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
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).
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 firstname.lastname@example.org.
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.