NGC 581

Messier 103 (NGC 581) is a compact, distant open star cluster in the constellation Cassiopeia, approximately 8,000 light-years from Earth — one of the more remote open clusters in Messier's catalog. It carries an interesting historical footnote: M103 was not actually observed by Messier himself but was added to the catalog from observing notes compiled by Pierre Méchain. Charles Messier included it as entry 103 in the catalog's 1784 edition, making it one of the last objects added. NGC 581 spans about 14 light-years across and is estimated to be somewhere around 20–25 million years old, a young cluster still populated by hot, luminous blue-white stars.

NGC 581 contains roughly 170 confirmed members. The brightest of these are hot B-type stars, the most massive members still on the main sequence at the cluster's young age. A bright visual binary star appears in the upper-right (northwest) portion of this image; it is not a cluster member but a foreground object that happens to lie along the same line of sight. At its distance of 8,000 light-years, M103 is so remote that it appears relatively small in the sky despite its actual physical size — what is visible fits within an image only 8.6 arcminutes across. Its richness and the scatter of stellar brightnesses among its members make it a more rewarding telescopic object than its modest apparent size might suggest.

In a small telescope M103 appears as a compact triangular or fan-shaped grouping of about a dozen brighter stars against a fainter stellar background. This true-color image was assembled from fifteen BVR exposures taken at the Kitt Peak National Observatory 2.1-meter telescope on the night of December 24, 2000.