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:
http://hubblesite.org
http://messier.seds.org/m/m045.html
http://www.sbau.org/

Comments in order of when they were received | (reverse order)

COMMENT 370241	2013-02-02 12:24 PM
Another excellent post--thank you so much--and keep 'em coming! (Love the politician reference:>)
JAZZEE	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/.
AUNTIE S.	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.
COMMENT 370441	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.
RDH	2013-02-03 09:01 AM
Wonderful tutorial with great illustrations. Thank you for sharing your knowledge and enthusiasm.

60% of comments on this page were made by Edhat Community Members.