THE PLEIADES ON APRIL 26, 2009

THE PLEIADES
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Known pre-historically. Mentioned by Homer about 750 B.C., by biblical Amos about 750 B.C., and by Hesiod about 700 B.C.


The Pleiades, also known as Messier 45 (M45), 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 (see below).

According to Kenneth Glyn Jones, the earliest known references to this cluster are mentionings by Homer in his Ilias (about 750 B.C.) and his Odyssey (about 720 B.C.), and by Hesiod, about 700 B.C.. According to Burnham, they were seen in connection to the agricultural seasons of that time. Also, and the Bible has three references to the Pleiades (the Hebrew "Kiymah"): Job 9:7-9, Job 38:31-33, and Amos 5:8; the prophet Amos is believed to have given his message in 750 B.C. or 749 B.C., while there is no consent on the dating of the book of Job: Some believe it was written about 1,000 B.C. (the regency of Kings David and Solomon in old Israel) or earlier (Moses, about 13th to 16th century B.C.), others give reasons that it may have been created as late as the 3rd to 5th century B.C.. The present author [hf] does not know if the cluster is mentioned in one of the earlier Assyrian or Sumerian sources.

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.

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Also note the Pleiades map.
Today, April 26, 2009 at sunset, they can be seen with the naked eye, to the West, midway between the Moon and Mercury.
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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.


About 1846, German astronomer Mädler (1794-1874), working at Dorpat, noticed that the stars of the Pleiades had no measurable proper motion relative to each other; from this he boldly concluded that they form a motionless center of a larger stellar system, with star Alcyone in the center.
This conclusion was to be, and was, rejected by other astronomers, in particular Friedrich Georg Wilhelm Struve (1793-1864). Nevertheless, the common proper motion of the Pleiades was a proof that they move as a group in space, and a further hint that they form a physical cluster.

Longer exposure photographs (and also short focal ratio, i.e. short focal length compared to their aperture, "rich field" telescopes of considerably good quality, especially good binoculars) have revealed that the Pleiades are apparently imbedded in nebulous material, obvious in one of the images, which was taken by David Malin with the UK Schmidt Telescope, and is copyrighted by the Royal Observatory Edinburgh and the Anglo-Australian Observatory.


The Pleiades nebulae are blue-colored, which indicates that they are reflection nebulae, reflecting the light of the bright stars situated near (or within) them.

The brightest of these nebulae, that around Merope, was discovered on October 19, 1859 by Ernst Wilhelm Leberecht (Wilhelm) Tempel at Venice (Italy) with a 4-inch refractor; it is included in the NGC as NGC 1435.
Leos Ondra has made the biography of Wilhelm Tempel available online together with a drawing of the Merope Nebula, and has agreed to include it in this database.

The extension to Maia was discovered by the brothers Paul and Prosper Henry in Paris on a photographic plate taken on November 16, 1885; this is NGC 1432 or the Maia Nebula.

The nebulae around Alcyone, Electra, Celaeno and Taygeta were found on photographs in the later 1880s.

The full complexity of the Pleiades nebulae was revealed by the first astro cameras, e.g. by that of the brothers Henry in Paris and Isaac Roberts in England, between 1885 and 1888.

In 1890, E.E. Barnard discovered a starlike concentration of nebulous matter very close to Merope, which found its way into the IC as IC 349.

The analysis of the spectra of the Pleiades nebulae by Vesto M. Slipher in 1912 reveiled their nature as reflection nebulae, as their spectra are exact copies of the spectra of the stars illuminating them.


Physically, the reflection nebula is probably part of the dust in a molecular cloud, unrelated to the Pleiades cluster, which happens to cross the cluster's way.
It is not a remainder of the nebula from which the cluster once formed, as can be seen from the fact that the nebula and cluster have different radial velocities, crossing each other with a relative velocity of 6.8 mps, or 11 km/sec.

According to new calculations published by a team from Geneva (Meynet et.al. 1993), the age of the Pleiades star cluster amounts 100 million years.
This is considerably more than the previously published "canonical" age of 60--80 million years (e.g., the Sky Catalog 2000's 78 million).

It has been calculated that the Pleiades have an expected future lifetime as a cluster of only about another 250 million years (Kenneth Glyn Jones); after that time, they will have been spread as individual (or multiple) stars along their orbital path.

The distance of the Pleiades cluster has been newly determined by direct parallax measures by ESA's astrometric satellite Hipparcos; according to these measurement, the Pleiades are at a distance of 380 light years (previously, a value of 408 light years had been assumed).
This value would have required an explanation for the comparatively faint apparent magnitudes of the Pleiades stars.

However, subsequent investigations with the Hubble Space Telescope and the Mount Palomar and Mount Wilson Observatories have finally shown that the Hipparcos distance is probably too small: By acurate parallaxes of Pleiades stars, this cluster is at a distance of 440 +/-6 light-years.

