Physics Photo of the Week

November 17, 2006
Andromeda - A Constellation and Galaxy.

High in the sky in the fall lies the star constellation Andromeda that is shown in the primary photograph.  Andromeda is especially interesting because it contains the most distant object visible to the naked eye from clear dark skies: the great galaxy in Andromeda - otherwise known as M31.  M31 is barely visible in the photograph of the whole constellation - indicated with the arrow in the image that contains the outline below.  Although M31 is naked-eye visible on exceptionally clear dark nights - away from city lights - it is easily visible through binoculars.

M31 is about 2 million light years distant - the most distant naked eye object in the sky.  This means that the light which was captured in the digital photographs left the galaxy 2 million years ago! 

The photograph was produced by overlaying multiple time exposures of 15 sec. each with a "standard" digital camera.  Because the sky moves during the whole process, each of the 15 second exposures has to be shifted by a small amount so that all the stars superimpose at the same place.  The co-adding of about 16 separate images minimizes the "speckle" or "noise" in the final image.  The contrast and color saturation is also adjusted with simple image editing programs.

The image at left was produced by either zooming the camera (10x optical zoom) and photographing a series of 15 second images as before.  Here one can begin to see the extended size of the disk galaxy near the center of the image.  Notice also that the core of the galaxy is not as blue as the surrounding field stars.  This indicates that the billions of stars that make up the core of M31 are old and evolved.

The image at left below is photographed through a small telephoto lens piggy-backed onto a telescope with a clock drive.  The photograph at right below used a digital SLR camera mounted so the sensor was at the prime focus of an 8-inch Schmidt-Cassegrain telescope.  Notice the fine detail of the dust clouds in the telescope image.

Photo by D. Collins

Photo by Dan Sockwell

The Andromeda Galaxy - at 2 million light years - is the closest spiral galaxy to the Earth.  Millions of similar galaxies exist in the known universe.  The Andromeda Galaxy is also believed to be very similar to the Milky Way - our own galaxy

The Andromeda Galaxy was featured in a Physics Photo of the Week almost two years ago on Dec. 10, 2006.

There will be no Physics Photo of the Week next week (November 24, 2006) due to Thanksgiving Vacation at Warren Wilson College.

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

Click here to see the Physics Photo of the Week Archive.

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