M103
NGC 581
Properties
Position & Identifiers
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About M103
Description
M103 is a small, bright open cluster in Cassiopeia, about 10,000 light-years from Earth. It contains about 40 stars spread across 15 light-years, with an age of about 25 million years. The cluster forms a distinctive triangle pattern, with a prominent red giant star (Struve 131) near one vertex providing a striking color contrast.
Observing Tips
Located about 1 degree northeast of Delta Cassiopeiae (Ruchbah). Easy to find in the rich Cassiopeia Milky Way. In binoculars it appears as a small, dense patch. A telescope at 50-80x reveals the attractive triangular pattern of about 25 stars. The red giant star at one corner of the triangle creates a beautiful color contrast. Several other open clusters lie nearby. Best observed from September through February.
History
Discovered by Pierre Mechain in March 1781. It was the last object in Messier's original 1781 catalog (M104-M110 were added later by other astronomers). Messier described it as 'a cluster of stars.'
Fun Facts
M103 was the last object in Charles Messier's personally published catalog — objects M104 through M110 were added posthumously by other astronomers based on Messier's and Mechain's notes. The red giant Struve 131 near the cluster is actually a foreground star, not a true cluster member, but it greatly enhances the visual appeal.
Community Photos (1)
Credit: Jim Mazur. License: CC BY-SA 4.0. (Wikimedia Commons)
Skybred Feb 28, 2026