NGC 7789, Open Star Cluster named "Caroline's Rose"
NGC 7789 is one of many open star clusters in the constellation "Cassiopeia" and belongs in a larger telescope surely to the most beautiful objects which
can be found in this celestial region. The object was discovered in October 1783 by Caroline Herschel and is therefore named "Caroline's Rose". With a total
brightness of mv=6.7mag the cluster is already visible as a hazy spot in binoculars. The distance is given with about 7,600 lightyears.
The brightest single stars reach an apparent magnitude of 10.6mag and belong to the group of "giant stars". Due to their spectral class K4 they shine in a
soft orange hue. Using the known distance the absolute brightness of these stars can be determined to Mv=-2.3mag - so they shine about 710x as bright as
our Sun.
The total number of stars in NGC 7789 is difficult to determine, but recent studies estimate an incredible number of 16,000 stars. Most of them are much
fainter than 14mag, but many giant stars are brighter than 13mag and therefore visible even in a smaller telescope. All stars together give the cluster a total
mass of 7,000 solar masses - therefore it is one of the most massive examples of their class. Because the stars are gravitationally strongly bound to the cluster,
NGC 7789 is quite insensitive to external disturbances. Despite its old age of 1.6 billion years it hardly shows any signs of dissolution and will surely survive
many more orbits around the Milky Way center.
Finally, we want to talk briefly about the physical dimensions. The total diameter of NGC 7789 is given with 65 light years. In the telescope, the apparent
diameter can be determined to be about 12' (minutes of arc), which corresponds to a distance of 27 light-years at a distance of 7,600 light-years. Thus, in visual
observation, we only really perceive the innermost region of NGC 7789 as a star cluster, although the cluster extends about twice as far into space.
