A group of stars are called star clusters. Star clusters are categorized into two types; globular star clusters and open star clusters.
Open clusters are loosely clustered groups of young stars containing a few hundred members. 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.
Globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound. Gravitationally bound objects are objects held together in orbit about each other by gravity. Globular clusters because of the gravitational energy, are shaped into a tight sphere. The stars in the cluster orbit a galactic core as a satellite and have high stellar densities going toward the center of the sphere.
Globular clusters are found in the halo of a galaxy and contain more stars and are older than less dense galaxies and open clusters that are found in the disk. In the Milky Way, there are 160 known globular clusters with more likely to be discovered. Other bigger galaxies, like Andromeda, has 500 while M87 has 13,000 globular clusters.
The first globular cluster discovered was M22 in the constellation Sagittarius by Abraham Ihle in 1665.
A Cluster with a Secret
The Milky Way galaxy is orbited by more than 150 globular star clusters that date back to the distant past of the Universe. One of the closest to the Earth is the cluster Messier 4 (also known as NGC 6121) in the constellation of Scorpius (The Scorpion). This bright object can be easily seen in binoculars, close to the bright red star Antares, and a small amateur telescope can show some of its constituent stars.
This new image of the cluster from the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory reveals many more of the cluster’s tens of thousands of stars and shows the cluster against the rich background of the Milky Way.
Astronomers have also studied many of the stars in the cluster individually using instruments on ESO’s Very Large Telescope. By splitting the light from the stars up into its component colours they can work out their chemical composition and ages.
Video: Zooming in on Messier 4
New results for the stars in Messier 4 have been surprising. The stars in globular clusters are old and hence not expected to be rich in the heavier chemical elements [1]. This is what is found, but one of the stars in a recent survey was also found to have much more of the rare light element lithium than expected. The source of this lithium is mysterious. Normally this element is gradually destroyed over the billions of years of a star's life, but this one star amongst thousands seems to have the secret of eternal youth. It has either somehow managed to retain its original lithium, or it has found a way to enrich itself with freshly made lithium.
This WFI image gives a wide view of the cluster and its rich surroundings. A complementary and more detailed view of just the central region from the orbiting NASA/ESA Hubble Space Telescope was also released this week as part of the Hubble Picture of the Week series.
Notes
[1] Most of the chemical elements heavier than helium are created in stars and dispersed into the interstellar medium at the end of their lives. This enriched material then forms the building blocks of future stellar generations. As a result very old stars, such as those in globular star clusters, which formed before significant enrichment had occurred, are found to have lower abundances of the heavier elements when compared to stars, such as the Sun, that formed later.
Open clusters are loosely clustered groups of young stars containing a few hundred members. 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.
Globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound. Gravitationally bound objects are objects held together in orbit about each other by gravity. Globular clusters because of the gravitational energy, are shaped into a tight sphere. The stars in the cluster orbit a galactic core as a satellite and have high stellar densities going toward the center of the sphere.
Globular clusters are found in the halo of a galaxy and contain more stars and are older than less dense galaxies and open clusters that are found in the disk. In the Milky Way, there are 160 known globular clusters with more likely to be discovered. Other bigger galaxies, like Andromeda, has 500 while M87 has 13,000 globular clusters.
The first globular cluster discovered was M22 in the constellation Sagittarius by Abraham Ihle in 1665.
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| This image from the Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory shows the spectacular globular star cluster Messier 4. This great ball of ancient stars is one of the closest of such stellar systems to the Earth and appears in the constellation of Scorpius (The Scorpion) close to the bright red star Antares. New observations with ESO's Very Large Telescope have revealed that one star — marked on this picture — has much more lithium than the other stars in the cluster that have been studied. The source of this lithium is mysterious. Normally this element is gradually destroyed over the billions of years of a star's life, but this one star amongst thousands seems to have the secret of eternal youth. It has either somehow managed to retain its original lithium, or it has found a way to enrich itself with freshly made lithium. Credit: ESO Acknowledgement: ESO Imaging Survey |
A Cluster with a Secret
The Milky Way galaxy is orbited by more than 150 globular star clusters that date back to the distant past of the Universe. One of the closest to the Earth is the cluster Messier 4 (also known as NGC 6121) in the constellation of Scorpius (The Scorpion). This bright object can be easily seen in binoculars, close to the bright red star Antares, and a small amateur telescope can show some of its constituent stars.
This new image of the cluster from the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory reveals many more of the cluster’s tens of thousands of stars and shows the cluster against the rich background of the Milky Way.
Astronomers have also studied many of the stars in the cluster individually using instruments on ESO’s Very Large Telescope. By splitting the light from the stars up into its component colours they can work out their chemical composition and ages.
Video: Zooming in on Messier 4
New results for the stars in Messier 4 have been surprising. The stars in globular clusters are old and hence not expected to be rich in the heavier chemical elements [1]. This is what is found, but one of the stars in a recent survey was also found to have much more of the rare light element lithium than expected. The source of this lithium is mysterious. Normally this element is gradually destroyed over the billions of years of a star's life, but this one star amongst thousands seems to have the secret of eternal youth. It has either somehow managed to retain its original lithium, or it has found a way to enrich itself with freshly made lithium.
This WFI image gives a wide view of the cluster and its rich surroundings. A complementary and more detailed view of just the central region from the orbiting NASA/ESA Hubble Space Telescope was also released this week as part of the Hubble Picture of the Week series.
Notes
[1] Most of the chemical elements heavier than helium are created in stars and dispersed into the interstellar medium at the end of their lives. This enriched material then forms the building blocks of future stellar generations. As a result very old stars, such as those in globular star clusters, which formed before significant enrichment had occurred, are found to have lower abundances of the heavier elements when compared to stars, such as the Sun, that formed later.
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