Known pre-historically. Mentioned by Hesiod between 1000 and 700 B.C.
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).
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.
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.
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. Also note our Pleiades map.
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 (image left).
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 our image, which was taken by David Malin with the UK Schmidt Telescope.
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 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, 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 reveled their nature as reflection nebulae, as their spectra are exact copies of the spectra of the stars illuminating them.
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.
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 new value requires an explanation for the comparatively faint apparent magnitudes of the Pleiades stars.
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.
These stars give rise to a specific problem of stellar evolution:
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.
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.
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 sometime 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.
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:
Such events demonstrate the relations of the apparent sizes of the Moon and the cluster:
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.