Messier 103 (NGC 581, OCL 326)
Open Cluster in Cassiopeia
RA: 01h 33.4min
Dec: +60° 40min
Mag: 7.4vmag
Size: 6'
Moon is rising, so for everyone still out there I will present one of the more easy objects in the sky - the open cluster Messier 103.
Discovered by Pierre Méchain in 1781, Messier add the cluster in his catalogue in 1783 without observing the cluster by himself. An interesting fact is, that M 103 was the last entry in Messier's original list. The first detailed description came from William Herschel who spoke from about 14-16 "large" stars. Admiral William Smyth was the first who described the red giant star near the middle of the cluster.
Astrophysical there is nothing spectacular about M 103. With its distance of around 7160 Ly (WEBDA) it has the same distance than the famous double cluster h & ? Persei. It belongs to the Cas OB8 association, together with its neighbours NGC 654 or NGC 663. Modern studies listed 172 members. The brightest star HD 9311 (Struve 131) at the NW tip is a multiple system (A/B 13.8"). The second brightest "star" is a red giant near the middle. It is in discussion whether both stars belongs to the cluster. Only the brighter star HD 9365 at the SE tip does not belong to the cluster. With its young age of around 25 million years it mostly consists of several type B blue super giants of around 10-11mag.
Someone may ask if the OOTW of the deepskyforum.com is the right place for a bright open cluster. In my mind yes, because its is simply a beauty in nearly all optics, no matter if 1000mm is your aperture or your focal length. But what makes the cluster so observable nice. First of all its unusual shape of a triangle with a red star in the middle. With 6' diameter the triangle becomes visible in a 10x50 binocular, while the red colour of its giant in the middle starts to shine from around 8-inch, maybe smaller? But there is more. Alan MacRobert calls M 103 in his nice book "Star-Hopping for Backyard Astronomers" the "M 103 family of clusters" and he is right. With a field of 3° we can find several bright clusters, like NGC 654, NGC 659 or the larger NGC 663. Taking a telescope, Trumpler 1 is another highlight only 0,7° NE. More challenging is IC 166 NE of NGC 663. Star charts also shows a "PK" object only 23' W of M 103. Sadly it is no Planetary Nebula but an "Emission Line Star". I add two sketches below. Sketching open clusters is not always possible, but for M 103 the opposite is true. For nearly all apertures it is a funny project to sketch the object, to see as many stars as possible and to manage to show the shape which delight us.
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