The stunning Hercules Cluster (M13, NGC 6205), also called
the 'Great globular cluster in Hercules' is perhaps the most famous globular
cluster residing in the Milky Way's halo. It is another astronomical jewel
which is viewed best either using binoculars or a small telescope.
It was discovered by Edmond Halley in 1714, who noted that
'it shows itself to the naked eye when the sky is serene and the Moon absent.'
Charles Messier catalogued this beautiful object in 1764. At magnitude +5.8, it
is possible from a dark site to observe it with the naked eye as a faint smudge
of light.
When you look at this group of several hundred thousand
stars (some astronomers reckon over a million!) it is amazing to think that it
is over 25,000 light years away -- and it's still just on the outskirts of our
galaxy!
M13 lies in the lower left shoulder in the constellation of
Hercules, which itself lies between Bootes, Corona Borealis and Lyra, and is
currently best seen in the late evening/early morning in mid northern
latitudes.
Towards the centre of the cluster, the stars are about five
hundred times more concentrated than in our solar neighbourhood. Such globular
clusters contain some of the oldest stars in the universe, indeed, the age of
M13 has been determined at about 12 billion years. However, this cluster and
other globulars possess a peculiar attribute. M13, for example, contains one
young blue star, Barnard 29, of spectral type B2 - totally at odds to the other
constituent stars of the cluster.
Recent research by astronomers has located many such young
blue and bright stars in ancient globular clusters - they've even been given a
name - 'blue stragglers'! It has been postulated that these young stars form
from the merger (or collision) of two or more of the ancient stars in each
densely packed globular cluster. The result is one young and hot blue giant
star.
M13 was selected in 1974 by Dr. Frank Drake of Cornell
University and Carl Sagan as a target for one of the first radio messages
addressed to possible extra-terrestrial intelligent life, and sent by the
Arecibo Radio Telescope, Puerto Rico. The message, when decoded, contains a
graphic which includes the atomic numbers of the elements which make up
deoxyribonucleic acid (DNA, the basis of all life on Earth), a graphic figure
of a human, the height of an average human, the human population of Earth and a
diagram of the Solar System.
The return journey time for a reply is 50,000 years, but
bearing in mind the density of such globular clusters, it is now in doubt
whether planets will reside in stable orbits around these ancient stars due to
gravitational perturbations from their stellar neighbours. Perhaps M13 wasn't
such a good target candidate for a message after all!
However, do go out and enjoy one of the splendours of the
northern night sky, you certainly won't be disappointed!
Andy Fleming is the author of the astronomy blog
AstronomyQuest at
http://astronomyquest.blogspot.com/
and also of the AstronomyCast podcast, available at:
http://astronomyquest.blogspot.com/p/astronomyquest-podcasts.html
The podcast is also available for FREE download from the iTunes store.
The AstronomyQuest blog and podcast aims to provide an educational resource for
the public in new developments and discoveries in astronomy and cosmology. It
also includes media reviews and tips on amateur observing and explanations of
various astronomical phenomena, and scientific theories pertaining to
astronomy.
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