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M56 (NGC 6779)
Nathan McReynolds

M56

Messier 56, or M56, is a globular cluster that is located 9.4 kpc away from our solar system in the constellation Lyra (Harris, William). Like the other Messier objects it was named after the French astronomer Charles Messier who discovered it on January 19, 1779. Globular clusters are a gravationally bound group of stars. Smaller than galaxies, these clusters typically orbit around a galactic center. A distinguishing trait between globular and open star clusters is the high density of stars in the center of the cluster. This, combined with the gravitational influence of the stars on each other, causes the globular cluster to have a spherical shape.

The above image was taken by Calvin's telescope in New Mexico. To make such an image, a series of photographs are taken through filters that let specified wavelengths through. For this image B,V,R, and I filters were used. After removing the noise from these images they can be stacked to form a color image. Since the images taken through each filter appear as a grey scale, the colors are determined by the intensity of the light in each filter. It was also necessary to reduce the data size of the image file, so by keeping the average pixel color in a general area, the size of the image was reduced so that it could be opened as a JPEG.

One of the most notible aspects of this image is the distribution of star colors. Generally stars are described as a range from blue to red. The blue stars in this image generally are the blue stars on the horizontal branch of the HR diagram (see chart above). However a few of the blue stars are foreground stars that are really just between us and the globular cluster. The diagram also indicates that the red stars seen are red giants (Vanderbei). For the most part, these stars are distributed evenly, however the blue stars are obscured by the red and yellow stars in the center of the cluster.

One of the measurements made on this object was a King profile which uses a defined radius to calculate the surface brightness of the object. By plotting the surface brightness as a function of the radius, a King profile can be made that provides the core radius of the object (see figure above). Using the Harris database as a source for distance (9.4 kpc), and the surface brightness profile, the calculated core radius was 29.8 +/- 6.3 arcsec (Harris, William). However, the accepted size is 26.4 arcsecs (Harris, William). Also from the distance and the angular size of the object with in the image, the linear size of the object was calculated to be approximately 90 light years across compared to the accepted 84 light years (Wikipedia).

References:

Harris, W.E. 1996, AJ, 112, 1487 <http://www.physics.mcmaster.ca/~harris/mwgc.dat>.

Vanderbei, Robert J.< http://www.princeton.edu/~rvdb/images/NJP/HRdiagMatlab.html>.

Wikipedia, "M56".

Right Ascension (J2000) 19:16:35.57
Declination (J2000) +30:11:00.5
Filters used B (Blue), I (Infrared), R (Red), V (Green)
Exposure time per filter B (90s x 60), V (90s x 10), R (90s x 3), and I (90s x 6)
Image dimension 644x644 pixels; 12.7x12.7 arcminutes
Date/time observed March 16, 2015, 12:19 UT

 

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