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
Hunting for Planets in the Dark
Nov 19, 2009 |
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A proposed space mission that aims to measure dark energy could also detect planets that current surveys are unable to find.
Maps Unveil the Source of Starburst Galaxy's Winds
Nov 18, 2009 |
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(PhysOrg.com) -- A research group at Kyoto University has discovered that shocks are the primary energy sources that excite the galactic wind region of starburst galaxy NGC 253. Their images of the center ...
NGC 4710 galaxy: Baffling boxy bulge (w/ Video)
Nov 18, 2009 |
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(PhysOrg.com) -- Just as many people are surprised to find themselves packing on unexplained weight around the middle, astronomers find the evolution of bulges in the centres of spiral galaxies puzzling. A ...
COBE Satellite Marks 20th Anniversary
Space & Earth / Space Exploration
Nov 17, 2009 |
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(PhysOrg.com) -- NASA's Cosmic Background Explorer (COBE) satellite rocketed into Earth orbit on Nov. 18, 1989, and quickly revolutionized our understanding of the early cosmos. Developed and built at Goddard ...
Mysterious X-rays from a Nearby Galaxy
Nov 13, 2009 |
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(PhysOrg.com) -- The nucleus of an active galaxy, an AGN, contains a massive black hole that is vigorously accreting material. In the process it typically ejects jets of particles and radiates brightly at ...
Do we need dark matter?
Nov 12, 2009 |
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It's the biggest problem in physics: the matter we can see in the universe accounts for just five per cent of the observed gravity that holds galaxies together.
Rapid star formation spotted in 'stellar nurseries' of infant galaxies
Nov 11, 2009 |
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The Universe's infant galaxies enjoyed rapid growth spurts forming stars like our sun at a rate of up to 50 stars a year, according to scientists at Durham University.
NASA's Great Observatories Celebrate International Year of Astronomy
Nov 10, 2009 |
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(PhysOrg.com) -- A never-before-seen view of the turbulent heart of our Milky Way galaxy is being unveiled by NASA on Nov. 10. This event will commemorate the 400 years since Galileo first turned his telescope ...
Swift, XMM-Newton satellites tune into a middleweight black hole
Nov 10, 2009 |
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(PhysOrg.com) -- While astronomers have studied lightweight and heavyweight black holes for decades, the evidence for black holes with intermediate masses has been much harder to come by. Now, astronomers ...
'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 ...
