Star Clusters Examined
Δημοσιεύθηκε στις 11 Δεκ 2013
A look at star clusters; how all
stars within them are formed from the same material and are
approximately the same age; a look at the two kinds of clusters -- "open
clusters" which are young and exist in the spiral arms of the galaxy,
and "globular clusters" which are old and exist in the outskirts of the
galaxy and possibly as old as the universe itself.
Star clusters
or star clouds are groups of stars. Two types of star clusters can be
distinguished: globular clusters are tight groups of hundreds of
thousands of very old stars which are gravitationally bound, while open
clusters, more loosely clustered groups of stars, generally contain
fewer than a few hundred members, and are often very young. Open
clusters become disrupted over time by the gravitational influence of
giant molecular clouds as they move through the galaxy, but cluster
members will continue to move in broadly the same direction through
space even though they are no longer gravitationally bound; they are
then known as a stellar association, sometimes also referred to as a
moving group.
Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster.
Globular
clusters, or GC, are roughly spherical groupings of from 10,000 to
several million stars packed into regions of from 10 to 30 light years
across. They commonly consist of very old Population II stars—just a few
hundred million years younger than the universe itself—which are mostly
yellow and red, with mass just less than two solar masses. Such stars
predominate within clusters because hotter and more massive stars have
exploded as supernovae, or evolved through planetary nebula phases to
end as white dwarfs. Yet a few rare blue stars exist in globulars,
thought to be formed by stellar mers in their dense inner regions; these
stars are known as blue stragglers.
In our galaxy, globular
clusters are distributed roughly spherically in the galactic halo,
around the galactic centre, orbiting the centre in highly elliptical
orbits. In 1917, the astronomer Harlow Shapley was able to estimate the
Sun's distance from the galactic centre based on the distribution of
globular clusters; previously the Sun's location within the Milky Way
was by no means well established.
Until recently, globular
clusters were the cause of a great mystery in astronomy, as theories of
stellar evolution gave ages for the oldest members of globular clusters
that were greater than the estimated age of the universe. However,
greatly improved distance measurements to globular clusters using the
Hipparcos satellite and increasingly accurate measurements of the Hubble
constant resolved the paradox, giving an age for the universe of about
13 billion years and an age for the oldest stars of a few hundred
million years less.
Super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters.
Our
galaxy has about 150 globular clusters, some of which may have been
captured from small galaxies disrupted by the Milky Way, as seems to be
the case for the globular cluster M79. Some galaxies are much richer in
globulars: the giant elliptical galaxy M87 contains over a thousand.
A
few of the brightest globular clusters are visible to the naked eye,
with the brightest, Omega Centauri, having been known since antiquity
and catalogued as a star before the telescopic age. The best known
globular cluster in the northern hemisphere is M13 (modestly called the
Great Globular Cluster in Hercules).
stars within them are formed from the same material and are
approximately the same age; a look at the two kinds of clusters -- "open
clusters" which are young and exist in the spiral arms of the galaxy,
and "globular clusters" which are old and exist in the outskirts of the
galaxy and possibly as old as the universe itself.
Star clusters
or star clouds are groups of stars. Two types of star clusters can be
distinguished: globular clusters are tight groups of hundreds of
thousands of very old stars which are gravitationally bound, while open
clusters, more loosely clustered groups of stars, generally contain
fewer than a few hundred members, and are often very young. Open
clusters become disrupted over time by the gravitational influence of
giant molecular clouds as they move through the galaxy, but cluster
members will continue to move in broadly the same direction through
space even though they are no longer gravitationally bound; they are
then known as a stellar association, sometimes also referred to as a
moving group.
Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster.
Globular
clusters, or GC, are roughly spherical groupings of from 10,000 to
several million stars packed into regions of from 10 to 30 light years
across. They commonly consist of very old Population II stars—just a few
hundred million years younger than the universe itself—which are mostly
yellow and red, with mass just less than two solar masses. Such stars
predominate within clusters because hotter and more massive stars have
exploded as supernovae, or evolved through planetary nebula phases to
end as white dwarfs. Yet a few rare blue stars exist in globulars,
thought to be formed by stellar mers in their dense inner regions; these
stars are known as blue stragglers.
In our galaxy, globular
clusters are distributed roughly spherically in the galactic halo,
around the galactic centre, orbiting the centre in highly elliptical
orbits. In 1917, the astronomer Harlow Shapley was able to estimate the
Sun's distance from the galactic centre based on the distribution of
globular clusters; previously the Sun's location within the Milky Way
was by no means well established.
Until recently, globular
clusters were the cause of a great mystery in astronomy, as theories of
stellar evolution gave ages for the oldest members of globular clusters
that were greater than the estimated age of the universe. However,
greatly improved distance measurements to globular clusters using the
Hipparcos satellite and increasingly accurate measurements of the Hubble
constant resolved the paradox, giving an age for the universe of about
13 billion years and an age for the oldest stars of a few hundred
million years less.
Super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters.
Our
galaxy has about 150 globular clusters, some of which may have been
captured from small galaxies disrupted by the Milky Way, as seems to be
the case for the globular cluster M79. Some galaxies are much richer in
globulars: the giant elliptical galaxy M87 contains over a thousand.
A
few of the brightest globular clusters are visible to the naked eye,
with the brightest, Omega Centauri, having been known since antiquity
and catalogued as a star before the telescopic age. The best known
globular cluster in the northern hemisphere is M13 (modestly called the
Great Globular Cluster in Hercules).
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