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M 69

Kroglasta kopica - Globular Cluster M69 (NGC 6637), class V, in sagittarius

Rektascenzija 18 : 31.4 (u:m)
Deklinacija -32 : 21 (sto:m)
Razdalja 26.7 (*1000 sv.l.)
Vizual. magnituda 7.6 (mag)
Zorni kot 7.1 (loc min)

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. Messier missed this southern cluster when he first looked for it in 1764, but found it with the better scope he had in 1780.

M69 is only 7.1 arc minutes in diameter in long exposure photos, corresponding to roughly 55 light years at its 27,000 light years distance. The visually bright compact core is less than half, only about 3'. Its stellar concentration is about average for a globular cluster, as it is of concentration class V. 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.

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.

Povzeto po:
Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

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