Authors



Egor Eremeev, Stanislav Pachkov, Artem Khvastunov
Irina Guseva

Introduction

Many a night I saw the Pleiades, rising thro' the mellow shade,
Glitter like a swarm of fireflies tangled in a silver braid.
- Alfred, Lord Tennyson, 1837-8, Locksley Hall


The Pleiades is an open star cluster, also known as the Seven Sisters and Messier 45. It is a conspicuous object in the night sky with a prominent place in ancient mythology.

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).

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". 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.

The old Russian name of the Pleiades is "stozhary" that means "100 fires".

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.

The Pleiades were the seven daughters of Atlas and Pleione, and half-sisters of the Hyades, whose mother was Ethra (`bright sky'; a different Ethra than the mother of Theseus). Both Pleione and Ethra were Oceanids, daughters of Oceanus and Tethys, the titans who ruled the outer seas before being replaced by Poseidon. Atlas, another titan, led their war against the gods, and was afterward condemned by Zeus to hold up the heavens on his shoulders. The Pleiades were also nymphs in the train of Artemis, and together with the seven Hyades (`rainmakers' or `piglets'; individual Hyad names are not fully agreed upon) were called the Atlantides.

One day the great hunter Orion saw the Pleiads (perhaps with their mother, or perhaps just one of them) as they walked through the Boeotian countryside, and fancied them. He pursued them for seven years, until Zeus answered their prayers for delivery and transformed them into birds (doves or pidgeons), placing them among the stars. Later on, when Orion was killed, he was placed in the heavens behind the Pleiades, immortalizing the chase.

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 Madler (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 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 image, which was taken by David Malin with the UK Schmidt Telescope.
A Peruvian cosmological chart from around 1613 seems to show the Pleiades. Pachacuti Yamqui, an Inca nobleman, drew the chart to show the objects that were depicted on the temple in Cusco, adding Spanish and Quechua notations

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. The extension to Maya was discovered in 1875 (this is NGC 1432), the nebulae around Alcyone, Electra, Celaeno and Taygeta in 1880. 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. 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.

Open clusters as phenomena

In 1767, Reverend John Michell (Michell 1767) derived that clusters were most probable physically related groups rather than chance collections of stars, by calculating that it would be extremely improbable (1/496,000) to find even one cluster like the Pleiades anywhere in the sky, not to speak of the number of then-known open clusters; moreover he presumed that all or at least most then-known nebulous objects actually were composed of stars. The finding of common proper motion by Madler for the Pleiades and other stellar groups, including the Ursa Major Moving Cluster by Proctor (Proctor 1869), further established the physical relationship between cluster stars. Finally, spectroscopy was needed to show the common radial motion (velocity) of the cluster stars, and to show that the stars perfectly match in a Hertzsprung-Russell diagram (HRD), indicating that they all lie at roughly the same distance. The final confirmation of the roughly common distance came only from the direct measurement of parallaxes for a number of nearby clusters, both from Earth-bound observatories and from ESA's astrometric satellite Hipparcos.

Open clusters are physically related groups of stars held together by mutual gravitational attraction. Therefore, they populate a limited region of space, typically much smaller than their distance from us, so that they are all roughly at the same distance. They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.

Open clusters are of great interest for astrophysicists because of these properties:

  • the stars in a cluster are all at about the same distance
  • the stars have approximately the same age
  • the stars have about the same chemical composition
  • the stars have different masses, ranging from about 80-100 solar masses for the most massive stars in very young clusters to less than about 0.08 solar masses.

    Therefore, they represent samples of stars of constant age and/or constant chemical composition, suited for study with respect to stellar structure and evolution, and to fix lines or loci in many state diagrams such as the color-magnitude diagram (CMD), or Hertzsprung-Russell diagram (HRD).

