Galaxy
hideA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting the galaxy's center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.
There are probably more than 100 billion (1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.
Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
For more information about Galaxy, read the full article at
Wikipedia.
This text uses material from Wikipedia and is available under the GNU Free Documentation License.
News tagged with galaxies
'Dropouts' pinpoint earliest galaxies
Nov 06, 2009 |
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Astronomers, conducting the broadest survey to date of galaxies from about 800 million years after the Big Bang, have found 22 early galaxies and confirmed the age of one by its characteristic hydrogen signature ...
In a Galaxy Far, Far Away...
Nov 06, 2009 |
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(PhysOrg.com) -- Astronomers have published the discovery of the farthest known object in the cosmos: a star that exploded when the universe was only 630 million years old -- only 4.6% of its current age. ...
Starring Intelligent Aliens
Nov 05, 2009 |
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The most probable place to find intelligent life in the galaxy is around stars very similar to our sun, a new study has found.
New type of supernova explosion reported; predicted by theoretical physicists at UCSB
Nov 05, 2009 |
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A new class of supernova was discovered by scientists at Berkeley and may be the first example of a new type of exploding star. A team of astrophysicists at UC Santa Barbara had predicted this kind of explosion in their t ...
Hubble Image Showcases Star Birth in M83, the Southern Pinwheel
Nov 05, 2009 |
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(PhysOrg.com) -- The spectacular new camera installed on NASA's Hubble Space Telescope during Servicing Mission 4 in May has delivered the most detailed view of star birth in the graceful, curving arms of ...
Shedding Light on the Cosmic Skeleton
Nov 03, 2009 |
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(PhysOrg.com) -- Astronomers have tracked down a gigantic, previously unknown assembly of galaxies located almost seven billion light-years away from us. The discovery, made possible by combining two of the ...
Fermi Telescope Detects Gamma-Ray From 'Star Factories' in Other Galaxies
Nov 02, 2009 |
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Nearby galaxies undergoing a furious pace of star formation also emit lots of gamma rays, say astronomers using NASA's Fermi Gamma-ray Space Telescope. Two so-called "starburst" galaxies, plus a satellite ...
Starburst galaxy sheds light on longstanding cosmic mystery
Nov 02, 2009 |
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(PhysOrg.com) -- An international collaboration that includes scientists from the University of Delaware's Bartol Research Institute in the Department of Physics and Astronomy has discovered very-high-energy ...
Dark Matter in a Galaxy
Oct 30, 2009 |
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(PhysOrg.com) -- Stars, the most familiar objects in the night sky, make up only a tiny percentage of the total amount of matter in the universe -- about 2%.
Discovery of dwarf galaxy a big find for astronomy team
Oct 20, 2009 |
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In some ways, discovering a new galaxy is all in a day's work for John Cannon, Macalester College assistant astronomy professor.
X-Ray Jets from Galaxies
Oct 19, 2009 |
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(PhysOrg.com) -- Some dramatic galaxies eject gigantic, collimated jets of ionized gas millions of light-years long, powered by the massive black holes at their centers. The ionized jets are detected at radio ...
Astronomers seek to explore the cosmic Dark Ages
Oct 15, 2009 |
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No place seems safe from the prying eyes of inquisitive astronomers. They've traced the evolution of the universe back to the "Big Bang," the theoretical birth of the cosmos 13.7 billion years ago, but there's still a long ...
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Cosmic archaeology: Astrophysicists use new spectrographs to look far back into the history of the universe
Oct 13, 2009 |
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(PhysOrg.com) -- The distant past of the universe is moving closer. Astronomers are using special spectrographs to investigate galaxies in the depths of the universe as part of the Sloan Digital Sky Survey ...
Sky merger yields sparkling dividends
Oct 13, 2009 |
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(PhysOrg.com) -- Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy ...
Invisible hand in invisible matter
Oct 06, 2009 |
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(PhysOrg.com) -- An international team of astronomers have found an unexpected link between mysterious 'dark matter' and the visible stars and gas in galaxies that could revolutionise our current understanding ...
