Physics Photo of the Week

December 5, 2008

Clusters over Swannanoa

Two prominent clusters appear in the eastern sky early in the evening when viewed in the northern hemisphere at this time of year - early December.  A time exposure allows the stars to be visible above the lights of the Swannanoa Valley at Warren Wilson College.  The most well-known of the clusters is the Pleiades in the top third of the picture.  A much larger, but more dispersed, cluster is the sideways "V"-shaped Hyades cluster in the lower part of the photo.  The sky in the distance is illuminated by the lights from "downtown" Swannanoa about 3 miles away.

The Pleiades are also known as "the Seven Sisters" - a  very
appropriate nickname because the stars are literally siblings. The stars in the cluster were all formed at approximately the same time - about 100 million years ago from a large cloud of hydrogen and dust that condensed due to self gravity, forming stars. In astronomical time, 100 million years is a relatively short time.  The Sun was formed about 5 billion years ago.  That's 50 times the age of the Pleaides.  Even though the Seven Sisters are true "sisters", there are many more than seven in the family.  The Pleiades consists of about 200 stars.  Gia Campanella, a student in Astronomy, photographed the image on the right with a simple digital camera two years ago.  On December 3, students in the 2008 astronomy class (Phil Hamilton, Jillian Levy, and Georgia Beasley) photographed the Pleiades with a more sophisticated digital camera with a 300 mm FL lens.  Students Rebecca Moss and Melissa Hahn processed the image to stack 11 4-second exposures to give the nice photograph on the left.  Clearly there are more than seven sisters in the Pleiades family.  How many stars can you count?


The Hyades cluster - the lower cluster from the top photo is shown again below appears much larger because it is much closer to the Solar System (150 light years,  about 1/3 the distance to the Pleiades).  Notice the color of the Hyades compared to the Pleiades.  There are more orange-colored stars in the Hyades whearas the Pleiades are predominantly blue.  The stars in the Hyades are less hot than the bright stars of the Pleiades.  This is a factor of the
relative ages of the clusters.  The Hyades is considerably older than the Pleiades (about 780 million years compared to 100 million years for the Pleiades.  As star clusters age, the stars evolve.  The hotter stars, being more massive, burn-up their hydrogen fuel much more quickly and evolve into red giants as they switch to the nuclear fusion of helium.  The astronomy class this fall will measure the colors of these two clusters and confirm the drastic difference in cluster ages.  Telescopes on these clusters are little use, however, because the telescope can only image a few stars at a time. 

The brightest star in the field of view of the Hyades is Aldebaran - the "eye of the bull".  However, Aldebaran is not a member of the Hyades - it is much closer. 

All the background information in this article can be found in the website "Students for the Exploration of Space" -

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 

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.

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

Observers are invited to submit digital photos to: