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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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