Discovering the Pleiades star cluster

Discovering the Pleiades star cluster

The Pleiades star cluster is a great classic of the starry sky. This cluster is on the edge of the constellation Taurus, about 24 degrees from the celestial equator. This particular position makes the Pleiades visible to almost all the peoples of the Earth. The distance was determined by different methods, involving also the Hubble space telescope and the Hipparcos satellite. More recently, thanks to a combined work by the National Radio Astronomy Observatory, using more radio telescopes, a very precise distance of 443 light years has been determined.

The stellar component of the Pleiades

The stellar cluster of the Pleiades is composed of over 1000 stars, almost all not visible to the naked eye: in particular, from suburban or polluted skies you can see with the naked eye 5 or 6 stars, while from clear and unpolluted skies you can see 14 stars. The main stars of the cluster, all of B class, are the following:

 

Star Apparent magnitude Curiosities
Alcyone 2,86
Asterope 5,64 e 6,41 Stella doppia
Atlante 3,62
Celeno 5,44
Elettra 3,70
Maia 3,86
Merope 4,17
Pleione da 4,77 a 5,50 Stella variabile di tipo Gamma Cassiopeiae
Taigete 4,29

The nucleus of the cluster is 8 light years wide. Almost all the stars are young and very hot, but there are other objects such as white dwarfs and brown dwarfs: the latter are like missing stars, having not reached the minimum mass necessary to trigger the thermonuclear fusion processes. The brown dwarfs have a mass of less than about 8% of the mass of our Sun and in the early part of their lives many of them produce a certain amount of energy by fusion of deuterium and lithium, easier than hydrogen fusion. Although they represent a quarter of the stellar population of the cluster, they contribute only 2% to the total mass.

Discovering the Pleiades star cluster

An image of the nebular component of the Pleiades in the infrared, taken by NASA’s Spitzer Space Telescope.

The nebular component of the Pleiades

Around the stars of the Pleiades there is a set of reflection nebulae, composed mostly of dust. The main components are:

  • NGC 1435 around the star Merope
  • IC 1990 at the north of the cluster
  • NGC 1432 located behind the stars further west of the cluster

One might think that these nebulae were born together with the cluster and that they are therefore simple remains that have not condensed to form stars. In fact, if we accept the age of the star cluster is about 100 million years, the great radiation pressure of the powerful young hot stars would have to clean up the surrounding area, wiping away the dust and dispersing it. In fact, thanks to a more in-depth examination of radial velocities, it was discovered that the Pleiades are probably simply passing through an area rich in interstellar dust and gas. The nebula is already hardly visible with many telescopes and with long exposure photography it is instead very visible and of blue color around the stars of the cluster.

The Pleiades in history

As mentioned earlier, the particular position of the Pleiades makes them visible to almost all the peoples of the Earth: it is therefore not surprising that there are myths and legends about being from Europe to America, from Asia to Australia. During the Bronze Age, for the Celts they were associated with pain and death, because during the period in which Halloween is still celebrated, the stars appeared for the first time at sunset. For the Greeks Pleiades were the Seven Sisters, while for the Vikings they were the hens of Freyja, a deity of Norse mythology. The Aztecs in Central America and the Maori in New Zealand based their calendar on the Pleiades: the former were based on the heliacal rise of the cluster, while the latter started the year as soon as the priests saw the Pleiades in the first light of dawn. In Australia, the Aboriginals believed that the Pleiades were a woman kidnapped by Kidili, the man of the Moon. In literature, we have to mention Giovanni Pascoli who in his writing “Il Gelsomino Notturno” calls the Pleiades “Chioccietta”, imagining a hen moving in a blue threshing ring surrounded by a chirp of stars. The list could still continue, this star cluster has really stimulated the imagination of the people!

The future of the Pleiades

What does the future hold for the Pleiades? Being an open cluster, without gravitational constraints, the dispersion process is already under way. It is supposed that within 250 million years the various components of the Pleiades will be completely dispersed. In that period of time, the motion of the cluster will bring it near the foot of the constellation of Orion.

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