Physics Photo of the Week

November 22, 2013

The Pleiades
The Pleiades is a cluster of stars highly visible in the late fall of every year in the Northern Hemisphere.  To the naked eye the Pleiades is quite faint, but noticeable in dark skies in the constellation Taurus, The Bull.  Averted vision gives a better view.  A common name for the Pleiades is "The Seven Sisters". 

"The Seven Sisters" is a pretty good name because the stars are truly "sisters" or siblings.  The stars were all "born" at about the same time - about 140 million years ago.  Stars are typically born out of large clouds of primordial gas - primarily hydrogen.  Starbirth is believed to be triggered by a compression shock wave traveling through the galaxy, compressing the gas, so that self gravity feeds on itself and condenses larger and larger modules of the gas eventually forming stars that sustain nuclear fusion.  The shock waves in the galaxy are revealed as the spiral arms of a galaxy (see PPOWs for May 2, 2008 and Sept 16, 2011).

The Pleiades are important for astronomers for calibrating their instruments for colors.  Several stars in the Pleiades and other famous clusters have been carefully measured for color for calibrating astronomical photometers.  I am currently using these standards for some research that I am currently engaged in.

The number of stars in the Pleiades cluster is much more than 7 as the picture above indicates.  By enhancing the video gain of the image (the photo at left) more faint stars can be seen.  Taking images for longer exposures will bring out even more fain stars.  It is believed that there are 500 - 1000 stars in the Pleiades.

Open clusters such as the Pleiades are relatively short lived as stars go.  There is not enough gravity to hold the cluster together from the tugs of other objects in the Galaxy.  Most open clusters disperse after a few billion years.  The Sun was also born in a cluster about 5 billion years ago, much older than the Pleiades.  Astronomers have searched for the Sun's "sisters" and found candidates scattered all across the Milky Way galaxy. 

Anyone with a digital camera (at least a "point and shoot") can mount the camera on a tripod, set the camera's sensitivity to maximum, and take about a 15 second exposure.  Experiment with various zoom factors.  This photo was made with the same set-up as the Andromeda Galaxy photo published last week (PPOW Nov. 20, 2013).

The Pleiades have been featured many times before on Physics Photo of the Week: November 19, 2010January 10, 2005November 16, 2012, and several others.

Due to the Thanksgiving break, there will be no Physics Photo of the Week next week.  The next Physics Photo will be posted on December 5.

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

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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