Sunday, September 02, 2007


As Molly mentioned yesterday (see Sept. 1- 'Lunar Occultation of the Pleiades Tomorrow') tonight the Moon will occult 5 of the "seven sisters" that make up the Pleiades star cluster. The event will begin shortly after midnight here in Central Daylight Time. For the time in other time zones visit the Time and Date.Com website. For a forecast of visibility conditions in numerous places across North America see the Clear Dark Sky website.
The Pleiades are an "open star cluster". These are star groups which were formed by the same interstellar molecular cloud and are still loosely gravitationally bound to each other. This is in contrast to "globular clusters" where the stars are tightly bound to each other. Open clusters form only in spiral or irregular galaxies, and are usually less than a few hundred million years old. This is because they are gradually disrupted by encounters with other objects in their home galaxy as well as by close encounters within the cluster itself. Over time radiation pressure drives the remaining gas within the cloud from which the stars form away from the cluster. Typically only about 10% of the material ends up condensing into stars. Star clusters are important for the study of stellar evolution because they are a 'control group' that are quite similar in age and chemical composition.

Some star clusters such as the Pleiades have been known since ancient times. Others appear merely as fuzzy patches to the naked eye. Globular clusters are more common towards the centre of galaxies, and consist of thousands of stars in a regular spherical distribution. Open clusters are found in any direction in the sky and have a much sparser stellar population. They do, however, form almost only in the plane of galaxies such as the Milky Way, and hence they are sometimes referred to as galactic clusters.

Stars always form in groups because only a cloud of gas containing the mass of many stars has enough matter to collapse under gravity. Many things such as shock waves from nearby supernovae or the graviational influence of other bodies may initiate the process of collapse. Several thousand stars are usually formed in the process. In the Milky Way such formations are estimated to occur every few thousand years. Once the process is underway the hottest and largests stars, of class O and B, emit radiation sufficient to ionize the surrounding gas, producing what is known as an H II region. The radiation from these ultrahot stars and the first few supernovae both act to expell the remaining gas from the area of the cluster, and in afew tens of millions of years there will no be enough to support further star formation.

Clusters can be classified by the Trumpler system devised by Robert Trumpler in 1930. The Roman numberals I to IV indicate the "concentration" of stars in the cluster, from highly concentrated to weakly concentrated). The arabic numerals 1 to 3 indicate the range of brightness of the stars, from a small to a large range. The letters "p", "m" or "r" are added to say if the cluster is poor, medium or rich in stars. The letter "n" is added if the cluster lies with enough gas to produce a visible nebula. By this method the Pleiades are an I3rn system. The Pleiades are one of over 1,000 known open star clusters in our galaxy. There are probably many times more yet to be discovered. As mentioned above open star clusters are found only in spiral or irregular galaxies. In elliptical galaxies star formation has essentially ceased many millions of years ago, and hence the young stars found on open clusters are not present.

Open clusters are essentially unstable. In many the average speed of the individual stars formed is greater than the escape velocity from the cluster. The stripping away of extrastellar gas alluded to above also reduces the mass of the cluster and hence its gravitational pull. Those clusters with enough mass to avoid early dispersal may remain in existence for tens of millions of years, but eventually processes both internal (close encounters between stars) and external (encounters with molecular gas clouds) lead to the disruption of the cluster.

Because the stars in an open cluster are of about the same age, the same distance from Earth and of roughly the same material they are useful in studying and testing models of stellar evolution. They are also useful in determining astronomical distance scales. Some clusters such as the Pleiades are close enough that their distance can be measured directly by parallex (the small change in apparent position when the Earth is at one or the other side of the Sun during a year. Many cluster distances have been determined by the Hipparcos satellite using this method. Another method is to use the "proper motion" of members of the cluster which converge on a "vanishing point". This along with the radial velocity from the Doppler shift of the stars and the angle from the cluster to its vanishing point give the distance by simple trignometry. Distances to clusters too far to estimate by these methods can be estimated by comparing a Hertzprung Russell diagram from a cluster of a known distance to a close fit for the more distant cluster of interest. A comparison of the apparent magnitude of stars in the distant cluster to the absolute magnitude of stars whose distance is known allows an estimate of distance to be made.
The Pleiades comprise not just the "seven sisters" but also stars named after their mythological parents, Atlas and Pleione(see diagram at the top of this page). You can find the Pleiades by looking down towards the horizon from the constellation Perseus (see articles in the August archive of this blog about the Perseid meteor shower). Both the Pleiades and Perseus are just above a line drawn due east from the two upper stars of the "Big Dipper". As the year advances the Pleiades will gradually come to lie more east than down from Perseus. You have to have good eyesight to distinguish individual stars in the Pleiades, but ancient people were able to distinguish at least 6 of the "7 sisters". Many cultures, both European and Native American, used the Pleiades as a sort of celestial eye-chart. The keenness of your vision could be estimated by how many stars you could see in the group.