    Comparing the "standard" HRD, derived from nearby stars with sufficiently wellknown distances, or the theory of stellar evolution, with the measured CMD of star clusters, provides a considerably good method to determine the distance of star clusters. Comparing their HRD with stellar theory provides a reasonable way to estimate the age of star clusters.

    The result that all the cluster HRDs can be explained by the theory of stellar evolution gives convincing evidence for this theory, and moreover for the underlying physics including nuclear and atomic physics, quantum physics and thermodynamics.

    Over 1100 open clusters are known in our Milky Way Galaxy, and this is probably only a small percentage of the total population which is probably some factor higher; estimates of as many as about 100,000 Milky Way open clusters have been given.

    Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years. The escaped individual stars continue to orbit the Galaxy on their own as field stars: All field stars in our and the external galaxies are thought to have their origin in clusters quite probably.

    Open clusters are often typized according to a simple scheme which goes back to Harlow Shapley, which describes richness and concentration roughly:

    c
    very loose and irregular
    d
    loose and poor
    e
    intermediately rich
    f
    fairly rich
    g
    considerably rich and concentrated

    Another important and more sophisticated scheme was introduced by R.J. Trumpler (Trumpler 1930). This scheme consists of three parts, characterizing the cluster's degree of concentration, the range of brightness of its stars, and the richness, as follows:

    Concentration
    I
    Detached; strong concentration toward center
    II
    Detached; weak concentration toward center
    III
    Detached; no concentration toward center
    IV
    Not well deteached from surrounding star field
    Range in Brightness
    1
    Small range in brightness
    2
    Moderate range in brightness
    3
    Large range in brightness
    Richness
    p
    Poor: Less than 50 stars
    m
    Moderately rich: 50 to 100 stars
    r
    Rich: More than 100 stars
    A "n" following the Trumpler class indicates that there is nebulosity associated with the cluster.

  • The information of the Pleiades

    Right Ascension 03 : 47.0 (h:m)
    Declination +24 : 07 (deg:m)
    Distance 380 (ly)
    Visual Brightness 1.6 (mag)
    Apparent Dimension 110.0 (arc min)

    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.

    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, G. Meynet, J.-C. Mermilliod, and A. Maeder in Astron, 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). The new value requires an explanation for the comparatively faint apparent magnitudes of the Pleiades stars.

    The Trumpler classification is given for the Pleiades as II,3,r (Trumpler, according to Kenneth Glyn Jones) or I,3,r,n (Gotz 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 leades 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 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, 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.

    Observations

    We observed the Pleiades with small CCD astrograph (AKD) at Pulkovo observatory (St Petersburg). Lens diameter of this telescope is 10 cm, focal length - 71 cm. Instrument operates with CCD camera ST-8 having 1500*1000 pixels, 9*9 mkm. Field of view of this telescope with CCD is 45 by 67 arc minutes, scale is 2.5 arcsec/pixel. The spectral responce of the CCD allows to use photometric B,V,R filter for our observations.

    Our goal was to create the color image of the Pleiades on the base of CCD frames obtained with blue, green and red filters.

    Observations were made on 13 December 2004 with B,V R photometric filters, 3 frames with each filter with exposure 2 minutes. The Field of View of our CCD astrograph is not so large to get the image of the whole cluster. We centered our telescope at Alcyone, the frames contain also Pleione, Atlas, Merope, Asterope, Maja and Taygeta.

    "Blue" image (B filter)

    "Green" image (V filter)

    "Red" image (R filter)

    After preliminary processing the frames (taking into account dark current, flat field, making some fitreing procedures, frame convertaton, e.t.c.) we combined blue, green and red images using the CCDOPS 5.0 software provided by SBIG.

    The final color image show that the Pleiades region contain large amount of blue and green stars. It is evident confirmation of well known fact that the Pleiades is young cluster containing a lot of young hot stars.

    Our Work

    The Pleiades is the thorougly investigated open cluster, one of the nearest to the Sun. It is well known that the Pleiades consist mainly of young stars with large common proper motion.

