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NGC 7331

NGC 7331 is a spiral galaxy found in the constellation Pegasus. Galaxies are star collections containing anywhere from several hundred million to a trillion or more stars. Spiral galaxies in particular are shaped like a large, flat disc with bulging centers. The center, called the nucleus, is where the oldest stars are located. In contrast, the spiral arms surrounding the nucleus contain large quantities of dust and gases. New star creation often occurs in this portion of a spiral galaxy. NGC 7331, discovered by William Herschel in 1784, is frequently used as the indicator of a small galaxy cluster known as Stephan's Quintet. As suggested, this is a collection of five galaxies, of which NGC 7331 is by far the brightest. It is among the first spiral galaxies ever recognized. Although it is approximately 50 million light-years away, this galaxy's appearance is often used to represent that of our own Milky Way galaxy. With its 15 to 20 degree tilt, this galaxy is viewed at a slight angle. The spiral arms are visible within a 12 arcminute radius of the compact galaxy core. NGC 7331 is visible with mere binoculars, at a brightness of 9.5 magnitude.

In the image above, the bright compact core is clearly visible at the center of the galaxy. This core is yellow in color, while faint blue spiral arms surround the core. This variation in color suggests the temperature of the spiral arms is higher than that of the stars in the core. Stars with higher temperatures emit light of shorter wavelengths, appearing bluer, while the light emitted by stars of a relatively lower temperature has a longer wavelength, resulting in yellow. However, the dust contained within a galaxy also contributes to coloration. Galaxies with a high concentration of dust tend to appear more blue, indicating that in this particular galactic instance there is more dust contained in the spiral arms than in the core. The distance to NGC 7331 is estimated to be 46,000,000 light-years; we find a maximum angular size of 187 arcseconds, which corresponds to a linear size of 41,800 light-years. On the left side of the image, three of the partner galaxies found in Stephan's Quintet are visible. These can be located by looking for objects with appearances similar to that of NGC 7331, only on a smaller scale - a bright yellow core surrounded by a hazy, bluish-yellow oval. There are, in fact, four other galaxies present in this photo, though only the three closest to NGC 7331 are members of Stephen's quintet. The higher brightness of NGC 7331 in comparison to its cluster counterparts is evident in this photo. Based on the difference in brightness from the left side to the right side of the galaxy, it can be concluded that the left side is closer to Earth. This brightness variation can be contributed to the effects of dust. Dust has the ability to absorb a majority of light passing through it. This results in the darkening effect seen on the more distant side of the galaxy, as well as in the dark lines.

References:

Ferris, Bill. "NGC 7331." Deep Sky Observer's Companion - the online database. <http://www.docdb.net/show_object.php?id=ngc_7331>.

Bennett, Jeffrey and Seth Shostak. Life in the Universe: Third Edition. 54-55. Addison: Columbus, 2012.

"Spiral Galaxies." Sloan Digital Sky Survey. <http://cas.sdss.org/dr6/en/proj/basic/galaxies/spirals.asp>.

"NGC 7331." <http://messier.seds.org/xtra/ngc/n7331.html>.

"Dust in Galaxies." <http://www.astr.ua.edu/keel/galaxies/dust.html>.

Right Ascension (J2000) 22:37:05.10
Declination (J2000) +34:25:13.0
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B, V, R, and C (60s x 5)
Date observed October 12, 2011

 

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