NGC 654, Open Star Cluster
NGC 654 is a wonderful star cluster in the constellation Cassiopeia, located midway between the two stars "epsilon Cas" and "delta Cas". Its distance
is given as 7,200 light-years. From the apparent angular dimension of 6' a true diameter of 12 lightyears can be calculated. A little more than 80 stars
belong to NGC 654, whose age is given as 14 million years. The reddening is E(B-V) = 0.86, which leads to a dimming of the stellar brightness
by 2 magnitudes. Responsible for this are two dust clouds at 700 and 3,100 light-years distance. Towards the center the dust seems to be less dense.
Only 2.5' southeast of NGC 654 lies the star "HD 10494" with a brightness of 7.3mag. It is by far the brightest star in the near vicinity. Both proper
motion and radial velocity suggest that it belongs to NGC 654. From its apparent magnitude and distance the absolute magnitude can be determined
to -4.4mag and the luminosity to 5,000 Suns. HD 10494 therefore belongs to the group of luminous giant stars. If it would be as far away as Sirius,
it would be shining brilliantly in the sky with an apparent brightness of -7.3mag.
Another star with the designation "HIP 8074" is also worth mentioning. It is located 3.5' southwest of the cluster with a magnitude of 9.5mag and is
surrounded by a very faint reflection nebula called "VdB 6". The star itself is an A0 supergiant.
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The star cluster NGC 654 is quickly found. Since the cluster is quite small, compact and starry, it can be magnified very well. The most beautiful view in
my 20" f/3 Dobsonian is at 270x. Most striking is of course the very conspicuous F0 supergiant, shining dazzling bright and yellowish in the eyepiece. The
cluster itself seems to be completely resolved - no grainy or diffuse background is visible. In total about 50-60 individual stars are visible, which have quite
different magnitudes. The star density increases towards the center. The overall shape is roundish to slightly oval, but the outer contours appear irregular.
The stars are arranged in several stellar nodes, in between there are star-poor areas.
The reflection nebula "VdB 6" is not easy to see and requires indirect vision even with 50cm aperture. Then it appears as a very faint, dull nebula cloud of low
extension without structure. If you didn't know it was there, you would probably miss it.
