NGC 1817

NGC 1817 is an open cluster of stars in the constellation Taurus. It was discovered by English astronomer William Herschel in February 1784.^[5] With an apparent magnitude of 7.7^[3] and spanning 9.3 arc minutes across the sky, it is separated from the NGC 1807 cluster by just 26 arc minutes.^{[6][circular reference]} Indeed, the two may actually be parts of a single extended cluster.^[5]

NGC 1817
NGC 1817 as seen by the DSS
Observation data (J2000 epoch)
Right ascension	05h 12m 15.(0)s[1]
Declination	+16° 41′ 2(4)″[1]
Distance	6,430 ly (1,972 pc)[2]
Apparent magnitude (V)	7.7[3]
Apparent dimensions (V)	9.3′[3]
Physical characteristics
Estimated age	0.8−1.2[4] Gyr
Other designations	C 0509+166, Collinder 60
Associations
Constellation	Taurus
See also: Open cluster, List of open clusters

The NGC 1817 cluster is around the same age as the Hyades, or perhaps a little younger at 0.8−1.2 billion years. The turnoff point for this cluster—where stars above a certain mass are evolving through the red giant stage—is twice the mass of the Sun.^[7] The cluster is situated in the opposite side of the sky from the Galactic Center at a separation of 32 kly (9.9 kpc) from the core,^[7] and is around 1.3 kly (0.4 kpc) away from the Galactic plane.^[4]

Measurements of the proper motion of 810 stars within a 1.5° region centered on the cluster suggest that it has at least 169 members.^[8] Of these, there is a total of 26 variable stars, including three candidate Gamma Doradus variable stars and sixteen Delta Scuti variables.^[9] This unusually high proportion of Delta Scuti variables is likely the result of the turnoff point being located within the instability strip.^[5]

Gallery

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  Open clusters NGC 1807 and 1817 showing separation of 26'