Seven Blue Sisters in SB Skies
updated: Feb 02, 2013, 10:00 AM
By Chuck McPartlin
We had some nice clear skies last week, but the Moon was bright, so it
was a good time to image a prominent star cluster. Well placed in
Santa Barbara skies these days, are the Pleiades, or the Seven Sisters.
They are overhead as darkness falls, next to brilliant Jupiter, but
about 400 light years away, while Jupiter is at about 40 light minutes.
The Pleiades are often mistaken for the Little Dipper, which is
actually the constellation Ursa Minor, the Little Bear. The
Pleiades form more of a "micro dipper" in the shoulder of the
constellation Taurus, the Bull. Taurus, by the way, represents
only half a bull - he ends at the waistline. The story is that
when he was placed into the sky, his rear half cracked off, and
fell back to Earth, and brought forth politicians.
Here is the Pleiades image, a mosaic of 80 individual photos
taken with a modified monochrome video camera used for security
systems, attached to a 5-inch telescope. Printed full size, it
would be about 3 feet across - way too much ink. This gives you an
idea of how small the field of view can be through a telescope.
As you can see, there are more than seven stars in the Seven Sisters.
There are thousands of stars in the cluster, gravitationally bound
and moving through space together. Because the cluster is easily seen
and near the ecliptic, the path of the Sun and the planets across the
sky, lots of cultures have stories about it.
One Greek story involves seven sisters who were being pursued by
Orion. Zeus turned them into doves, and they flew up into the sky
to become these stars. One of sisters is dimmer than the rest,
because she married a mortal, and longed to return to Earth, while
the other sisters had the foresight to marry immortal gods.
The video camera is more sensitive to red light, and the Pleiades
are hot, young stars, and thus blue. Here is an image generated from
planetarium software, showing more what your eye would see. The named
stars of the cluster have been labeled, but are rather hard to read.
From left to right they are: Atlas, Pleione, Alcyone, Merope, Maia,
Sterope, Taygeta, Electra, and Celaeno. Atlas and Pleione are the parents
of the sisters. The little line of stars trailing down from Alcyone
is known as "Ally's Braid".
Finally, here is an image assembled with big scopes - the Palomar Observatory
Sky Survey's Oschin Telescope and the Hubble Space Telescope.
The strange superimposed hieroglyphic represents the fields of view of the
instruments. Here you can see lots of color, not only in the stars, but in
the reflection nebulae surrounding them. The Pleiades are colliding with a
cloud of space dust, very small particles of carbon and silicates puffed off
and left behind by an earlier generation of stars. These particles preferentially
bend and scatter blue wavelengths of light, amply supplied by the cluster.
Earth's daytime skies are blue for the same reason. Our atmospheric particles
scatter the blue wavelengths of sunlight down to our eyes.
References for a cloudy day:
Comments in order of when they were received | (reverse order)
2013-02-02 12:24 PM
Another excellent post--thank you so much--and keep 'em coming! (Love the politician reference:>)
2013-02-02 12:29 PM
Thank you, Chuck and Pat McPartlin, for the wonderful free astronomy events you and your colleagues with the Santa Barbara Astronomical Unit host for the community at various locations every month. To learn more, visit http://www.sbau.org/.
2013-02-02 04:47 PM
Thank you so much for sharing the knowledge and photos. I learn so much on Edhat - especially about politicians.
2013-02-03 08:47 AM
Forget the lame joke about politicians; "... the Pleiades are hot, young stars, and thus blue ..." takes the cheesecake award.
2013-02-03 09:01 AM
Wonderful tutorial with great illustrations. Thank you for sharing your knowledge and enthusiasm.
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