Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 December 27 
Posted on 12/26/2003 10:14:47 PM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 December 27
Explanation: Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes were added to the image for aesthetic reasons, produced by kite string donated by Rob Gendler's kids and placed over the telescope dew shield.)
Pleiades
![[m45.jpg]](http://www.seds.org/messier/Jpg/m45.jpg)
| Right Ascension | 03 : 47.0 (h:m) |
|---|---|
| Declination | +24 : 07 (deg:m) |
| Distance | 0.38 (kly) |
| Visual Brightness | 1.6 (mag) |
| Apparent Dimension | 110.0 (arc min) |
Known pre-historically. Mentioned by Hesiod between 1000 and 700 B.C.
The Pleiades are among those objects which are known since the earliest times. At least 6 member stars are visible to the naked eye, while under moderate conditions this number increases to 9, and under clear dark skies jumps up to more than a dozen (Vehrenberg, in his Atlas of Deep Sky Splendors, mentions that in 1579, well before the invention of the telescope, astronomer Moestlin has correctly drawn 11 Pleiades stars, while Kepler quotes observations of up to 14).
Modern observing methods have revealed that at least about 500 mostly faint stars belong to the Pleiades star cluster, spread over a 2 degree (four times the diameter of the Moon) field. Their density is pretty low, compared to other open clusters. This is one reason why the life expectation of the Pleiades cluster is also pretty low.
According to Kenneth Glyn Jones, the earliest known reference of this cluster is a mention by Hesiod, about 1000 BC (according to Burnham, they were seen in connection to the agricultural seasons of that time). Homer mentions them in his Odyssee, and the Bible has three references to the Pleiades.
The Pleiades also carry the name "Seven Sisters"; according to Greek mythology, seven daughters and their parents. Their Japanese name is "Subaru", which was taken to christen the car of same name. The Persian name is "Soraya", after which the former Iranian empress was named. Old European (e.g., English and German) names indicate they were once compared to a "Hen with Chicks". Other cultures tell more and other lore of this naked-eye star cluster. Ancient Greek astronomers Eudoxus of Knidos (c. 403-350 BC) and Aratos of Phainomena (c. 270 BC) listed them as an own constellation: The Clusterers. This is also referred to by Admiral Smyth in his Bedford Catalog.
Burnham points out that the name "Pleiades" may be derived from either the Greek word for "to sail", or the word "pleios" meaning "full" or "many". The present author prefers the view that the name may be derived from the mythological mother, Pleione, which is also the name of one of the brighter stars.
According to Greek mythology, the main, visible stars are named for the seven daughters of "father" Atlas and "mother" Pleione: Alcyone, Asterope (a double star, also sometimes called Sterope), Electra, Maia, Merope, Taygeta and Celaeno. Bill Arnett has created a map of the Pleiades with the main star names. These stars are also labeled in a labeled copy of the UKS image which appears in this page. Also note our Pleiades map.
In 1767, Reverend John Michell used the Pleiades to calculate the probability to find such a group of stars in any place in the sky by chance alignment, and found the chance to be about 1/496,000. Therefore, and because there are more similar clusters, he concluded correctly that clusters should be physical groups (Michell 1767).
On March 4, 1769, Charles Messier included the Pleiades as No. 45 in his first list of nebulae and star clusters, published 1771.
Unfortunately, the Beagle 2 has yet to bark. The latest update from Spaceflightnow.com:
SATURDAY, DECEMBER 27, 2003
0050 GMT (7:50 p.m. EST Fri.)
The 76-meter (250-foot) Lovell Telescope at Jodrell Bank Observatory in Cheshire, U.K., was unable to hear any transmissions from the Beagle 2 lander tonight, the project has confirmed.
The radio telescope performed a scan of the sky while Mars was visible to the observatory.
"At present, Beagle 2 should be sending a pulsing on-off signal once a minute (10 seconds on, 50 seconds off). Some 9 minutes later, this very slow "Morse Code" broadcast should reach Earth after a journey of some 98 million miles (157 million km)," Beagle officials said in a statement.
"Although the Beagle's transmitter power is only five watts, little more than that of a mobile phone, scientists are confident that the signal can be detected by the state-of-the-art receiver recently installed on the Lovell Telescope. However, a significant drop in signal strength would require rigorous analysis of the data before it could be unambiguously identified.
"Although the ground-based radio telescopes will not be able to send any reply, the new information provided by detection of the transmission from Beagle 2 would enable the mission team to determine a provisional location for Beagle 2. This, in turn, would allow the communications antenna on Mars Odyssey to be directed more accurately towards Beagle 2 during the orbiter's subsequent overhead passes."
The next Odyssey flight over the landing site will occur in a few hours.
FRIDAY, DECEMBER 26, 2003
2355 GMT (6:55 p.m. EST)
Thus far this evening the Jodrell Bank radio observatory has not detected any signals from Beagle, project officials report.
2055 GMT (3:55 p.m. EST)
Mars Odyssey heard no signal from Beagle when it flew over the landing site earlier today.
Jodrell Bank radio telescope in the U.K. will continue listening this evening in hopes of detecting a transmission from the tiny lander that is supposed to be operating on the surface of Mars.
Odyssey will try again at 0657 GMT (1:57 a.m. EST) and 1814 GMT (1:14 p.m. EST) on Saturday.
"While initial attempts to detect a signal from the tiny spacecraft have failed, further efforts are scheduled whenever possible during the next few days," the European Space Agency says.
1835 GMT (1:35 p.m. EST)
If this current Mars Odyssey shot to find the Beagle lander is unsuccessful, there are several more attempts planned over the next few days.
According to the European Space Agency, the next sweep by Jodrell Bank will occur between 2320 and 2400 GMT (6:20-7 p.m. EST) tonight, although this giant radio telescope likely will begin looking earlier than that window.
On Saturday, Mars Odyssey passes over the landing site again at 0657 GMT (1:57 a.m. EST). Jodrell Bank becomes available between 2316 and 2356 GMT (6:16 and 6:56 p.m. EST).
Odyssey will continue the search daily and the Stanford University radio telescope in the U.S. is expected to also join in the effort, ESA said.
"If all those attempts are unsuccessful, then Mars Express itself flies over the landing site in the first week of January 2004. Of all these potential signal detectors, Mars Express is the only one that has been specially designed and tested to transmit and receive signals from Beagle 2.
"The hope is strong that the Mars Express orbiter will be successful in this task," ESA officials said.
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Psalm 19:1
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