M42 the Orion Nebula (above). At the center
of the bright area is the Trapezium Cluster.
Credit: Mike Barrett
M13, the Great Globular
Cluster in Hercules
Credit: Mike Barrett
Within the center of the Orion Nebula, M42, is an area
known as the Trapezium Cluster. The Trapezium Cluster
contains just a few stars, but they are very bright and
very hot. They shine through the nebula and can be easily seen in a small telescope.
Globular Clusters
Globular Clusters are categorised by the number of
stars in the group ranging from a few thousand to many
millions of individual stars. These are all congregated in a
relatively small area and are bound together by gravity.
Most Globular Clusters exist outside the main ecliptic of
the galaxy in the halo of the galaxy and orbit the galactic core.
In contrast to the Open Clusters the Globular Clusters
are formed from very old stars and will typically be seen
ranging from white to orange in color. The predominant
star type in a Globular cluster is spectral type G which
is the same as our Sun. There can still be some younger
stars in the cluster, but images will mainly show white and
orange stars.
There are around 150 Globular Clusters orbiting our
galaxy. The precise number of the Globular Clusters
is difficult to ascertain as they are not all visible so the
quantity has been estimated based on the number of
observable clusters. Globular Clusters are not unique to
our galaxy, we can observe similar star clusters orbiting
other galaxies.
The best examples of Globular Clusters are M13: The
Great Globular Cluster in Hercules, and Omega Centauri
in Centaurus. Both of these can be seen in dark skies
without a telescope. M13 is a classic northern hemisphere object visible throughout summer and autumn.
Omega Centauri is visible from more southerly sites, and
in the southern hemisphere.
To the naked eye the clusters will appear like a faint
fuzzy blob, not dissimilar from a galaxy. When viewed
with binoculars the circular form can be observed,
without a distinct edge. Telescopes reveal more of the
structure of the cluster, including some of the individual
stars in the core extending out to the diffuse stars of
the extremities. As with all astronomical observations of
faint objects the real detail and beauty of these clusters
is revealed in astrophotography. Using the technique
of combining a number of long exposures and careful
processing the structure and form of the cluster can be
teased out.
Good luck hunting for these star clusters, the best
ones to look for in the summer are M13 in the Northern
Hemisphere and Omega Centauri in the Southern Hemisphere.
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