NGC 6256 | |
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Observation data ( J2000 epoch) | |
Constellation | Scorpius |
Right ascension | 16h 59m 32.68s [1] |
Declination | −37° 07′ 17.1″ [1] |
Distance | 22 kly (6.8 kpc) [2] |
Apparent magnitude (V) | 11.3 [3] |
Apparent dimensions (V) | 7′ [4] |
Physical characteristics | |
Absolute magnitude | −7.15[ citation needed] |
Estimated age | 13.0±0.5 Gyr [2] |
Other designations | NGC 6256, GCL 49.1, ESO 391-SC6 and vdB-Hagen 208 [5] |
NGC 6256 is a globular cluster of stars in the southern constellation of Scorpius. It was discovered by the Scottish astronomer James Dunlop on Aug 2, 1826. [6] In J. L. E. Dreyer's New General Catalogue annotation it is described as, "very faint, very large, very gradually bright in the middle, well resolved clearly consisting of stars." [7] The cluster is located at a distance of 22 thousand light-years (6.8 kpc) from the Sun. [2]
This is an ancient cluster with an estimated age of about 13 billion years; it was formed during the very early stages of assembly of the Milky Way galaxy. The cluster is orbiting within the galactic bulge with a low orbital eccentricity. It is heavily reddened by extinction due to interstellar gas and dust. [2] The structure is very concentrated at the center, showing a post core collapse morphology. [8]
The HR diagram for this cluster displays two sequences of blue straggler stars, which are the products of stellar mergers. The bluer sequence is well-defined and narrow, most likely being generated over a short time span. The redder sequence is more sparse, being the result of a continuous process of formation. It is hypothesized that the bluer sequence was formed around the time of the cluster's core collapse about one billion years ago; an event that made stellar collisions more likely. [9]
The cluster contains a millisecond X-ray pulsar, designated IGR J16597-3704. [10]