Globular clusters are
group of stars bound into a spherical structure by gravitational force. They
are typically found in the halo of a galaxy. Our galaxy, the Milky Way, has
about 150
known globular clusters. There must be more of these yet to be discovered as
they might be hidden due to the galactic dust clouds.
Globular cluster NGC 1866 (Image credit: esahubble.org)
Globular
clusters vary in size depending on the number of stars they contain. They
typically contain a few lakh stars (few hundred thousand stars). Omega
Centauri, the largest known globular cluster contains about one crore stars (10
million stars).
Stars in
globular cluster are very old. Actually, they are postulated to be formed at
the same time when the Milky Way galaxy was formed. They are used to infer the
age of our galaxy itself. We know that the stars in globular clusters are very
old because they contain very little amount of elements that are heavier than
helium. Elements heavier than helium are formed as a result of death of a star.
A star dies when
it has used up all of its hydrogen and fused it into helium. While it takes
million or even billions of years, the helium is further fused into heavier
elements in a relatively short amount of time. Vary massive stars die by a
spectacular explosion called a supernova. For a brief period of time, the star
shines brighter than the whole galaxy. The heavy element formed inside these
stars get distributed and contribute to the mass of stars and planets formed
after its death.
Stars in
globular clusters have very little heavy elements. Such type of stars are
called Population
II stars.
In 2018, some evidence
of the presence of a black hole about 4 times the mass of sun was found in the
globular cluster NGC 3201. Simulation of the evolution of globular clusters by
several research groups have indicated the presence of black holes of mass similar
to Sun, or even 100 to
10000 the mass of our Sun.
Globular cluster
are densely packed with stars. Therefore, any point inside a globular cluster
would be well lit. A plant inside a globular cluster will probably have no
prominent day and night cycle as we have on Earth. Although there have been
several exoplanet candidates found in globular clusters, it is very hard to
study them in detail due to the distances involved.
The stars in globular
clusters are very old. Almost as old as he galaxy itself. This gives a long
time for life there to emerge and evolve. However, high luminosity and temperature
deep inside the clusters might be detrimental for life. May be some star at the
periphery of the clusters where there is possibility of daily and seasonal
cycle might harbour life.
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