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The Question

(Submitted June 16, 1997)

I thought, if I am not mistaken, that the Pleiades open cluster was formed in the Orion nebula, and then drifted away to their present location. Is this true, or is it a false memory?

The Answer

I have not heard of the suggestion that the Pleiades originated in Orion. The two associations are both quite distant, and in different directions (angular separation about 80 degrees on the sky), and a quick calculation shows that they would have to be receding from each other at about 10 km/s in order to reach their current positions. This is not a large velocity, but the scenario would also require that the Pleiades maintain its integrity over the course of its lifetime, and this seems somewhat unlikely on the face of it.

A few more facts: The Pleiades is a loose cluster of approximately 100 stars with an average age estimated at 78 million years. The stars in the Pleiades are approximately 125 parsecs or 407.5 light years from our solar system. These are very young stars, much younger than our own Sun, estimated at 5 billion years old, much younger even than our own planet, Earth. These are very hot, bright stars of spectral type B, much hotter and about 10 times more massive than our Sun, spectral type G. They have not yet moved away from the interstellar gas cloud, or nebula, from which they formed. Remnants of this nebula can readily be seen in photographs of the group. Studies of the proper motions of these stars, or their movement through space, have shown that they are in the process of dispersion.

I hope this helps,

Tim Kallman for the Ask an Astrophysicist Team

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