NGC 2204

NGC 2204 is an open cluster of stars in the Canis Major constellation. It was discovered by the German-English astronomer William Herschel on 6 February 1785.^[5] The cluster has an integrated visual magnitude of 8.6 and spans a diameter of 10.0′. Resolving the individual member stars is a challenge with a 10 to 12-inch amateur telescope.^[3] It is located at a distance of approximately 13,400 light years from the Sun.^[2] The cluster shows a mean radial velocity of +91.38±0.30 km/s relative to the Sun,^[6] and is orbiting the inner galactic disk region about 1 kpc below the galactic plane.^[2]

NGC 2204
NGC 2204, Digitized Sky Survey
Observation data (J2000.0 epoch)
Right ascension	06h 15m 31.7s[1]
Declination	−18° 40′ 12″[1]
Distance	13.4 ± 1.3 kly (4.1 ± 0.4 kpc)[2]
Apparent magnitude (V)	8.6[3]
Apparent dimensions (V)	10.0′[3]
Physical characteristics
Radius	16.25ly
Estimated age	2.0±0.3 Gyr[2]
Other designations	Collinder 88, Melotte 44[4]
Associations
Constellation	Canis Major
See also: Open cluster, List of open clusters

This is a rich but diffuse cluster with a Trumpler class of III 3m,^[7][8] spanning a physical diameter of about 55 ly (17 pc).^[9] It is an older cluster with an estimated age of 2.0±0.3 billion years.^[10] The metallicity is correspondingly poor,^[11][8] showing an abundance of iron about 59% of that in the Sun.^[2] There is a prominent giant branch clump on the HR diagram.^[7] The cluster has a significant population of blue stragglers,^[12][11] an indicator of past stellar mergers. It has a pair of candidate chemically peculiar stars,^[8] and five variable stars have been discovered, including four eclipsing variables.^[9]