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.

open cluster, see star cluster.

globular cluster: see star cluster.

galactic cluster: see star cluster.

cluster munitions or cluster bombs, air-dropped or ground-launched weapons that open in mid-air and scatter dozens, hundreds, or thousands of smaller submunitions (or bomblets) over a wide area. Such munitions are effective against targets that do not have fixed locations, such as enemy soldiers or vehicles, and also against precise positions, such as airfields and missile sites. Artillery shells that employ principles similar to cluster munitions have existed for decades. There are eight types of submunition-based weapons: antipersonnel, which rely on explosive fragmentation to kill troops and destroy unarmored targets; antitank, which utilize shaped-charge warheads to pierce the armor plate of tanks and armored fighting vehicles; incendiary, which are intended to start fires by employing white phosphorus and napalm bomblets; anti-electrical, which interfere with electric power transmission lines by creating short circuits with carbon fiber or aluminum-coated glass fiber bomblets; mine laying, which like land mines do not explode immediately but wait to be triggered by pressure or magnetism; antirunway, which are designed to penetrate concrete before detonating so as to crater and shatter runway surfaces; leaflet dispensing. which drop large quantities of propaganda materials behind enemy lines; and chemical/biological weapons of mass destruction. For humanitarian reasons the last was banned by the Chemical Weapons Convention of 1993, and stockpiles amassed by six nations are being destroyed. Antipersonnel mines deployed by mine-laying cluster munitions are banned under the 1997 Mine Ban Treaty (also known as the Ottawa Convention), which prohibits the production, stockpiling, and use of antipersonnel mines. Because unexploded bomblets scattered by cluster munitions can remain dormant for years after a conflict ends and then be triggered by a noncombatant (often a child), the Cluster Munition Coalition, the International Committee of the Red Cross, United Nations organizations, other organizations, and many nations began negotiations to produce a treaty banning submunition-based weapons in 2007. A draft treaty that would outlaw cluster bombs and give ratifying nations eight years to destroy such weapons was approved by 111 nations in May, 2008. Among the nations that did not participate in the conference that adopted the draft were the United States, Russia, China, India, Pakistan, and Israel.

cluster, in astronomy: see star cluster; galaxy.

Hercules cluster, giant globular star cluster in the northern constellation Hercules; cataloged as M13 or NGC 6205. The cluster is just visible to the naked eye and is the best example of a globular cluster visible in the Northern Hemisphere. Its angular diameter is about 2/3 that of the full moon, and its linear diameter is 100 light-years. The entire cluster contains perhaps 100,000 or more stars. In its central portion, the stars are so close together that they cannot be resolved, despite the cluster's relatively near distance of 35,000 light-years; the separation between stars is estimated to be only a few astronomical units as compared to the normal interstellar separation of a few light-years.