Pleiades (star cluster)

The Pleiades (also known as M45 or the Seven Sisters) is an open cluster in the constellation of Taurus. It is among the nearest to earth of all open clusters, probably the best known and certainly the most striking to the naked eye.

Age and future evolution

Ages for star clusters can be estimated by comparing the H-R diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher ages.

Related Topics:
H-R diagram - Stellar evolution - Convective

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Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to its very low ignition temperature of 2.5 million K, the highest-mass brown dwarfs will burn lithium eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years {{ref|Basri}}.

Related Topics:
Main sequence - Lithium - Nuclear fusion - Brown dwarf

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Like most open clusters, the Pleiades will not stay gravitationally bound forever, as the component stars are moving faster than the escape velocity of the cluster. Calculations suggest that the cluster will take about 250 million years to disperse.

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