Messier 103: Open Cluster in Cassiopeia

M103 is a small, tight open cluster in the W of Cassiopeia, sitting just a degree from the bright star Ruchbah. It is one of the easiest Messier objects to locate.

What it is

An open cluster is a group of stars born together from the same cloud of gas, still loosely held by their mutual gravity. M103 is a young one, with hot blue-white stars as well as a few that have already swelled into red giants. Gaia measurements put it about 8,240 light-years away, and at that distance its 6′ apparent size corresponds to roughly 14 light-years across. It lies in a rich stretch of the Milky Way in Cassiopeia that holds many other young clusters, among them NGC 654, NGC 659 and NGC 663 a short way to the east, and M52 on the other side of the W.

When to see it

M103 is an autumn and early-winter object, highest around midnight in mid-October and high in the evening sky from October through January. From about 40°N it climbs to roughly 69° and never sets, so you can find it on any clear night of the year, though it is low in the north on spring evenings. From the Southern Hemisphere it is visible only from the tropics, very low in the north, and not from mid-southern latitudes.

How to find it

Find the W of Cassiopeia. Ruchbah (Delta Cassiopeiae) is the fourth of its five stars counting from the western end, where Caph sits. M103 lies about 1° northeast of Ruchbah, a little off the line from Ruchbah toward Segin (Epsilon Cassiopeiae), the star at the eastern end of the W. Ruchbah and the cluster fit in the same binocular field.

What you will see

In binoculars M103 is a small, hazy knot with one or two stars resolved, easy even from suburban skies. A 70–100 mm telescope at low to medium power resolves a few dozen stars in a compact fan or arrowhead shape, with a brighter star at one end and a reddish star among the members. A 200 mm or larger telescope shows many fainter members filling in the fan, though the cluster is small enough that you should use moderate magnification to frame it. No filter is needed.

History

Pierre Méchain discovered the cluster in 1781, and Messier added it as the last entry in the version of his catalog published that year. In the 20th century, M104 to M110 were appended from Messier's and Méchain's notes, so M103 remains the last object of Messier's own published list.