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NGC 869
NGC 869 is an open cluster located 7600 light years〔 away in the constellation of Perseus. The cluster is most likely around 13〔 million years old. It is the westernmost of the Double Cluster with NGC 884. NGC 869 and 884 are often designated h and χ Persei, respectively.〔Some confusion surrounds what Bayer intended by these designations. It is sometimes claimed that Bayer could not have resolved the pair into two patches of nebulosity, and that Chi refers to the Double Cluster and h to a nearby star; see Stephen James O'Meara and Daniel W.E. Green, 2003, "The Mystery of the Double Cluster", ''Sky and Telescope'', Vol. 105, No. 2 (February 2003), p. 116–119. Bayer's ''Uranometria'' chart for Perseus does not show them as nebulous objects, but his chart for Cassiopeia does, and they are described as ''Nebulosa Duplex'' in Schiller's ''Coelum Stellatum Christianum'', which was assembled with Bayer's help; see Morton Wagman, ''Lost Stars'', McDonald & Woodward, 2003, ISBN 0939923785, p. 240.〕 Located in the Perseus OB1 association both clusters are located physically close to one another, only a few hundred light years apart. The clusters were first recorded by Hipparchus, but have likely been known since antiquity.
The Double Cluster is a favorite of amateur astronomers. These bright clusters are often photographed or observed with small telescopes. Easy to find, the clusters are visible with the unaided eye between the constellations of Perseus and Cassiopeia as a brighter patch in the winter Milky Way.
In small telescopes the cluster appears as a beautiful assemblage of bright stars located in a rich star field. Dominated by bright blue stars the cluster also hosts a few orange stars that add to the visual interest.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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