The Trumpler classification is given for the Pleiades as II,3,r (Trumpler, according to Kenneth Glyn Jones) or I,3,r,n (Götz and Sky Catalog 2000), meaning that this cluster appears detached and strong or moderately concentrated toward its center, its stars are spread in a large range of brightness, and it is rich (has more than 100 members).

Some of the Pleiades stars are rapidly rotating, at velocities of 150 to 300 km/sec at their surfaces, which is common among main sequence stars of a certain spectral type (A-B).
Due to this rotation, they must be (oblate) spheroids rather than spherical bodies. The rotation can be detected because it leads to broadened and diffuse spectral absorption lines, as parts of the stellar surface approach us on the one side, while those on the opposite side recede from us, relative to the star's mean radial velocity.

The most prominent example for a rapidly rotating star in this cluster is Pleione, which is also variable in brightness between mag 4.77 and 5.50 (Kenneth Glyn Jones).

It was spectroscopically observed that between the years 1938 and 1952, Pleione has ejected a gas shell because of this rotation, as had been predicted by O. Struve.

Cecilia Payne-Gaposhkin mentions that the Pleiades contain some white dwarf (WD) stars.

These dwarf stars give rise to a specific problem of stellar evolution:
How can white dwarfs exist in such a young star cluster ?

As it is not only one, it is most certain that these stars are original cluster members and not all field stars which have been captured (a procedure which does not work effectively in the rather loose open clusters anyway).

From the theory of stellar evolution, it follows that white dwarfs cannot have masses above a limit of about 1.4 solar masses (the Chandrasekhar limit), as they would collapse due to their own gravitation if they were more massive. But stars with such a low mass evolve so slow that it takes them billions of years to evolve into that final state, not only the 100 million year age of the Pleiades cluster.

The only possible explanation seems to be that these WD stars were once massive so that they evolved fast, but due to some reason (such as strong stellar winds, mass loss to close neighbors, or fast rotation) have lost the greastest part of their mass.
Possibly they have, in consequence, lost another considerable percentage of their mass in a planetary nebula.
Anyway, the final remaining stars (which was previously the star's core) must have come below the Chandrasekhar limit, so that they could go into the stable white dwarf end state, in which they are now observed.

New observations of the Pleiades since 1995 have revealed several candidates of an exotic type of stars, or starlike bodies, the so-called Brown Dwarfs.

These hitherto hypothetical objects are thought to have a mass intermediate between that of giant planets (like Jupiter) and small stars (the theory of stellar structure indicates that the smallest stars, i.e. bodies that produce energy by fusion somewhen in their lifetime, must have at least about 6..7 percent of one solar mass, i.e. 60 to 70 Jupiter masses).

So brown dwarfs should have 10 to about 60 times the mass of Jupiter.
They are assumed to be visible in the infrared light, have a diameter of about or less that of Jupiter (143,000 km), and a density 10 to 100 times that of Jupiter, as their much stronger gravity presses them tougher together.

Even with the naked eye and under modest conditions, the Pleiades are rather easily found, roughly 10 degrees north-west of the bright red-giant star Aldebaran (87 Alpha Tauri, mag 0.9, spectral type K5 III).

Apparently surrounding Aldebaran is another, equally famous open cluster, the Hyades; Aldebaran is known to be a non-member foreground star (at 68 light years distance, compared to 150 ly for the Hyades).

The cluster is a great object in binoculars and rich-field telescopes, showing more than 100 stars in a field about 1 1/5 degrees in diameter.
In telescopes, it is frequently even too large to be seen in one lowest magnification field of view.
A number of double and multiple stars are contained in the cluster.

The Merope Nebula NGC 1435 requires a dark sky and is best visible in a rich-field telescope (Tempel had discovered it with a 4-inch telescope).

As the Pleiades are situated close to the ecliptic (4 degrees off), occultations of the cluster by the Moon occur quite frequently:
This is a very appealing spectacle, especially for amateurs with less expensive equipment (actually, you can observe it with the naked eye, but even the smallest binoculars or telescopes will increase observing pleasure -- the March 1972 Pleiad occultation was one of the first amateur astronomical experiences of the present author).

Such events demonstrate the relations of the apparent sizes of the Moon and the cluster: Burnham points out that the Moon may be "inserted into the quadrangle formed by" Alcyone, Electra, Merope and Taygeta (Maia, and possibly Asterope, is occulted in this situation).

Also, planets come close to the Pleiades cluster (Venus, Mars, and Mercury even occasionally pass through) to give a conspicuous spectacle.


As mentioned in the description for the Orion Nebula M42, it is a bit unusual that Messier added the Pleiades (together with the Orion Nebula M42/M43 and the Praesepe cluster M44) to his catalog, and will perhaps stay subject to speculation.











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