Wednesday, February 3, 2021

The Pleiades

The Pleiades are one of the most beautiful and famous open clusters in the sky. Located just 400 light years away from Earth, the Pleiades can be easily observed even with the naked eye. Its brightest star, Alcyone, has an apparent magnitude of 2.86.

Observing under a sky with a lot of light pollution such as that of a city center, you can see at most four or five stars, while if you observe from a dark place the visible components become ten. If you use an instrument such as a telescope you can observe many more stars. Furthermore, in long exposure photographs you can note that all components are surrounded by reflection nebulae.

The stars of the Pleiades are gravitationally bonded and have a common origin. It is estimated that the cluster is made up of a thousand components, enclosed in a sphere with a diameter of about forty light years. The main stars are all very bright blue giants, but there are also some white dwarfs and numerous brown dwarfs, with a mass just 8% of that of the Sun.

Through the stellar evolution models it has been estimated that the Pleiades have an age of about 100 million years. However, the Pleiades will not be eternal: it is believed that within 250 million years the cluster will no longer exist. Some of its stars will in fact be ejected following close encounters with other components, while the rest of the cluster will be slowly disintegrated due to external gravitational fields, such as those of giant molecular clouds. Other stars being very massive will end their lives within a few tens of millions of years, either by exploding into a supernova or becoming a white dwarf.

Credit: NASA, ESA, AURA/Caltech, Palomar Observatory.

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