The Pleiades (pleye'-a-deez or plee'-a-deez, also known as M45, or the Seven Sisters) is an open cluster in the constellation of Taurus. It is among the nearest to earth of all open clusters, probably the best known and certainly the most striking to the naked eye.
The distance of the cluster is very important as it is a crucial step in determining the distance scale of the whole universe. The Hipparcos satellite measured a distance for the cluster which was 10% smaller than most previous measurements, but was later found to have suffered from a systematic error when observing the Pleiades. The cluster is now known to lie at a distance of about 135 parsecs (440 light years).
The cluster is dominated by hot blue stars, which have formed within the last 100 million years. Dust remaining from the formation of the cluster forms faint reflection nebulosity around the brightest stars. The cluster will in time disperse due to gravitational interactions with the spiral arms of the galaxy and giant molecular clouds. The cluster's lifetime will probably be about 250 million years.
The Pleiades are a prominent sight in the northern hemisphere in winter, and have been known since antiquity. They are mentioned in Homer's Iliad and Odyssey, and also by Hesiod. They are also mentioned three times in the Bible.
They have long been known to be a physically related group of stars rather than any chance alignment. The reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related.
Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious (), as most of Messier's objects were much fainter and more easily confused with comets - something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so added some bright, well known objects to boost his list.
The distance to the Pleiades has been estimated by many methods, as it is an important step in calibrating distance scales for the whole universe. Accurate knowledge of the distance to the Pleiades allows astronomers to plot a Hertzsprung-Russell Diagram for the cluster, which when compared to that plotted for clusters whose distance is not known allows their distance to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and galaxies, and a 'distance ladder' can be constructed.
Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster - a technique which should yield the most direct and accurate results. Later work has consistently found that the Hipparcos result was in error, but it is not known how the error was caused. The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs.
The cluster is about 12 light years in diameter, and contains approximately 500 stars in total. It is dominated by young hot blue stars, up to 14 of which can be seen with the naked eye, depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to the Plough and Ursa Minor (the Little Dipper). The total mass contained in the cluster is estimated to be about 800 solar masses.
The cluster contains many brown dwarfs. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.
Also present in the cluster are several white dwarfs. At the young age of the cluster, normal stars would not be expected to have had time to evolve into white dwarfs, a process which normally takes several billion years. It is believed that rather than being individual low to intermediate mass stars, the progenitors of the white dwarfs must have been high mass stars in binary systems. Mass transfer from the high mass star to its companion during its rapid evolution would result in a much quicker route to the formation of a white dwarf.
Age and future evolution
Ages for star clusters can be estimated by comparing the H-R diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher ages.
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to its very low ignition temperature of 2.5 million K, the highest mass brown dwarfs will burn lithium eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.
Like most open clusters, the Pleiades will not stay gravitationally bound forever, as the component stars are moving faster than the escape velocity of the cluster. Calculations suggest that the cluster will take about 250 million years to disperse.
Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot young stars.
It is often thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.
Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.
Names and technical information
The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Asterope, Merope, Electra, Maia, Taygete, Celaeno and Alcyone, along with their parents Atlas and Pleione. The Hyades were sisters of the Pleiades. The following table gives details of these stars:
Pleiades Bright Stars
| Name || Pronunciation || Designation || m || Type || Catalog
| Alcyone || al-seye'-a-nee || eta (25) Tauri || 2.86 || B7e III || vdB 23
| Atlas || at'-las || 27 Tauri || 3.62 || B8 III || Ced 190
| Electra || e-lek'-tra || 17 Tauri || 3.70 || B6e III || vdB 20
| Maia || may'-a, also meye'-a || 20 Tauri || 3.86 || B7 III || NGC 1432
| Merope || mair'-a-pee || 23 Tauri || 4.17 || B6 IV || NGC 1435, IC 349
| Taygeta || tay-ij'-i-tee || 19 Tauri || 4.29 || B6 V || Ced 19e
| Pleione || pleye-oh'-nee || BU (28) Tauri || 5.09 (var.) || B8e p || Ced 19p
| Celaeno || se-lee'-noh || 16 Tauri || 5.44 || B7 IV || Ced 19c
| Asterope || a-stair'-a-pee || 21 and 22 Tauri || 5.64/6.41 || B8e V/B9 V ||
- m: visual magnitude
- Catalog: catalog number of associated nebulosity
Pronunciation guide: a as in cat (when stressed), or in sofa (when not); ay as in day; ai as in air; e as in pet; ee as in feet; i as in bit; eye as in bite; oh as in bone; s as in hiss
Stress in indicated by an appostrophe after the stressed syllable (af'-ter).
The name of the cluster itself has uncertain etymology, with suggested origins being from plein, to sail, making the Pleiades the 'sailing ones'; from pleos, full or many; or from pelaiades, flock of doves (in the Greek legend, the sisters are transformed into birds).
The Pleiades in folklore
The Pleiades' high visibility in the night sky has guaranteed it a special place in many cultures, both ancient and modern:
The word has acquired a meaning of "multitude", hence the name of the French literary movement La Pléiade and an earlier group of Alexandrian poets.
- Adams, Joseph D.; Stauffer, John R.; Monet, David G.; Skrutskie, Michael F.; Beichman, Charles A. (2001), The Mass and Structure of the Pleiades Star Cluster from 2MASS, The Astronomical Journal, v.121, p.2053
- Basri G., Marcy G. W., Graham J. R. (1996), Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars, Astrophysical Journal v.458, p.600
- 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
- Moraux, E.; Bouvier, J.; Stauffer, J. R.; Cuillandre, J.-C. (2003), Brown dwarfs in the Pleiades cluster: Clues to the substellar mass function, Astronomy and Astrophysics, v.400, p.891
- Ovid XIII, 293.
- Percival, S. M.; Salaris, M.; Groenewegen, M. A. T. (2005), The distance to the Pleiades. Main sequence fitting in the near infrared, Astronomy and Astrophysics, v.429, p.887
- Soderblom D.R., Nelan E., Benedict G.F., McArthur B., Ramirez I., Spiesman W., Jones B.F. (2005), Confirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades, The Astronomical Journal, v. 129, pp. 1616-1624.
- Zwahlen, N.; North, P.; Debernardi, Y.; Eyer, L.; Galland, F.; Groenewegen, M. A. T.; Hummel, C. A. (2004), A purely geometric distance to the binary star Atlas, a member of the Pleiades, Astronomy and Astrophysics, v.425, p.L45