Physics Photo of the Week

March 8, 2013

North Carolina Milky Way
Dave Wilson, of northern Buncombe County near Asheville, NC photographed this excellent picture of the Milky Way in December, 2012.  I had never seen the Milky Way in the Winter from my home in North Carolina.  From North Carolina I have only seen the Milky Way in the the late summer and early fall when the much brighter parts of the Milky Way are visible near the southern horizon.  Congratulations to Dave Wilson for the excellent photo!  Click on the image to see a larger picture. 

Dave's camera (a Nikon D90) is so sensitive and shows so many stars that picking out the familiar constellations and stars is nearly impossible.  At right the contrast has been reduced to make the brighter stars of Cassiopeia more visible along with the constellation familiar stick figure as an "M".  I have also outlined part of Cygnus, the Northern Cross, in the lower left part of the modified photo.  Click on the photo to see a larger version.

The Milky Way is the disk-shaped spiral galaxy where the Sun and the Solar System reside.  The milky appearance crossing through the night skies arises from the millions of stars (like the Sun) that populate the disk.  Think of the galaxy resembling a double sided Frisbee - a flat disk with a thick hub in the middle.  Since the Sun and the solar system are in the galaxy disk, looking in any direction along the plane of the disk sees more stars than in the other directions.  The "milky river of stars" is our galaxy's disk seen edge-on from inside the disk.  We also see dust clouds causing the dark patches along the Milky Way.  Dust in the disk obscures some stars that are further away and also provide the fuel to form new stars over time scales of hundred of millions of years.  The Sun was "born" out of a dust cloud about 4 billion years ago.  The Milky Way galaxy is huge!  The disk is about 100 thousand light years across. 

Dave's photo is also very interesting in that it captured another galaxy.  At the top of the main photo, the elongated smallish cloud is the Milky Way's closest spiral galactic neighbor - M31, the Great Galaxy in Andromeda.  The Andromeda galaxy is similar in structure to the Milky Way (a huge spiral of 100 billion stars and dust clouds, 100,000 light years across), but the Andromeda galaxy is about 2 million light years distant.  Thus the starlight from the Andromeda galaxy captured by Dave's camera last December left the galaxy when human ancestors were just beginning to roam the Earth - long before any recorded history.  The light from the the bright stars in Cassiopeia (within our galaxy) left those stars only about 400 years ago at the dawn of modern astronomy when Galileo first used a telescope to examine stars and planets.

Also visible in Dave's magnificent photograph in the upper right center is the double cluster (h and chi Persei).  These two clusters are about 7000 light years distant - still within the Milky Way disk - containing large bright stars that formed about 10-100 million years ago.  That is very young for stars compared to the Sun's 4 billion years' age.   Thank you, Dave!


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 dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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