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
Newly discovered star one of hottest in Galaxy (w/ Video)
Dec 02, 2009 |
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(PhysOrg.com) -- Astronomers at The University of Manchester's Jodrell Bank Centre for Astrophysics have discovered one of the hottest stars in the Galaxy with a surface temperature of around 200,000 degrees ...
The Energy Sources of Ultraluminous Galaxies
Nov 27, 2009 |
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(PhysOrg.com) -- Ultraluminous infrared galaxies ((ULIRGs) are galaxies whose luminosity exceeds that of a trillion suns; for comparison, the Milky Way galaxy has a typical (and much more modest) luminosity ...
'Cosmic fruit machine' matches collisions
Nov 24, 2009 |
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(PhysOrg.com) -- A new website will give everyone the chance to contribute to science by playing a 'cosmic fruit machine' and compare images of colliding galaxies with millions of simulated images of galactic ...
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 ...
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%.
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 ...
Stripped down: Hubble highlights two galaxies that are losing it
Sep 30, 2009 |
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(PhysOrg.com) -- Ram pressure is the drag force that results when something moves through a fluid -- much like the wind you feel in your face when bicycling, even on a still day -- and occurs in this context ...
New Vista of Milky Way Center Unveiled
Sep 22, 2009 |
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(PhysOrg.com) -- A dramatic new vista of the center of the Milky Way galaxy from NASA's Chandra X-ray Observatory exposes new levels of the complexity and intrigue in the Galactic center. The mosaic of ...
Zooming to the centre of the Milky Way -- GigaGalaxy Zoom phase 2
Sep 21, 2009 |
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(PhysOrg.com) -- The second of three images of ESO's GigaGalaxy Zoom project has just been released online. It is a new and wonderful 340-million-pixel vista of the central parts of our home galaxy as seen ...
Galaxy-Sized Observatory for Gravitational Waves
Sep 17, 2009 |
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(PhysOrg.com) -- Astronomers are making plans to create a galaxy-sized observatory to look for gravitational waves. The project is part of a joint effort with astronomers from Australia and Europe, who also ...
A Theory of Dark Matter
Sep 08, 2009 |
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Among the most astounding, unexpected, and important achievements of the past century (or even more) have been the discoveries of dark matter and dark energy, collectively dubbed the "dark sector."
No strain for Andromeda: Galaxy is cosmic cannibal (w/ Video)
Sep 02, 2009 |
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(PhysOrg.com) -- A huge galaxy neighbouring our own Milky Way appears to have expanded by "digesting" smaller galaxies nearby, a new study has shown.
Is the Milky Way doomed to be destroyed by galactic bombardment? Probably not
Aug 31, 2009 |
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(PhysOrg.com) -- As scientists attempt to learn more about how galaxies evolve, an open question has been whether collisions with our dwarf galactic neighbors will one day tear apart the disk of the Milky ...
Galaxies Demand a Stellar Recount
Aug 19, 2009 |
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(PhysOrg.com) -- For decades, astronomers have gone about their business of studying the cosmos with the assumption that stars of certain sizes form in certain quantities. Like grocery stores selling melons ...
Galaxy Zoo Hunters Help Astronomers Discover Rare 'Green Pea' Galaxies
Jul 27, 2009 |
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A team of astronomers has discovered a group of rare galaxies called the "Green Peas" with the help of citizen scientists working through an online project called Galaxy Zoo. The finding could lend unique ...
