Star Cluster

Star clusters are groups of stars that occur close to each other in space (and, thus, are gravitationally bound to one another), appear to have roughly similar ages, and therefore, seem to have had a common origin. Star clusters are typically classified into one of two large sub-groups: galactic clusters and globular clusters. Galactic clusters are sometimes also known as open clusters. Astronomers have identified thousands of galactic star clusters in the Milky Way galaxy, but no more than about 200 globular star clusters.

The two types of star clusters found in the Milky Way galaxy differ from each other in a number of ways. First, galactic clusters occur in the plane of the galaxy, while globular clusters are found outside the galactic plane in the region known as the galactic halo. Second, globular clusters tend to be much larger than galactic clusters, with an average of a few thousand to a million stars in the former and a few hundred stars in the latter. In fact, some galactic clusters contain no more than a half dozen stars. Probably the most famous of all galactic clusters is the Pleiades, or the Seven Sisters. This grouping consists of six or seven stars as seen by the naked eye, but of many more when viewed by a telescope. Third, globular clusters, as their name suggests, tend to have a rather clearly defined spherical shape with a higher concentration of stars at

the center of the sphere. In contrast, galactic clusters, as their alternative name—open clusters—suggests, tend to be more open and lacking in any regular shape.

Fourth, the compositions of stars found in each kind of cluster are quite different. The stars that make up galactic clusters tend to consist primarily of hydrogen (more than 90%) and helium (almost 10%), with small amounts of heavier elements (less than 1%). Stars in globular clusters, on the other hand, contain even smaller amounts of heavier elements. This difference suggests that the stars in galactic clusters are much younger stars than those in globular clusters. When the latter were formed, the universe still consisted almost entirely of hydrogen and helium, so those were the only elements used in the formation of globular cluster stars. Much later in the history of the universe, some heavier elements had been formed and were present at the formation of galactic cluster stars.

Astronomers found a new type of star cluster in 2005 within the Andromeda galaxy. Although similar in some ways to globular clusters (for instance, it contained hundreds of thousands of stars and the stars showed similar characteristics with other globular clusters), this star cluster was much larger across than what is normally seen. This cluster was also several hundred light-years across (where one light-year is the distance that light travels in vacuum in one year). In addition, the star cluster was hundreds of times less dense. There is much to be learned from this star cluster. The information yet to be discovered may very likely help scientists learn more about how stars are created, evolve, and eventually die out.

Star clusters are important to astronomy because of the fact that these stars developed at about the same time. Thus, any differences seen within these stars is basically due to differences in mass. Theories of stellar evolution are often times based on studies of galactic and globular star clusters.

Star cluster

Star clusters are groups of stars close to each other in space that appear to have roughly similar characteristics and, therefore, a common origin. Some of the over 100 billion stars in the Milky Way, our home galaxy, are grouped together in either tight or loose star clusters. More than 100 tight groupings, called globular clusters, surround the galaxy's spiral arms in a great halo. Loose groupings, called open clusters or galactic clusters, are far more numerous and are found toward the center of the galaxy.

Globular clusters radiate with a continuous glow. These nearly spherical (ball-shaped) star systems contain anywhere from tens of thousands to one million stars. They are most heavily concentrated at the center of the cluster. While in reality there is a great distance between stars in these clusters, an observer on Earth may find it impossible to pick out individual stars.

In contrast to globular clusters, which are 10 to 13 billion years old, open clusters are quite young. These groups, formed just a few million to a few billion years ago, contain hot young stars and some stars still in formation. Open clusters have far fewer members than globular clusters (usually just a few hundred) and have no particular shape.

Over 1,000 open clusters have been identified in our galaxy. However, many more may be undetected because of interstellar matter that blocks our view of the Milky Way's plane. One the most famous of the open clusters is the Pleiades (pronounced PLEE-a-deez), or the Seven Sisters. This grouping in the Taurus constellation consists of six or seven stars visible to the naked eye, but many more when viewed through a telescope.

Words to Know

Cluster of galaxies: A group of galaxies that are bound together by gravity.

Galaxy: A large collection of stars and clusters of stars containing anywhere from a few million to a few trillion stars.

Globular cluster: A tight cluster of tens of thousands to one million very old stars.

Light-year: The distance light travels in one year in the vacuum of space—roughly 5.9 trillion miles (9.5 trillion kilometers).

Open cluster: A loose cluster of roughly a few hundred young stars.

Supercluster: A connected group of clusters of galaxies that may extend for hundreds of millions of light-years.

Void: Region of space extending for hundreds of millions of light-years that contains few if any galaxies.

Superclusters

Superclusters are currently the largest structures known in the universe. As stars and clusters of stars group together into galaxies, galaxies collect into groups known as clusters of galaxies. On a larger scale, superclusters are clusters of clusters of galaxies. As clusters of galaxies group into superclusters they leave empty spaces called voids between the superclusters. Superclusters and voids typically extend for hundreds of millions of light-years.

Beyond the Milky Way

Star clusters are certainly not limited to the Milky Way galaxy. In 1924, U.S. astronomer Edwin Hubble identified globular clusters in what, at the time, were believed to be nebula (clouds of dust particles and hydrogen gas) within the Milky Way. He discovered that the distance from Earth to these globulars, and the nebula in which they were located, was so great that these globulars had to be entirely separate galaxies. Thus, Hubble proved that the Milky Way was not the only galaxy in the universe.

[See also Star ]

Star cluster

Star clusters are groups of stars that occur close to each other in space , appear to have roughly similar ages, and therefore, seem to have had a common origin. Star clusters are typically classified into one of two large subgroups, galactic clusters and globular clusters. Galactic clusters are sometimes also known as open clusters. Astronomers have identified thousands of galactic star clusters in the Milky Way , but no more than about 200 globular star clusters.

The two types of star clusters found in the Milky Way differ from each other in a number of ways. First, galactic clusters occur in the plane of the galaxy , while globular clusters are found outside the galactic plane in the region known as the galactic halo. Second, globular clusters tend to be much larger than galactic clusters with an average of a few thousand to a million stars in the former and a few hundred stars in the latter. In fact, some galactic clusters contain no more than a half dozen stars. Probably the most famous of all galactic clusters is the Pleiades, or the Seven Sisters. This grouping consists of six or seven stars as seen by the naked eye (depending on the accuracy of one's eyesight), but of many more when viewed by telescope . Third, globular clusters, as their name suggests, tend to have a rather clearly defined spherical shape with a higher concentration of stars at the center of the sphere . In contrast, galactic clusters, as their alternative name also suggests, tend to be more open and lacking in any regular shape.

Fourth, the compositions of stars found in each kind of cluster are quite different. The stars that make up galactic clusters tend to consist primarily of hydrogen (more


than 90%) and helium (almost 10%), with small amounts of heavier elements (less than 1%). Stars in globular clusters, on the other hand, contain even smaller amounts of heavier elements. This difference suggests that the stars in galactic clusters are much younger than those in globular clusters. When the latter were formed, the universe still consisted almost entirely of hydrogen and helium, so those were the only elements used in the formation of globular cluster stars. Much later in the history of the universe, some heavier elements had been formed and were present at the formation of galactic cluster stars.