Anyone who has attended a “star party”, where experienced observers conduct guided tours of the wonders of the night sky, has seen at least a few star clusters. Several are easily visible with the unaided eye, notably the Pleiades and the Hyades in Taurus, Praesepe in Cancer, and the lovely Jewel Box in the Southern Cross; many are within the range of binoculars or small telescopes.
Clusters were born together out of enormous clouds of gas, and the stars of a cluster are all the same age. They lie in and near the arms of our Milky Way Galaxy and many are quite young (some are forming still). More than 1000 star clusters are known, and they may contain anything from a few dozen to a few hundred stars. Each has its own characteristics, some have unusual appearances (like the Pleiades, which resemble a very little dipper), and no two look entirely alike.
Many clusters are named by their number in the New General Catalog (NGC), which was compiled between 1864 and 1908. Our highlight deep sky object this month is a little-known member of this catalog – NGC 2169 in Orion.
NGC 2169, nicknamed “The 37 Cluster” due to its striking resemblance to the number 37, is located about 3,600 light years away from us. As far as open star clusters go, NGC 2169 is a small one, spanning only 7 light years across. Its stars are about 8 million years old and are expected to disperse over time as they encounter other stars, interstellar clouds, and experience gravitational tides.
NGC 2169 contains 11th-magnitude GSC 00742–02169, a chemically peculiar variable “Ap” star. Its variations (of less than 0.1 magnitudes) are caused by misalignment between the magnetic field poles and the rotation axis, which is a common phenomenon with Ap stars.
Finder map – field width 20°, stars to magnitude +7.