In my blog, we've already taken a look at two famous star clusters, the Pleiades and Hyades. Today, we'll be looking at a star cluster that is mostly ignored by backyard astronomers, partly because it is not listed among the famous Messier objects, nor is it found in even the much more comprehensive NGC catalogue. It doesn’t appear on many sky atlases and it is rarely mentioned in articles. many beginning stargazers learning their way around the deep sky may not be aware of this spectacular gem. That object is known as The Alpha Persei Cluster, also cataloged as Collinder 39 and Melotte 20.
The Alpha Persei Cluster is a very easy star cluster to locate. This time of the year, it should be around the northeastern horizon, and quite high above the horizon when it's dark. Everyone knows the 'W' shape of Cassiopeia and the Pleiades' sprinkling of jewels. Now just look for the very bright Capella and its three 'kids' and, as you can see from my picture below, the Alpha Persei Cluster should be smack in the middle of the triangle formed by those three objects.
As you can see from my picture below, the cluster appears as a lovely stream of stars trickling down the night sky, with its brightest member alpha Persei (Mirfak.), at its central star. Many astronomers refer to this cluster as an association because it is loosely bound by gravity. This loose type of open cluster is also called an OB-Association since the clustered stars are mainly of the young, massive, and hot spectral types O and B. These associations are quite unstable and have short lifetimes before they evaporate into space. The cluster is not rapidly dispersing but its members are moving in the same direction.
A closer look at the star will reveal the brightest members of this cluster, including Alpha, Delta, Sigma, Iota and Psi (see below). A rich-field telescope, such as my Meade ETX-80, gives the best views of the Alpha Persei cluster - keep the magnification under 20x. Even at such low power, under dark cloudless and moonless skies, you’ll see about 30 bright stars - perhaps somewhat less under light polluted skies such as at Longstanton. All told, there are more than 100 young stars brighter than magnitude 12 spread across its 3° width.
If you take multiple exposures of the clusters and stack them together, the two clusters are more evident, as in the picture below.
Finally, perhaps the most famous star in Perseus is Algol (in the centre of my picture below). This is a variable star - regular as clockwork, every 2.867 days, the brightness of the star plummets from a relatively bright magnitude of 2.1 to a dim 3.4 (30 percent of normal). The whole event takes only a few hours. The next night time the Algol minima takes place is at 1:21 am on October 10th, so I'll see if I can take a few pictures of that and tell you all about it - weather permitting, of course!
All photographs on this page © Sabri Zain 2012.
