M69 was discovered by Abbe Nicholas Louis de la Caille, and he included it in his southern catalogue, which was numbered Lacaille I.11.In 1764, Charles Messier failed to find this southern star cluster for the first time, but in 1780, he found it with a better telescope and recorded it on August 31, 1780.
Physical characteristics
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M69Spectral typeIf it is judged as G2 or G3, itsColor indexB-V = 1.01。It is one of the globular clusters with the highest metal abundance, indicating that the content of elements heavier than helium in its stars is relatively high.However, this value is significantly lower than that of a younger star like our sun (family I), indicating that even this globular cluster was formed in the early universe.At that time, the universe contained fewer heavy elements, because these heavy elements had to be formed in stars.The distance of M69 is almost the same as that of its apparent nearest neighbor, M70. The former is about 29700 light years, while the latter is 29400 light years.This indicates that these two globular clusters are just physical neighbors;The distance between them is calculated to be only about 1800 light years.On the contrary, the distance of another nearby globular cluster M54 in the line of sight direction is almost three times of this distance.There are not many variable stars in M69: Shapley has not found even one. At present, the number of known variable stars is only 8, of which 2 are the constraint type variable stars with a period of about 200 days.
M69，Like its neighbor M70, it is one of the smaller and darker globular clusters in the Messier catalog.In the dark sky, 7x50 or 10x50 binoculars can just see it, if the observation site is not very far north.However, in the observation site of Messier - Paris, this is a more difficult object to observe.
In the long exposure photos, the diameter of M69 is only 7.1 minutes, and the corresponding size is about 61 light-years at the distance of 29700 light-years.Ultra long exposure photos show a larger diameter: 9.8 cm, corresponding to about 85 light-years.The visible bright compact nucleus is less than half the size, only about 3 '.Because M69 is quite close to the center of the Milky Way (only 6200 light years away), its tidal gravitational radius is relatively small, 8.25 'or 72 light years.Its concentration level is V, indicating that its star convergence is close to the average level of globular clusters.Its central core has a diameter of 0.68 ', or about 6 light years, and its half mass radius is 0.83', or about 7.2 light years.
