City of the stars
and many make superb photographic targets. The red colouration mainly
comes from light emitted by hydrogen atoms, the clouds of gas being
predominantly made of hydrogen. The hot young stars forming within the
cloud ionize the atoms of hydrogen and cause them to give off specific
wavelengths of light. One of the strongest wavelengths emitted is that
called hydrogen alpha (656.28nm) and this lies in the red part of the
coloured spectrum.
The bright Orion Nebula which lies at the heart of Orion’s sword, is
visible to the naked eye, one of only a few. At the nebula’s heart lies a hot
young cluster of stars known as the Trapezium Cluster, stars which have
literally been born out of the nebula material.
Another interesting type of object occurs when a dark cloud sits in
front of a brighter one. One of the best examples of this is the Horsehead
Nebula, again located in the constellation of Orion. Here a finger of dark
gas protrudes in front of a curtain of emission nebulosity. The finger looks
like a knight chess piece in silhouette and this is what gives the nebula its
distinctive name.
Open clusters and nebulae provide many hours of enjoyment for
astronomers. Hunting them down and revelling in their appearance is
one of the great joys of amateur astronomy. With one or two notable
exceptions, these objects lie within our own Milky Way Galaxy. Scattered
in a halo around the core of the Milky Way is another type of cluster,
known as a globular cluster. Here, tens of thousands or even millions
of stars are clustered together in a tight region of space. Each star is in
orbit around the common centre of gravity of the cluster but collisions
are very rare. There are many great examples of globular clusters within
amateur reach, including the Great Globular in Hercules which is visible
in the Northern Hemisphere. It has to be said though, that the SouthernClusters and Nebulae
Stars form out of collapsing clouds of predominantly hydrogen gas.
Huge clouds of gas may collapse to form many stars which are all
moving through space in the same direction and at the same speed; a
quantity known as proper motion. Stars formed in this manner form
what is known as an open cluster and there are many examples of this
type of association in the night sky. Probably one of the most famous
open clusters known is the Pleaides or Seven Sisters, located in the
constellation of Taurus, the Bull.
Best seen in the autumn and winter skies, to the naked eye, the Pleiades
shine like brilliant diamonds scattered on black velvet. Despite being called
the Seven Sisters, those with keen sight may be able to make out many
additional members of the cluster. The author Pete Lawrence has seen
15 under good conditions from his home in Selsey, England. Binoculars or
a small telescope on a low power will show many further members, the
cluster containing around 250 stars in total. A long exposure photograph of
the Pleiades shows the stars to be surrounded by delicate blue gas. This was
once thought to be left over material from the formation of the stars but the
current belief is that Pleiades stars, moving through our galaxy, have simply
encountered the cloud of material. The light from the stars reflecting off the
dust in the cloud gives the nebula its blue appearance.
The two main types of nebula (Latin for cloud) are emission and
reflection. Reflection nebulae, such as that which permeates the Pleiades
open cluster, tend to be bluish in colour while emission nebulae have
a reddish component to them. There are many superb examples of
emission nebulae visible in the night sky, including the Eta Carina Nebula,
Tarantula Nebula, Orion Nebula, Lagoon Nebula and the list goes on.
Many of these beautiful objects are within reach of amateur equipment
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