The Pleiades in reality… the Pleiades star cluster

The Pleiades have inspired a wealth of mythology and legends: fascinating as these are the reality the star cluster is profoundly more wonderful. Historically, the Pleiades were seen as a group of 'seven' stars – its brightest stars: Alcyone, Atlas, Electra, Maia, Merope, Taygeta and Pleione are visible to the keen naked eye. However modern observations show that this most famous of open clusters is comprised of several hundred stars wreathed in intricately structured nebulosity.

At a distance of about 440 light years from the Earth, the Pleiades are one of the nearest galactic open clusters. The brightest stars in the cluster (Alcyone is magnitude +2.8, and Pleione +5.1) are distributed over about seven light years and although faint to naked sight these stars are from 40 to 1000 times brighter than our Sun. From the Earth the cluster's apparent size is 110 minutes of arc (almost 2°) in the plane of the ecliptic: in comparison, the diameter of a full Moon is about 0.5°.

The cluster has an apparent motion relative to the Earth of an angular rate of just over five seconds of arc per century towards the star lambda Tauri, that is, in a south-easterly direction. Thus the Pleiades takes some 60,000 years to traverse one degree.

The Pleiades exhibits one of the finest and nearest examples of a reflection nebula associated with a cluster of young stars. The nebulosity seen here is light reflected from the particles in an interstellar cloud of cold gas and dust into which the cluster has drifted. The apparent blue colour is due to the preferential scattering of blue light by these tiny interstellar particles and is 'streaky' in structure since the particles have been aligned by the magnetic fields between the stars.

The Pleiades - copyright AAO/ROE

This beautiful image of the Pleiades cluster was produced by David Malin of the Anglo-Australian Observatory and is the copyright of the Anglo-Australian Observatory and the Royal Observatory, Edinburgh. The image is part of the AAO UK Schmidt series: the width is about 100 arc minutes, with the north east in the upper right. It is reproduced with the consent of the AAO. Resource: A high resolution image (113k).

Other images of the cluster:

The Location of the Pleiades in the sky

Right Ascension: 3h 47'
Declination: 24° 07'

The structure of the Pleiades

Modern observing methods have revealed that over 400 mostly faint stars form the Pleiades star cluster. These are spread over a 110 minutes of arc or nearly 2 degree field at a low density in comparison to other nearby open clusters. The total mass contained in the cluster is estimated to be about 800 solar masses.

The Trumpler classification for the Pleiades as II,3,r (Trumpler, according to Kenneth Glyn Jones) or I,3,r,n (Götz and Sky Catalog 2000) – that is, the cluster appears detached and strong to moderately concentrated toward its centre, the stars exhibit a wide range of brightness, and it is rich (having more than 100 members). The central part of the cluster is spherical in form, with a core radius of slightly over 4.5 light years, however the outer part is markedly elliptical, with an ellipticity of 0.17. The tidal radius of the entire cluster is estimated at about 52 light years. [ Investigation of the Pleiades cluster IV. The radial structure. Authors: Raboud, D.; Mermilliod, J.-C. 1998: Astronomy & Astrophysics. ]

The nebulae

The brightest of these nebulae, around Merope, was discovered in 1859 by Ernst Wilhelm Leberecht (Wilhelm) Tempel at Venice with a 4-inch refractor. The nebula to Alcyone was discovered in 1875, and the nebulae around Electra, Celaeno and Taygeta in 1880. Their full complexity was revealed by the first astronomical cameras of the brothers Henry in Paris and Isaac Roberts in England, between 1885 and 1888.

The analysis of the spectra of the Pleiades nebulae by Vesto Slipher in 1912 showed them to be reflection nebulae, as their spectra correspond to the spectra of the associated stars.

Radio and infrared observations in the 1980s established that the nebulosity associated with the Pleiades results from a chance collision of the young stars with an interstellar cloud, rather than being the residual debris from the cluster's formation, as was previously believed.

A 3 car train wreck?

New data obtained at Kitt Peak National Observatory suggest that the Pleiades are actually encountering two clouds – a rare three-body collision in the vast emptiness of interstellar space.

Richard White, [note 1] of Smith College in Northampton, MA, interprets the new observations of sodium atoms in the Pleiades region in the context of other recent observations of the Pleiades region. These observations include significant new optical images of the Pleiades from the Burrell Schmidt telescope on Kitt Peak, published earlier this year in the Astrophysical Journal by Steven Gibson of the University of Calgary and Kenneth Nordsieck of the University of Wisconsin, [note 2] and applied Dr. Gibson's radio maps of neutral hydrogen in the cluster.

Sodium atoms in gas found in interstellar space absorb two specific wavelengths of yellow starlight. Because of the Doppler effect, the motion of the gas along our line of sight produces subtle shifts in the observed wavelengths.

