Physics Photo of the Week

April 27, 2007

Galaxy M51
Messier 51, the Whirlpool Galaxy, a favorite among amateur astronomers, is currently situated very high in the evening sky in the constellation Canes Venatici near the end of the handle of the Big Dipper.

This is the second color image published that is produced with the new CCD camera that the Physics Department has acquired with the help of the Small Projects Grant of the American Astronomical Association. (The earlier color image consists of the Great Nebula in Orion published on Feb. 23, 2007)  The color image was produced by photographing through three separate filters - red, green, and blue.  The three colors are then combined in a computer using the appropriate colored dots for the corresponding images.  The image was taken on April 22, 2007 on Warren Wilson Campus using a Celestron 8-inch telescope donated by Bernard Arghierre.  The imaging session lasted about 1 hour in which several 2-minute exposures were made in each color.  The images of each color were aligned and co-added to minimize the noise before combining the different colors.

This galaxy is believed to be between 31 - 37 million light years distant - way beyond the outskirts of our home Milky Way galaxy and considerably further than our nearest spiral neighbor - the Great Galaxy (M31) in Andromeda (see PPOW for November 17, 2006).  The Whirlpool Galaxy has a close neighbor - a smaller galaxy known as NGC 5195 near the top of the photo.  It is believed that these two galaxies collided in the "recent" past - about 100 million years ago.  Notice how one of the spiral arms of M51 appears to be stretched until it merges with the smaller galaxy.  The spiral arms in galaxies such as the Whirlpool Galaxy are made prominent and visible by the existence of young stars in the spirals.  The spirals are believed to be shock fronts of compressed interstellar matter - dust and hydrogen gas.  The compression wave initiates the gravitational condensation of clouds into clusters of stars.  Young stars clusters are characterized by hot, blue stars, thus making the spiral arms visible.  Compare the color of the spirals with the reddish hues of the neighboring galaxy - which consists of old cool stars.  Galaxies such as this consist of about 100 billion (1011) stars.  The other star-like objects in the photo are foreground stars relative close by in our own Milky Way. 

Much of the astronomical observations currently conducted with the CCD camera consist of measuring the light output from cataclysmic variable stars.  Watch this page for more news.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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