Roaming the Deep Sky #5
The Arrowhead Cluster, M71
By Don Clouse
Actually, I’ve never seen M71 called "The Arrowhead Cluster" anywhere, I just made it up. But as you will see it is an apt name for M71. This unusual and distinctive globular cluster, can be found along the eastern edge of the Summer Triangle (formed by Deneb, Vega and Altair), about halfway between Altair and Albireo (the beautiful double star which marks the head of Cygnus, the Swan). The Summer Triangle is high in the east by midnight during early summer evenings. Two charts are provided to locate and identify M71. If you feel like doing a little "experimenting", use this chart link for locating M71 on the sky. Click on "Find object in catalog", and then follow the links to select M71. Then click the gray "Update" button. This is a nice piece of sky charting software. It’s quite flexible. You can zoom the chart in and out and set numerous display parameters.
M71 had been classified by some astronomers as a large, rich, open cluster. Others had classified it as a loose, poor globular. Recent data indicate that M71 is indeed a globular cluster. Analysis of its Hertzsprung-Russell (color/temperature) diagram reveals that M71 is a young, 9-10 billion year old, globular. This youth explains other physical characteristics that are atypical of a globular cluster. These include a dearth of RR Lyrae variables and relatively high metallicity (i.e., all elements heavier than hydrogen and helium). M71 is about 12,000 light years from the sun and is currently 22,000 light years from the galactic center.
The shape is the first thing that you notice when viewing this object. It doesn’t appear spherical at all. At first the shape appeared to me to be rather amorphous. However, closer observation reveals a quite distinctive shape – an arrowhead. (An arrowhead shaped cluster within the constellation of Sagitta, the Arrow, would seem amble reason to dub M71 "The Arrowhead Cluster"!) The arrowhead points west with a shallow gulf open to the east at the base of the arrowhead. At 58x (8" SCT, 35mm eyepiece, 50’ true field), an 8’ wide, roughly triangular, faintly glowing haze is visible with four bright stars in a row across the face. At 101x (20mm eyepiece, 30’ true field), the number of stars visible with direct vision jumps to a dozen.
Also visible in the same low power field of view are open cluster Harvard 20 (H20) and a pretty asterism of three stars. The combination of my 8", f/10, Schimdt-Cassegrain telescope and a 35mm eyepiece, yields a field of view of 50 arcminutes at 58 power. M71, H20, and an asterism led by 9 Sagittae (Sge) are all visible in this 50’ field. The circle shown in the accompanying chart represents the 50’ field of view. (The chart has the normal, unaltered orientation – north at top and east to the left.) Now H20 is not a spectacular object. It’s Trumpler classification of "4 2 p" (not well detached from surrounding star field, moderate range in brightness, poor – less than fifty stars) pretty well describes it. Nonetheless, it does add interest to the field. It seems to me to be a rather stretched out, meandering, cluster. Each of two brighter stars in the cluster, lead a discontinuous chain of stars which converge to the southeast. About 12 stars are visible. A magnitude 6.1 star, 9 Sge, is a nearly equal distance west of both M71 and H20. With a spectral class of O5, 9 Sge is a very hot, young star. North-northeast from 9 Sge, in a line, is a magnitude 7.6, class B8 star followed by a magnitude 9.2 K-class dwarf. Thus proceeding from 9 Sge, each star is successively dimmer, cooler and older – but mostly the asterism just adds a nice (to continue the theme, arrow-like?) aspect to the field. Overall, M71, H20 and the 9 Sge asterism make for a rather pleasing field. If you haven’t observed this field yet, take a few minutes to do so at your next opportunity. I think you’ll enjoy the view. Clear skies!
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