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Astr111 Photography Projects, Fall 2009

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M71, Quinn Harr


M71, located within the constellation Sagitta at a distance of 13,000 light years (LY), was first discovered by Philippe Loys de Cheseaux in 1746 and first resolved into component parts by William Herschel in 1783. It is thought to be a globular cluster, though this has been a matter of some dispute. Globular clusters differ from open clusters in age, density of stars, galactic location, and size, among other factors. These and other significant criteria have led astronomers alternatively to classify M71 as a condensed open cluster and (more recently) as a loose globular cluster. For example, globular clusters move in a highly elliptical orbit around (and sometimes outside) the periphery of a given galaxy: this area is called the galaxy's halo or bulge. Open clusters, on the other hand, are more centrally located within a given galaxy, orbiting within what is called its disk. One way to estimate the location of a cluster within a galaxy is through its radial velocity measurement (RVM). Clusters outside the central disk rotation have higher RVM's, reaching upwards of 100's of km/sec. M71's RVM was once estimated at 80 km/sec, but more recent measurements have put the value at 23 km/sec, a value consistent with either type of cluster. Another relevant factor in determining a cluster-type is its elemental composition (or metalicity). Globular clusters have low metalicity because they are formed apart from the heavier elements abundant in a galaxy's disk. M71, however, has the second highest metalicity for a globular cluster. This fact can be explained in a manner consistent with M71's classification as a globular cluster if we note that galactic location is not the sole determining factor for metalicity: the presence of a certain variable star (RR Lyrae) contributes to low metalicity--to date, none of these types of stars has been found in M71. Other important factors lead us to classify M71 conclusively as a globular cluster. For example, its diameter estimated to extend up to 90 (LY), a value much higher than the standard <30 LY for open clusters.

If we examine the image above, we find conclusive proof that M71 is a globular cluster. The density of the stars within M71 (radiating outward) is typical of the older globular clusters, which, as gravitationally bound concentrations of stars similar in age and distance, are believed to have formed from ancient star populations composed of some of the oldest matter in the galaxy. Open clusters, on the other hand are far less dense (stars even within their center usually appearing distinctly) because their formation from cosmic gas or dust clouds--relatively younger material--produces mutual gravitational attraction rather than a bound concentration. The color most prevalent is yellow, though some larger outer stars are bluish-white. The larger blue stars are scattered throughout the photo, which leads us to believe that these are not part of the cluster but instead form a foreground through which we see M71. This result is consistent with M71's currently passing through Milky Way's disk. Our observation of M71 as it travels through the galactic disk had led to the earlier speculation that it might be an open cluster. If we understand the scattered blue stars as forming the foreground for the yellow stars in the cluster, this demonstrates M71's status as a globular cluster--the brightest stars in the older globular clusters are pale yellow (having completed the main phases in their evolution) while the younger blue stars (which we now understand as part of the foreground) would have already expired in globular clusters as they burn at a faster pace. The linear diameter of M71 is .33 KLY.

Fraknoi, Andrew, Morrison, David and Wolff, Sidney. Voyages to the Stars and Galaxies, 3rd ed. 293-295.

Frommert, Hartmut and Krongberg, Christine. "Globular Star Clusters." The Munich Astro Archive. <http://www.maa.clell.de/Messier/glob.html>.

________. "Messier 71." Students for the Exploration and Development of Space. <http://www.seds.org/messier/m/m071.html>.

________. "Star Clusters." The Munich Astro Archive. <http://www.maa.clell.de/Messier/cluster.html>.

Right Ascension (J2000) 19:53:48
Declination (J2000) +18:47:00
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 5x60 seconds in C, 300 seconds in BVR
Date observed

November 1, 2009 (CBVR)