M103 - NGC 581

Messier 103 (NGC 581) is an open star cluster located approximately 10,000 light-years from Earth in the constellation Cassiopeia. It was discovered by Pierre Mechain in 1781 and was the last object that Charles Messier personally added to his original catalog (M104 through M110 were added later by other astronomers). M103 is a relatively young open cluster with an estimated age of about 25 million years. It contains approximately 40 confirmed member stars spread across a diameter of about 15 light-years, though the cluster appears superimposed on a rich background of Milky Way stars that can make membership determination challenging. The brightest member of the cluster is a red giant star of about 7th magnitude, named Struve 131, which provides a pleasing color contrast against the predominantly blue-white younger stars in the cluster. This red giant is nearing the end of its stellar life and has expanded and cooled considerably from its original hot blue state. M103 has a visual magnitude of about 7.4 and spans approximately 6 arcminutes on the sky. The cluster displays a distinctive triangular or fan-shaped pattern that makes it easy to recognize. M103 lies about 1 degree northeast of the star Ruchbah (Delta Cassiopeiae), making it straightforward to locate via star-hopping. In binoculars, the cluster appears as a small, hazy patch with a few resolved stars. A 4-inch telescope provides an attractive view of the cluster's triangular pattern with a dozen or so stars resolved, while larger instruments reveal fainter members. M103 lies in a rich area of Cassiopeia near several other open clusters, making the region rewarding for extended exploration.

M103 contains roughly 40 member stars within a diameter of about 15 light-years, located approximately 10,000 light-years from Earth.