NASA ADS)

Observing Reports for M69 (IAAC Netastrocatalog)

NGC Online data for M69

Hartmut Frommert
Christine Kronberg
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