Messier 23

Messier 23
Messier 23
	Open cluster Messier 23 in Sagittarius
Observation data (J2000 epoch)
Right ascension	17h 57m 04s
Declination	−18° 59′ 06″
Distance	2,050 ly (628 pc)
Apparent magnitude (V)	5.5
Apparent dimensions (V)	35′
Physical characteristics
Mass	1,206 M☉
Radius	8 ly
Estimated age	330±100 myr
Other designations	M23, NGC 6494, 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 constellation of Sagittarius.^[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]

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^ Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.

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[15] Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.

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