Messier 103, Open Star Cluster
The open star cluster "Messier 103" is located in the middle of the constellation Cassopeia at a distance of 8,500 light years, whereas latest data from GAIA
indicate a distance of 9,500 light years. The cluster was discovered in 1781 by P. Méchain, a friend of Messier. With an apparent brightness of 7.4mag and
an size of 6 arcminutes Messier 103 is already visible as a nebulous spot in small binoculars.
Altogether about 170 stars are attributed to the star cluster with a true diameter of 15 light-years. In the center for example there is a red giant (BD+59° 274)
with a brightness of 8.5mag, which corresponds to a luminosity of 2,800 suns and an absolute magnitude of -3.8mag. The temperature at its surface is a cool
4,300 Kelvin - therefore it belongs to spectral type K7.
The brightest star "HD 9311" at the upper right corner of the cluster was not counted to Messier 103 so far, but considered as a foreground object. But recent
data from GAIA raise reasonable doubts, because the parallax of 0.344mas puts the star in the same distance as the other stars of Messier 103. Second, its
proper motion also shows the same values for amount and direction as the star cluster. So there is a lot of evidence that HD 9311 is indeed a member of the
cluster. The apparent magnitude of 7.3mag corresponds to an absolute magnitude of -5.0mag at a distance of nearly 9,500 light-years. Seen from a hypothetical
planet in Messier 103, HD 9311 would reach a brightness of -10.1mag and would therefore be visible even in the daytime sky. With a surface temperature of
about 20,000 Kelvin it is very hot and belongs to spectral class B5.
The age of Messier 103 was determined to 25 million years, so the cluster is still quite young.
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Messier 103 is a beautiful sight in a 20 inch Dobsonian. Since it is not so big, you can use high magnifications very well without losing the cluster character.
The first thing you notice in the eyepiece is an orange glowing star in the center of the cluster. And indeed this is a so called "red giant" - a star with a very cool
surface of 4,300 Kelvin and a diameter that can be many hundred times the diameter of the sun. Also very unusual is the shape of the cluster, which resembles
a triangle. At high magnifications of 300x and more many faint stars flash out of the dark sky background - in total I could count about 50-60 single stars at 370x.
The shape and the red giant star in the center make Messier 103 a very exciting object.