    We tried to understand how different stars are distributed in the region of the Pleiades and in the cluster and how they move in space. To do this we choose all stars in the region of Pleiades from Hipparcos and Tycho-2 catalogues (from Cartes du Ciel software). Then we draw charts that show the proper motion of each star in the field.
    (Our C++ program)

    Our work consist of two parts: in the first we used Hipparcos data, in the second - Tycho-2.

    First of all, we choose all stars in the Pleiades region that have parallaxes (distanses) measured by Hipparcos satellite.

    Then we distributed these stars (total - 58) by their distances from the Sun.

    The common opinion is that the Pleiades are located at the mean distance ~ 380 ly. So, first we divided all stars in 3 groups:

    1. Distances less then 300 ly
    2. Distances betweeen 300 and 480 ly
    3. Distances more then 480 ly

    But it appears that some stars behind 480 ly have proper motions and colors (b-v) very similar to the main part of the Pleiades cluster. So, finally, we decided to distribute all stars in the following groups:

    1. Distances < 300 - nearest stars
    2. Distances 300-600 - stars of the Pleiades cluster
    3. Distances > 600 - background stars

    The following animation show these stars and their properties. Stars with different B-V (colors) are shown by different colors (blue - B-V < 0.20, green - 0.2 < B-V < 0.6, orange - 0.6 < B-V < 1.0, red - B-V > 1.0). One can see that the nearest stars have large proper motions without evident common direction. But almost all stars located between 300 and 600 ly are blue or green and move in a common direction, that confirms their physical connection. It is stressed by the background stars, that are of different colors and very small proper motions.




    Taking into account the results obtained on the distribution of the cluster stars by their proper motions and colors, we decided to extract from the Tycho-2 catalogue all stars that have similar characteristics. Proper motions should be between 0 and 0.05 by right ascention, between -0.11 and -0.02 by declination and B-V less than 1. Next picture shows the difference between chosen stars and other in the Pleiades region.




    The Tycho-2 catalogue contains much more stars than Hipparcos, but we don't know parallaxes of these stars. We can expect only that the major part of stars with similar parameters belongs to the cluster. First, we determined the mean proper motion of all stars that supposed to be in the cluster. Then we excluded this mean proper motion from that of each star and distributed all stars in three groups by their colors: blue - B-V less than 0.2, green - 0.2 < B-V < 0.6, yellow - 0.6 < B-V < 1.

    The following animation shows an interesting result. One can see that younger (blue) stars are located mainly in the center of the cluster, while green and yellow stars are dissipated from the center.




    Certainly, we cannot say that all chosen stars belong to the cluster, and we cannot say, that we choose all stars of the cluster. Such investigation may be done if we know distances to each star, but, unfortunately we have this data only for the Hipparcos stars.

    Conclusions

    We obtained evident (visual) confirmation of known facts about open clusters and the Plejades particularly. We show that almost all stars in the cluster have similar colors and proper motions, distinguished from the nearest and background stars.

    We could not expect that the picture of the open cluster dissipation will be so evident - the younger are stars, the closer is concentration of stars to the center of cluster. It seems like the different color stars form the different color concentric rings, that especially confirmed by the picture with Hipparcos stars.

    References

    1. www.ras.ucalgary.ca/~gibson/pleiades
    2. www.crystalinks.com/pleiades.html
    3. antwrp.gsfc.nasa.gov/apod/ap021201.html
    4. www.seds.org/messier/open.html
    5. www.sbig.com
    6. www.astrosurf.com/astropc
    7. P.G.Kulikovskij "Spravochnik lyubitelya astronomii"; Editorial URSS, Moskva, 2002
    8. "Fizika kosmosa", redaktor S.B.Pikel'ner; Sovetskaya enciklopediya, Moskva, 1976
    9. Eckhard Slawik, Uwe Reichert "Atlas der Sternbilder"; Berlin, 1998