17h56m48s, −19° 01′00″| Messier23 | |
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Open cluster Messier 23 in Sagittarius
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| Observation data (J2000 epoch) | |
| Right ascension | 17h57m04s[1] |
| Declination | −18° 59′ 06″ |
| Distance | 2,050 ly (628 pc)[2] |
| Apparent magnitude (V) | 5.5[3] |
| Apparent dimensions (V) | 35′[2] |
| Physical characteristics | |
| Mass | 1,206[4] M☉ |
| Radius | 8 ly |
| Estimated age | 330±100 myr[5] |
| Other designations | M23, NGC 6494,[6] Cr 356, C 1753-190 |
| Associations | |
| Constellation | Sagittarius |
| See also: Open cluster, List of open clusters | |
Messier 23, also known as NGC 6494, is an open cluster of stars in the northwest of the southern constellationofSagittarius.[7] It was discovered by Charles Messier in 1764.[a][3] It can be found in good conditions with binoculars or a modestly sized telescope.[3] It is in front of "an extensive gas and dust network", which there may be no inter-association.[4] It is within 5° the sun's position (namely in mid-December) so can be occulted by the moon.[8]
The cluster is centered about 2,050 light years away. Estimates for the number of its members range from 169[9] up to 414, with a directly-counted mass of 1,206 M☉; 1,332 M☉ by application of the virial theorem.[4] The cluster is around 330 million years old with a near-solar metallicity of [Fe/H] = −0.04.[5] The brightest component (lucida) is of magnitude 9.3.[10] Five of the cluster members are candidate red giants,[11] while orange variable VV Sgr in the far south,[b] is a candidate asymptotic giant branch star.[12]
A 6th-magnitude star, shown in the top-right corner, figures in the far north-west as a foreground star – HD 163245 (HR 6679). Its parallax shift is 9.8912±0.0518 mas, having taken into account proper motion, which means it is about 101 parsecs (330 ly) away.[13]
Stars
Outer space|
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