Physics Photo of the Week

April 4, 2014

Galaxy M51 - The Whirlpool Galaxy
This impressive spiral galaxy is a favorite target for amateur and professional astronomers alike.  It consists of a "whirlpool" of about 100 billion stars.  M51 is estimated at about 23 million light years from Earth, which means the light we see today is the light that originated 23 million years ago.

From the focal length of the telescope that photographed this galaxy we can find the angular size in degrees, or other angular units.  Simple geometry from the angular size we find a diameter of the spiral to be 35,000 light years - considerably smaller than our own Milky Way Galaxy's diameter of about 100,000 light years.  The spiral structure of many galaxies arises from the piling-up of interstellar gas and dust into bigger clouds eventually forming new stars in the clouds.  The clouds exhibit spiral structure because the inner clouds orbit the core faster than the outer clouds.  Eventually - after several billion years - the spiral structure becomes more and more tightly wound.

The photograph actually contains two galaxies.  The cloud near the top of the photo is a separate galaxy (NGC 5195) that is either approaching M51 or is moving past it.  The outer spiral arm of M51 actually extends all the way and connects with the smaller galaxy.  The other spiral arm close to the satellite appears to be distorted by the tidal gravitational interaction with the satellite.  All the distinct stars scattered throughout the image are members of our own Milky Way galaxy.  M51 and its companion are too distant and the stars are too numerous to see the individual stars.

Compare this image of M51 with the Andromeda Galaxy (PPOW November 15, 2013).  The Andromeda Galaxy is only 1/10 the distance (2 million LY) of the Whirlpool and about 100,000 LY across.  The image of Andromeda Galaxy last November was made with a much smaller telescope in order to fit the whole large galaxy into the frame of the camera.

This image was made with the 14 inch telescope in the College View Observatory on March 23, 2014.  The telescope was donated to Warren Wilson College for the College View Observatory by alumnus Gary Starkweather class of 1978.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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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