The dark evenings of November provide a great opportunity to view the Milky Way as it flows through the northern constellations of Cassiopeia and Perseus. There are so many star clusters to see! As a young astronomer 30 years ago, I remember looking with binoculars from my bedroom window and discovering a nice semi-circle of stars below a brighter star. The view is shown in the large red circle below.
This scattered cluster of stars was not listed in any of the deepsky object catalogues that I knew about (Messier, New General Catalogue) and it was many years until I discovered that this object had a name. The bright star at the focus of the semicircle is called Mirfak (or alpha Perseii) and the cluster of stars is called the “alpha Persei Moving Cluster“. It is also known as Cr39; it is the 39th entry in a catalogue compiled by the Swedish astronomer Per Collinder in 1931. It also goes by the name Melotte 20.
So what is it? Well “cluster” is too strong a word. Astronomers call this an “association” instead. The stars are more loosely gravitationally bound than a cluster and the stars may eventually go their separate ways. For now, they’re all moving in roughly the same direction through space.
The cluster is approximately 600 light-years away from us and the 50 or so stars are mostly massive, very hot blue-white stars. The exception is the brightest member, Mirfak, which is slightly cooler and more yellowish. The age of the stars is estimated to be about 70 million years. Binoculars show the cluster very well; telescopes have a field of view just a bit too big to do this group of stars justice.
There are many other clusters of stars to be found in this part of the sky. Just go outside with your binoculars and take a look for yourself!
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.