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 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." UXL Encyclopedia of Science. . Encyclopedia.com. (June 28, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/star-cluster
"Star Cluster." UXL Encyclopedia of Science. . Retrieved June 28, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/star-cluster
star cluster, a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction. Two types of clusters can be distinguished—open clusters, also called galactic clusters because of their wide distribution in our galaxy (the Milky Way), and globular clusters. More than one thousand open clusters have been cataloged in the Milky Way, most of which are found in the spiral arms of the galaxy. Typically, an open cluster contains from a few dozen to a thousand loosely scattered stars and exists in a region rich in gas and dust. Among those which can be detected with the unaided eye are the Hyades cluster in the constellation Taurus, the Coma Berenices cluster, the Pleiades cluster, and the Praesepe cluster. Globular clusters are spherical aggregates of from thousands to hundreds of thousands of densely concentrated stars. Rather than lying on the galactic plane, these clusters are members of the outer halo, moving around the nucleus of our galaxy in highly inclined orbits. Because of their distribution around the galaxy, they provide an outline of its shape. About 150 globular clusters are known in the Milky Way galaxy, and others have been found in nearby galaxies. Visible to the unaided eye are Omega Centauri and 47 Tucanae, both in the southern skies, and M13 in the northern sky. Star clusters are cosmologically important as a first step to understanding the distance scale of the universe (see Hyades); and theoretical astronomers use observations of globular clusters to investigate the evolution and life span of stars. Because all the stars in a particular cluster are coeval (the same age), astronomers can infer that massive stars change more rapidly over time than less massive ones. X-ray sources have been detected recently in some globular clusters. Millisecond pulsars have also been found.
"star cluster." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (June 28, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/star-cluster
"star cluster." The Columbia Encyclopedia, 6th ed.. . Retrieved June 28, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/star-cluster