Examples of Open
Clusters (2) ...
The nebulosity around the stars of the
Pleiades cluster
is not thought to be gas and dust left over from their
formation, but rather a region of dust that the cluster happens to be passing through at this
time.
The Open Cluster M6
The right figure shows a National Optical Astronomy Observatory (NOAO)
image of the
the Butterfly Cluster (M6) in Scorpius. At
apparent visual magnitude 5.3, it
is just bright enough to be visible to the naked eye.
(When we quote a magnitude for a cluster, we mean a magnitude
corresponding to the total
brightness for all stars in the cluster.) This cluster is
at a distance of some 600 parsecs
and has a
diameter of about 4.5 parsecs. The estimated age of 100 million years
for M6 is moderately
young
for an open cluster and comparable to that of the Pleiades.
The Hyades Cluster
The closest open star clusters cover large
areas of the sky, and so can escape easy identification.
The Hyades are an example of a nearby open cluster. The approximately 200
stars of the Hyades
form much of the
face of the bull in the constellation
Taurus, and are only about 50 parsecs from Earth. The age of the Hyades cluster is
estimated to be about 500 million years.
Here is an animated
3D model of the Hyades
cluster based on Hipparcos high-precision distance measurements.
A Wild Duck
Since open clusters are relatively large and easy to observe, they have been
studied by astronomers for centuries. In many cases have been given
fanciful names. An example is The Wild Duck, also known as M11, shown in the adjacent
left image.
This distant cluster has about 3000 stars, and is
almost 2 kpc (6 million light years)
from Earth. It has an estimated age of 250 million years.
The name apparently comes from the resemblance
to a flight of birds in the sky.
This cluster is rather bright (apparent visual magnitude 6.3), and it was discovered in 1681 by
Gottfried Kirch in Berlin, about a century before Messier invented his catalog. Because it
is so densely packed with bright stars, it is estimated that if the Earth were placed at
the center of M11
we would see several hundred stars of apparent magnitude 1 or brighter in our sky. For
reference, we have only 16 stars of apparent magnitude 1 or higher in our present sky.
