Roaming the Deep Sky #15
Melotte 20
By Don Clouse
Melotte 20 is a large, nearby group of related stars. Not exactly an open cluster, it is also known as the Alpha Persei Moving Group1 and the Perseus OB3 Association2. It consists of a group of over 100 stars1. It appears that this OB association does not have any type O stars remaining3. Spectral types seem to range from B3 to A33. Presumably all the hotter, more massive stars (O1 to B2) have already burned themselves out. This group of young blue-white stars, along with the orange supergiant, Mirphak (Alpha Persei), all share a common proper motion1 through the outer portions of our Local Spiral Arm2. They move together since they formed together in the same region of the galaxy only some 30 million years ago2. As the more massive stars in the group blew up as supernovae, the expanding shock waves compressed the surrounding gas and dust triggering a wave of star formation. Some of these "second generation" stars went supernovae in turn triggering yet another round of star birth. This expanding ring of star formation continues today. The sun is currently moving through the midst of this system that bears the name Gould’s Belt after astronomer Benjamin Gould who first described the structure in detail. Tilted at an angle of about 18 degrees with respect to the plane of the galaxy, Gould’s Belt contains many of the bright naked-eye stars in our night skies including many of those in a number of prominent constellations. Gould’s Belt (consisting of stars, star clusters, OB Associations and gas/dust clouds) defines a circle around the sky passing through Crux, Centaurus, Lupus, Scorpius, Ophiuchus, Hercules, Lyra, Cygnus, Cepheus, Cassiopeia, Perseus, Taurus, Orion, Canis Major, Puppis, Vela, Carina, and finally, back to Crux to complete the belt. Melotte 20, the center from which this ring of star formation has expanded, is some 5401 to 6002 light years distant and is at least 33 light years across1.
Not only is this association known as Melotte 20 intrinsically large, because it is so close to us it subtends a large angle on the sky as well. The entire complex is about three degrees in diameter and several of the brighter members are easily visible to the naked eye. These include Psi, 29, 31 and 34 Persei1. It’s a lovely area for scanning with binoculars. Even though this cluster is best seen through tripod mounted binoculars, that didn’t stop me from trying my 80mm, f/5 refractor on it. A 24mm eyepiece gave me a magnification of 17x along with a 183’ field of view, which was just slightly too small to contain the cluster’s cataloged diameter of 185’. (I have no idea why I didn’t try my 35mm eyepiece (11x) with its 4o 17’ field of view! Argggh!) Though, in truth, it’s difficult to say just where the cluster ends. It’s very large and dispersed. In this field of view with Mirphak on the northwest edge, I estimated 200 stars. Even so, the cluster is not really dense at all since these stars are scattered more or less evenly across such a great area. Nonetheless, it is a rather dazzling array of stars and quite lovely. (Note: this cluster is listed3 as having 50 stars, so many of those seen are apparently foreground stars.)
A chart is not included with this article since Melotte 20 is such an obvious naked-eye object sitting in the center of Perseus. Perseus is in prime viewing position from mid-fall into winter. Try spotting Melotte 20 by eye the next time you are at an even moderately dark site. At a good dark site, you may be able to see as many as eight or nine stars with your unaided eye. Use averted vision to see several more stars. Then sweep your binoculars over to it. I think you’ll enjoy the view. Clear Skies!
References
- The Night Sky Observer’s Guide, Kepple and Sanner, Willmann-Bell, Inc., 1998.
- The Guide to the Galaxy, Henbest and Couper, Cambridge University Press, 1994.
Meagstar 4.0, E.L.B. Software, Willmann-Bell, Inc., 1998. I used MegaStar to determine that no stars hotter than B3 remain in the association. They seem to be mostly B8 through A2 with a number of F, G, and K types, many of which may be foreground stars.
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