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Pinwheel Galaxy M101
Cassie Westrate


Like our Milk Way, the Pinwheel Galaxy M101 is a spiral galaxy. Spiral galaxies are flat white disks filled with stars, cool gas and dust, interspersed with hotter ionized gas. These galaxies are named for their beautiful spiral arms. M101 is estimated to hold hundreds of billions of stars, many of which are smaller, cooler, and redder than our Sun. While these cooler, older stars can be found in the center bulge of the galaxy, the hot, blue stars in M101 are younger and can be found in the spiral arms. Three supernovae have also been discovered. M101 is 25 million light-years from Earth in the Northern Constellation Ursa Major. Since it is so far away, the image of M101 above is the galaxy as it looked 25 million years ago in the Earth's Miocene Period. M101 is the brightest of a group of nine galaxies called the M101 Group and is 170,000 light-years in diameter. The disk of M101 is mostly empty space and very thin, allowing the Hubble telescope to see more distant galaxies lying behind it.

In the image of M101 above, we see a bright center bulge with several spiral arms extended far out from the center. Some of these spiral arms become more transparent and are more difficult to see. The center of M101 is very small compared to other galaxies. Because it is so small, it is easier to see the disk and the expansive spiral arms. The distance to M101 is estimated to be 25 million light-years; we find a maximum angular size of 1403 arc seconds, which corresponds to a linear size of 170,000 light-years. Because M101 is so large, it does not fit in our field of view. As a result, this image is a mosaic of four separate clear images of the M101 spiral galaxy. In order to accomplish this, we divided the galaxy into four sections and took five images of each section. For each section the five images were stacked together to form one image of the section. The four sections were then blended together to form one cohesive image of the M101 spiral galaxy.


Bennett, Jeffrey O., Meghan Donahue, Nicholas Schneider, and Mark Voit. The Essential Cosmic Perspective. Sixth ed. San Francisco: Pearson Education, 2012. Print.

"M101: The Pinwheel Galaxy." .NASA, n.d. Web, Apr. 2011. <>.

Messier 101. N.P., 30 Aug. 2007. Web. 18 Apr. 2011. <>

Right Ascension (J2000) 14:03:39
Declination (J2000) +54:17:34
Filters used C (Clear)
Exposure time per filter 60s
Date observed March 22, 2011



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