Open clusters are
found in the disk of the Galaxy, and therefore lie largely
in the plane of the Milky Way. Many
of the closer ones, including those presented here, are easily
visible to the naked eye (Messier 35 a fainter exception). Some
are angularly so large that they make constellations of their own, or at least
significant parts of them.
Though their sizes vary greatly, open clusters typically contain a
few hundred loosely arranged stars packed within a diameter 10 or
so light years across. And though bound together by their own
gravity (the false Coathanger below an exception), most open
clusters gradually break up as a result of random encounters among
stars that speed members to the escape velocity, and because of
stretching by tides raised by the Galaxy. Following the set of
naked-eye clusters is a link to a photo that shows the Hyades,
Pleiades, and Messier 35 in context, as well as 15 fainter clusters
near the Galaxy's anticenterin Taurus, Auriga, Gemini, and Orion.
At the bottom are three contrasting globular clusters:
Messier 13 in Hercules (which is also
visible to the naked eye, though just barely), more massive but
more distant (and thus visually fainter) Messier 15 in Pegasus, and less massive (and also
fainter) Messier 10 in Ophiuchus.
While there are thousands of open clusters in the Galaxy, there are
but 150 or so known globulars. Distinct from open clusters, their
home is in a huge spheroidal halo that surrounds the
Galactic disk. And while open clusters are sparse, loose, and
compartively young (the Alpha Persei cluster only 50 million years
old, the Pleiades just 130 million), globulars are compact, closely
spherical, and can contain over a million stars packed into a
volume only a hundred or so light years across. With ages of 11 to
12 billion years, formed when the metal content of the Galaxy was
much less than it is today (the increase the result of stellar
evolution), they are among the oldest things known and among the
first things to be created after the Big Bang, the event that
formed our Universe.
Return to STARS.
Open Clusters Visible to the Unaided Eye
Copyright © James B. Kaler, all rights reserved.
Except where otherwise indicated, these contents are the property
of the author and may not be reproduced in whole or in part without
the author's consent except in fair use for educational purposes.
Opening illustration: Chi Persei, courtesy of Mark Killion.
Thanks to visitor number .