This star group is prominant in many cultures across the world. In Japan the group is known as Subaru which gave its name to the car manufacturing company which uses the stars in its logo. In China they are the Hairy Head of the white Tiger of the West. In India they are known as the Kuttika which means "the cutters". They are also called the star of fire and are associated with the Vedic God Agni, the fire god. Other Hindu myths say they are the six mothers of the war god Skanda who developed six faces for each one of them. The star group is mentioned in both Homer's Illiad and Odyssey, as well as in Hesiod's Works and Days. They mentioned three times in the Bible (Job 9:9, 38:31 and Amos 5:8), and some Muslim scholars believe that they are the Star of Najm mentioned in the Koran.

Indiginous Australians have many myths about the group. One of these is that the Pleiades is a woman who escaped being raped by the 'Man in the Moon'. Another myth is that they are seven sisters who are being pursued by a man named Jilbi Tjapaltjarri. They were transformed into stars to escape his pursuit. In return Jilbi trandformed himself into one of the stars in Orion's belt so that he could continue his pursuit across the sky.

The Vikings referred to them as Freyja's hens , and their name in many European languages compares them to a hen with chicks. To the Celts the stars were associated with mourning and funerals. This was because during the Bronze Age this cluster rose in the eastern sky just as the Sun was setting in the west at the time of Samhain (our modern Halloween), the festival of remembrance of the dead. Because of precession this no longer holds true, but it held true long enough to become embedded in Celtic mythology.

In Africa the Pleiades are known as kilimia in the Swahili language. This translates as the "digging stars" as it was time to start digging as the onset of the annual rains was near when they were visible. In southern Africa the Basotho people call the Pleiades seleme se setshehadi , the "female planter" because their disappearance in april and the appearance of the star Achernar signals the beginning of the cold season. Most peoples in southern Africa associated the visibility of the Pleiades with the time of agriculture and plenty.

In the New World the Seris of northwestern Mexico call the Pleiades Cmaamc ie women as they are supposed to be seven women giving birth. Mayan cultures in Guatemala used the Pleiades and also the star Eta Draconis as references for astronomy. The Pleiades were believed to be their original home by some Mayans. The Aztecs of Mexico borrowed this method of reckoning, and based their calender on these stars. Their year began when they were first observed the heliacal rising in the east just before being obliterated by the rising Sun. This was generally in our month of June. Both the Maoris of New Zealand and the original Hawaians also set their new year by this method.
The modern names of the stars in this cluster are taken from Greek mythology. According to this story the "seven sisters" are the daughters of the titan Atlas and the sea-nymph Pleione (see the diagram for the stars named after this pair). They were also sisters to other figures from Greek mythology including Calypso, Hyas, the Hyades and the Hesperides. The Pleiades were nymphs in the service of the goddess Artemis, and along with the Hyades served as the nursemaids and tecahers of the infant god Bacchus. As the story goes when Atlas was forced to hold the world on his shoulders Orion began to pursue them. To help them escape Zeus first changed them into doves, and then,when this proved insufficient, into stars. orion the hunter is said to still pursue them through the sky.

The star Atlas is a blue giant star about 290 lys from Earth. His consort Pleione is blue subgiant about 95 light years (lys) away. Atlas has a magnitude of 3.6, and Pleione shines at magnitude 5.1. Their daughters mated with several of the gods and produced a number of children.

The eldest of the seven sisters was Maia. She mated with Zeus to give birth to the god Hermes. Maia is another blue giant that lies about 390 lys from Earth and has an apparent magnitude of 3.9. Electra became the wife of Corythus but was seduced by Zeus and gave birth to Darnanus (the Dardanelles) who became the founder of the city of Troy. Electra is a blue giant as well, lieing about 390 lys away and shining with a 3.7 magnitude. Alcyone is yet another blue giant star 240 lys distant and with a magnitude of 2.9- the brightest of this star group. She mated with Poseidon and bore him many children.

Taygete was raped by Zeus on Mount Taygetos where she was in attendance on Artemis. The goddess changed her into a doe to escape the god's pursuit, but Zeus continued anyways. The issue of this union was supposed to have been Lacedaemon, the mythical founder of Sparta. Taygete is a blue main sequence star about 360 lys away with an apparent magnitude of 4.3. Another of the stars is Celaeno. She married Poseidon and bore him several children.Celaeno is a blue subgiant about 590 lys away and has an apparent magnitude of 5.5. Sterope mated with the god Ares and produced the son Oenomaus. This star Sterope, also called Asterope, is a blue main sequence star about 490 lys away and has an apparent magnitude of 5.8. The final star in the cluster Merope was the only one of the sisters who escaped divine attention. She was the wife of sisyphus and bore him one son, Glaucus. This star is a blue main sequence star about 390 lys away and has an apparent magnitude of 4.2.

As Molly mentioned yesterday only 5 of the seven sisters are being occulted tonight. The Moon will pass above the stars Alcyone and Merope, as well as Atlas and Pleione. These will be visible throughout the occultation. Have fun and see if you can identify each star in this group.

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