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Astr110 Photography Projects, Fall 2006

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M72, Ben Ipema


The picture is of the magnificent globular cluster known as M72. Globular clusters are easily recognizable because of the tight collection of stars. Generally these gravitationally bound stars are spread over a volume of 10 to 200 light years in diameter. Globular clusters can contain between ten thousand and one million stars. Our galaxy has approximately 200 globular clusters. Most other galaxies contain globular clusters as well. Spectroscope studies have shown that they contain less heavy elements then stars that form in the disks of galaxies (like the sun). This is expected because Globular clusters are very old and have already lost their heavier elements. Meaning that they are primarily composed of helium and hydrogen.

M72 is located within the constellation Aquarius. Discovered in 1780 by Pierre  Mechain, it has a distance from Earth of 54.4 Kly. M72 is not as bright as or as large as other globular clusters, and at 53,000 light years, it is located a considerable distance from the center of the galaxy.

The picture consists of a clear filter and it was taken at 11:12PM on October 10. It is a compilation of 30 images, each having a 20 second exposure time. The camera temperature was -20(C). The estimated angular size of the image is 6 arc minutes. Using the small angle formula I was able to determine that the linear size of the object to be 94.9 light years.


Fix, John D. Astronomy: Journey to the Cosmic Frontier. 4th ed. New York: McGraw Hill, 2006.

Right Ascension (J2000) 20:53:30
Declination (J2000) -12:32:00
Filters used clear(C)
Exposure time per filter 30 x 20 seconds in C,
Date observed

November 10, 2006 (C)