Starting in astronomy
beginner’s guide to stargazing
By Mike Barrett
Clusters of jewels in the night sky
Stellar Clusters are groups of stars that fall into two
broad categories: Open Clusters or Globular Clusters.
Both types of cluster are formed from gas clouds and
dust coalescing into a multitude of stars, but this is where
the commonality ends.
Open Clusters
Open Clusters can contain up to a thousand stars
which are normally very young, in cosmic terms. They are
generally found in the stellar nurseries of the spiral arms
of the galaxy. As the Open Clusters contain young stars
they are often hot stars, blue to white in color.
Some Open Clusters appear to to the eye as just a
collection of disassociated stars. A typical example of
this would be the Beehive Cluster, M44, in Cancer. This is
an example of an older Open Cluster where the gasses
and dust have either been ejected or incorporated into
the cluster members. These may even have some older
orange stars in the group and be very spread apart like
the Hyades in Taurus. I spent ages when I first started in
astronomy looking for the Hyades Cluster, not realising
how big it was and how far apart its members were.
Middle-aged Open Clusters such as the Pleiades or
M45 have bright blue stars of spectral type B. Although
the Pleiades stars are surrounded by blue nebulosity this is
reflection nebulosity in interstellar matter rather than the
gaseous remains of the stellar nursery. The Pleiades are
a very visible cluster to the naked eye, even from areas
with heavy light pollution. They form a question mark in
the winter skies of the northern hemisphere.
Young Open Clusters can be found within the heart of
the star generating nebulae, such as the Heart Nebula
(IC1805) , and the Orion Nebula (M45). These are surrounded by gasses and stellar dust. The gas is predominantly hydrogen giving these nebulae the characteristic
red color.
The Heart Nebula actually has a number of Open Clusters associated with it, the main one having the same
name as the entire nebula. This is found in the Heart of
the Heart at the top of a pillar of gas.The dramatic coloring of the heart nebula image is because the data was
captured using narrowband filters and creating a false
color final result.
Star Cluster (above) at the center of the Heart Nebula IC1805.
Credit Dave Smith www.AstroSnaps.co.uk
M45 the Pleiades (right), an open cluster visible to the naked eye.
Credit: Mike Barrett
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