PSPC: The Pleiades
The Pleiades is an open stellar cluster that is easily visible to the naked eye in the northern hemisphere. Its stars, numbering more than 500 members, lie only about 130 parsecs (1 parsec = 3.26 light years) away from the sun. The cluster is only about 70 million years old; its member stars are very young compared to the 4.5 billion year old sun. This PSPC image of the central region of the Pleiades shows that many of these very young stars are X-ray sources at levels that are up to a thousand times higher than that of the sun. Systematic X-ray surveys of the Pleiades by ROSAT have now identified of order of 170 cluster stars as X-ray emitters.
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