Physics Photo of the Week
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
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
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
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" - www.seds.org.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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the world around us. Students, faculty, and others are invited to
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explanation. Atmospheric phenomena are especially welcome.
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