Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 581

Type Open cluster

Class II2m

Distance 7150 ly (K2005) 9790 ly (CMD, 1999) 5220 ly (CMD, 1998)

Size 17 ly

Constellation Cassiopeia

R.A. 1h 33.4min

Decl. +60° 40′

Magnitude 7.4

Surface brightness –

Apparent diameter 6′

Discoverer Méchain, 1781

History M 103 is the last entry of Messier's original list. It made it into a last-minute annex with M 101 and M 102, after its discovery by Pierre Méchain on the 27th of March 1781, when the catalog of originally 100 nebulous objects was already set up for printing. Méchain recognized it correctly as a “Cluster of stars between ε and δ of the leg of Cassiopeia.”

50 years later, Admiral Smyth described M 103 as a “fan-shaped group, diverging from a sharp star in the north following [northeast] quadrant. The cluster is brilliant from the splash of a score of its largest members, the four principle ones of which are from the 7th to the 9th magnitude; and under the largest, in the southeast, is a red star of the 8th magnitude.” A similar view, expressed in fewer words, had d'Arrest: “A pretty, reddish star stands out, of a rosé tint. An uneven cluster, consisting of stars of 9th, 10th, and 11th magnitude.” Leo Brenner observed M 103 with a 7-inch telescope and noticed its double star: “pretty with magnification 93×, triangular, with pretty double star magnitudes 6 and 10, 13″ to 14″ separation, furthermore a beautiful red star of 8th magnitude, otherwise stars of 9th to 10th magnitudes.”

Astrophysics With a likely distance of 7200 light-years, M 103 is about as far away as the famous double cluster h and χ in the Perseus arm of the Milky Way, and it is the farthest open cluster in the Messier catalog.