The orientation of features in the optical and radio imagery provides clues to gas and dust motions across the sky, which can be combined with the spectroscopically measured velocities from Kitt Peak to allow astronomers to reconstruct the three-dimensional configuration of the interstellar matter near the Pleiades.

The sodium absorption lines reveal that there always is one feature between Earth and the Pleiades stars. This is moving toward the cluster with a line of sight velocity of about 10 kilometers per second. White associates this feature with the Taurus-Auriga interstellar cloud complex, the bulk of which lies about 40 light-years to the east.

However, toward some stars there are two or more absorption features. White argues that a shock-wave from the collision between the Pleiades and gas associated with the Taurus-Auriga complex can account for splitting of one feature into three in some areas, primarily on the south and east sides of the Pleiades. Even so, White concludes that the presence of an additional feature in the data, primarily on the west side and moving into the cluster at about 12 kilometers per second, defies understanding unless a second cloud also is converging The Pleiades.

The wake of the Pleiades

A large emission 'cavity' has been recently discovered whose bright rims extend about 5 deg eastward from the Pleiades and is pressurized by the soft-UV radiation of the cluster. This structure delineates the wake of the Pleiades as it moves through the Interstellar Medium [note 3]. The trajectory of the principal gas cloud forming the Pleiades nebulae can be traced back to an origin in Gould's Belt some 15 Million years ago, in a blowout of gas from an exploding star (PSR 1919+21) into the Galactic halo.

Star types in the cluster

Alcyone (eta Tauri) is a significant example of a nearby multiple star. It is a giant star of more than 10 Solar masses and is almost a thousand times more luminous than the Sun. It is orbited by 3 faint companion stars. The entire cluster contains numerous double stars and a few triples.

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 regular 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 of a rapidly rotating star in the cluster is the varible star Pleione (variablity mag 4.77 to 5.50). Spectroscopic observations have shown that between 1938 and 1952 the rapid rotation of Pleione caused the ejection a gas shell.

Named stars in The Pleiades - copyright AAO/ROE

Cecilia Payne-Gaposhkin has observed that the Pleiades contain several White Dwarf stars. These stars give rise to a specific problem of stellar evolution: How can white dwarfs exist in such a young star cluster? As they are numerous, it is highly probable that the stars are original cluster members and not field stars which have been captured. 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 if more massive they would collapse due to their own gravitation field. But stars with such a low mass evolve so slowly that it takes billions of years to attain that final state, not the mere 100 million year age of the Pleiades cluster.

A possible explanation may be that these white dwarf stars were formerly very massive and therefore evolved rapidly. Under intense effects, such as strong stellar winds, mass loss to close neighbours, or rapid rotation, these stars may have lost a high proportion part of their mass in planetary nebulae. Thus the residual stars – previously the stars' cores – have come below the Chandrasekhar limit, forming the observed stable white dwarfs.

An exotic type of star has been revealed in observations of the Pleiades since 1995. These Brown Dwarfs were, until recently, hypothetical objects thought to have a mass intermediate between that of giant planets (like Jupiter) and small stars. According to the theory of stellar structure, bodies that produce energy by fusion sometime in their lifetime, must have at least about 6 to 7 per cent of a solar mass, that is, some 60 to 70 Jupiter masses. Following this theory, brown dwarfs should have 10 to about 60 times the mass of Jupiter. They are assumed to be visible in the infrared spectrum, with diameters less than or similar to that of Jupiter (143 000 km), and densities of 10 to 100 times that of Jupiter, since they have stronger gravitation fields.

Many of stars in the cluster are X-ray sources at levels that are up to a thousand times higher than that of the sun. Systematic X-ray surveys of the Pleiades by ROSAT [note 4] have now identified some 170 stars in the Pleiades as X-ray emitters.

The age of the Pleiades

The relatively tight grouping is a sign of the youthfulness of the Pleiades cluster, although it is drifting apart and will gradually disperse. Its youth is also indicated by the absence of red giant stars in the group. None of the stars has yet had time to reach that stage of maturity, although the brightest member stars are hot B-type blue-white giants.

According to a recent calculation [G. Meynet, J.-C. Mermilliod, and A. Maeder in Astron. Astrophys. Suppl. Ser. 98, 477-504, 1993], the age of the Pleiades cluster is about 100 million years, whereas the "canonical" age is considered to be 60 to 80 million years (for instance, the Sky Catalog 2000 gives 78 million). The Pleiades may have an anticipated lifetime as a cluster of only some 250 million years – after then they will have been dispersed as single (or multiple) isolated stars along their orbital path.

The distance of the Pleiades from the Earth

There is an unresolved dispute as to the distance of the Pleiades cluster from the Earth.

The distance of the Pleiades cluster from the Earth was determined in 1997 using direct parallax measurements from the European Space Agency Hipparcos astrometric satellite. On this basis the Pleiades are about 118 parsecs (380 light years) distant from the Solar System. The previously accepted estimated distance was some 408 light years. This lower value requires an explanation for the comparatively faint apparent magnitudes of the stars in the cluster.

