Perfectly visible to the naked eye with a magnitude of 1.6 and a surface area of almost 2° (four times the Full Moon), it is the object in the sky whose representations are the oldest in the history of humanity; if we exclude the Sun and the Moon. Concordant sources indicate in fact that the shamans of the Neolithic period (-9000/-3300) saw, through its appearance in the sky, the announcement of the harvest period.
A representation of this cluster can be found on the “Nebra Disc”, dated 1600 BC (Bronze Age), which constitutes the oldest representation of the celestial vault known to date.
It is mentioned all around the world: among the Aztecs, the Hebrews, the Maoris, the Australian Aborigines, the Hindus, the Persians, the Chinese, the Japanese, the Egyptians, the American Indians… And, of course, among the ancient Greeks who named it “Pleiades” in honor of the seven daughters of the titan Atlas and Pleione (also parents of the Hyades, nymphs of the Rains, who gave their name to another open cluster near the Pleiades…).
This is why the Pleiades are also sometimes called “the seven sisters”, each of them having given its name to one of the main stars of the cluster (Maia, Alcyone, Asterope, Celeno, Electra, Taygetus and Merope) and this, despite the fact that we can see 9 stars with the naked eye! The other 2 stars are named after their parents.
There are even references to the Pleiades in Homer’s “Odyssey” (Canto V) and in the Bible…
Nothing surprising, however, for anyone who has already had the chance to observe this magnificent cluster that immediately captures the eye and attention! It is possible to distinguish, with the naked eye, at least the 7 main stars without much difficulty. The most seasoned observers (and those with very good eyesight) can count 9, or even up to 11.
With a simple pair of 7 x 50 binoculars, the Pleiades offer a sumptuous spectacle, revealing not only the main stars, but a myriad of fainter ones.
An “open” cluster implies a “young” cluster: unlike globular clusters, which are denser and able to maintain coherence for several billion years, open clusters end up being dispersed quite quickly due to gravitational interactions with the surrounding stars. Thus, the Pleiades formed less than 100 million years ago, and will be dispersed in 250 million years.
The brightest stars in the cluster are thus young, very hot, blue B-type supergiants. But most of the 3,000 or so stars in the cluster consist of red dwarfs, which are less massive and less bright, with the total mass of the cluster estimated at around 800 solar masses.
The central part of the cluster is contained within a sphere 16 light-years in diameter, but the cluster as a whole extends over 86 light-years.
The photograph reveals another unusual aspect of the Pleiades, namely the presence of large dust clouds around certain stars. These nebulae diffuse the light of the blue supergiants by reflection, thus taking on the same hues as the latter.
The Pleiades, an open cluster consisting of approximately 3,000 stars at a distance of 400 light-years (120 parsecs) from Earth in the constellation of Taurus. Image: NASA, ESA, AURA/Caltech, Palomar Observatory. Credit: D. Soderblom and E. Nelan (STScI), F. Benedict and B. Arthur (U. Texas), and B. Jones (Lick Obs.
Although the Pleiades are the result of the contraction of a large gas cloud (like the Orion Nebula), the nebulae visible around the stars are not the residues of this cloud; contrary to what one might believe. Formed over a hundred million years, the intense radiation of the supergiant stars has already had time to disperse, by radiation pressure, all the residues of the original cloud. In reality, studies show that the cluster, whose stars follow coherent trajectories, is currently crossing an interstellar dust cloud.