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.

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.

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.

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. 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.

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.

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.

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).
The extension to Maya was discovered in 1875, 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.

According to new calculations published in1993, the age of the Pleiades star cluster amounts 100 million years. This is considerably more than the previously published age of 60--80 million years. It has been calculated that the Pleiades have an expected future lifetime as a cluster of only about another 250 million years; 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. 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 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. It was spectroscopically observed that between the years 1938 and 1952, Pleione has ejected a gas shell because of this rotation.

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. Apparently surrounding Aldebaran is another, equally famous open cluster, the Hyades; Aldebaran is known to be a non-member foreground star.

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.

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. 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.