The implication of this reduced estimate was that either current stellar models were flawed or Hipparcos was giving incorrect distances. However recent research [note 5] has derived a distance of 133-137 parsecs (434-446 light years), thus reaffirming the fidelity of current stellar models.

A brief history of observation

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, the astronomer Moestlin had correctly drawn 11 Pleiades stars, while Kepler quotes observations of up to 14).

The cluster was first examined telescopically by Galileo, who recorded more than 40 member stars. It was an early subject for astronomical photography, being first photographed by Paul and Prosper Henry in 1885.

The cluster was the final entry, as M45, in an early modern astronomical catalogue by Charles Messier, Tables des Nebuleuses, ainsi que des amas d'Etoiles, que l'on decouvre parmi les Etoiles fixes sur l'horizon de Paris; observes a l'Observatoire de la Marine; Memoires de l'Academie des Sciences for 1771, Paris (Table of nebulae and star clusters, which have been discovered between the fixed stars over the horizon of Paris; observed at the Marine Observatory).

In 1846 Johann von Maedler of the Estonian Dorpat Observatory, an eminent lunar cartographer, systematically measured the motions of various stars in the Pleiades. Finding that they showed no observable relative motion within the cluster, he concluded that they were at the centre of the Galaxy, and that Alcyone was at the centre of the known universe. This later finding was somewhat reckless with the evidence: although obviously prior to the discoveries of Edwin Hubble et al on the scale and distribution of galaxies, it ignored Immanuel Kant's famous and suprisingly prescient theory on the nature of galaxies as remote "cities of stars". For a brief period, until this erroneous argument was refuted, the Pleiades were the subject of intense and specious public debate and much baseless speculation.

Recent observations

Recent advances in telescopy are revealing some of the fine structure of our galactic neighbours – such as the Pleiades cluster. Eerie though it appears, the picture below is not the product of a fertile imagination. This is a photograph of an interstellar cloud in the process of disintegration by intense radiation from an adjacent hot star. The cloud – IC 349 or Barnard's Merope Nebula – is illuminated by Merope in the Pleiades star cluster.

IC 349 or Barnard's Merope Nebula - copyright NASA

The image was produced by the Space Telescope Science Institute and is yet another informative [note 6] and beautiful product of the Hubble Space Telescope. It is the copyright of NASA, the National Aeronautics and Space Administration.

The cloud, which is part of Tempel's Nebula or NGC 1432, is drifting through the Pleiades star cluster. Since the cluster is itself dispersing and moving through space, the combined velocity of Merope and the nebula is some 11 kilometres per second.

As a result of its close proximity of the cloud and the star – in astronomical terms – of about 0.06 light-years (say 550 billion kilometres) the cloud has been extensively deformed. The is an effect of the phenomenon of 'radiation pressure' due to the intense stellar radiation emitted by the nearby star, which acts differentially on the dust particles composing the cloud. It selectively decelerates the particles: less massive dust particles are subject to greater deceleration than larger particles. The radiation pressure thus acts as a sieve, sifting the particles by size.

The clearly formed linear structures directed toward the star are streams of larger particles, whereas the smaller and thus more decelerated particles are, for the moment, retained within the main body of the cloud. If the nebula is not entirely dispersed or absorbed by the star during its close passage, it will pass Merope by and continue on into interstellar space.

Note 1

Astrophysical Journal Supplement, October 2003. White, Richard E.
Return to [note 1]

Note 2

Gibson, Steven J., Nordsieck, Kenneth H. (2003), The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry, The Astrophysical Journal, v.589, p. 362
Return to [note 2]

Note 3

White, Richard E., Bally, John. Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 409, no. 1, p. 234-247, 1993.
Return to [note 3]

Note 4

The ROSAT Mission (1990-1999). ROSAT, the Röntgen Satellite, was an X-ray observatory developed through a cooperative programme between Germany, the United States, and the United Kingdom. The satellite was proposed by the Max-Planck-Institut für extraterrestrische Physik (MPE) and designed, built and operated in Germany and launched by the United States.
Return to [note 4]

Note 5

Letters: Nature 427, 326 - 328 (22 January 2004). A distance of 133-137 parsecs to the Pleiades star cluster. Xiaopei Pan1, M. Shao1 & S.R. Kulkarni2
1 Jet Propulsion Laboratory, California Institute of Technology. 2 Caltech Optical Observatories, California Institute of Technology.

The researchers applied the orbital parameters of the bright double star Atlas in the Pleiades, using long-baseline optical/infrared interferometry. From the data they derived a firm lower bound of 127 pc, with the most likely range being 133 – 137 pc.
Return to [note 5]

Note 6

The apparent rays of light focused on the star are not present in reality, they are an optical phenomenon produced in the apparatus.
Return to [note 6]


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