Related topics: black holes , stars , milky way , hubble space telescope , universe
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
Forming the present-day spiral galaxies
Feb 04, 2010 |
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(PhysOrg.com) -- Using data from the NASA/ESA Hubble Space Telescope, astronomers have created a demographic census of galaxy types and shapes from a time before the Earth and the sun existed, to the present ...
On the Trail of a Cosmic Cat (w/ Video)
Jan 20, 2010 |
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(PhysOrg.com) -- ESO has just released a stunning new image of the vast cloud known as the Cat’s Paw Nebula or NGC 6334. This complex region of gas and dust, where numerous massive stars are born, lies near ...
Snowflake-Shaped Galaxy From Hubble Helps Ring in the New Year
Jan 18, 2010 |
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(PhysOrg.com) -- As part of its Hubble Heritage program, NASA has released an image, taken by a team led by UA astronomer Rodger Thompson, of a galaxy that resembles a snowflake.
Dense Gas in Ultraluminous Infrared Galaxies
Jan 15, 2010 |
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(PhysOrg.com) -- Ultraluminous infrared galaxies have luminosities that exceed a trillion suns.
Alien Planet Safari
Jan 14, 2010 |
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The premiere observatory of the next decade, the James Webb Space Telescope, will launch in 2014 in search of "big game"--namely, the first stars and galaxies ever formed in our Universe. But the "little game" ...
How Galaxies Came To Be: Astronomers Explain Hubble Sequence
Jan 12, 2010 |
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(PhysOrg.com) -- For the first time, two astronomers have explained the diversity of galaxy shapes seen in the universe. The scientists, Dr Andrew Benson of the California Institute of Technology (Caltech) ...
Surveying the X-ray Sky
Jan 11, 2010 |
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(PhysOrg.com) -- Astronomers have only modest laboratories to probe the mysteries of the cosmos. Mostly they have to rely on meticulous and clever observations of remote phenomena.
Flying Telescope Passes Key Test
Space & Earth / Space Exploration
Jan 08, 2010 |
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Most astronomers wouldn't dream of opening their observatory's doors in 100 mph winds. Yet NASA's new SOFIA telescope recently flew in an airplane at 250 mph with doors wide open.
Astronomers: We could find Earth-like planets soon
Space & Earth / Space Exploration
Jan 07, 2010 |
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(AP) -- Astronomers say they are on the verge of finding planets like Earth orbiting other stars, a key step in determining if we are alone in the universe.
Team Shines Cosmic Light on Missing Ordinary Matter
Jan 07, 2010 |
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(PhysOrg.com) -- An international team of scientists, led by University of Maryland astronomer Stacy McGaugh, has found that individual galactic objects have less ordinary matter, relative to dark matter, than does the Universe ...
WISE Eye Spies First Glimpse of the Starry Sky
Jan 06, 2010 |
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(PhysOrg.com) -- NASA's Wide-field Infrared Survey Explorer, or WISE, has captured its first look at the starry sky that it will soon begin surveying in infrared light.
Astronomers Detect Cosmic 'Dark Energy' in Galaxies Nearest Earth
Jan 06, 2010 |
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(PhysOrg.com) -- An international team of astronomers, including a University of Alabama researcher, have detected effects of "dark energy" within the Local Group of galaxies containing our own Milky Way Galaxy.
Galaxy History Revealed in This Colorful Hubble View (w/ Video)
Jan 06, 2010 |
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(PhysOrg.com) -- More than 12 billion years of cosmic history are shown in this unprecedented, panoramic, full-color view of thousands of galaxies in various stages of assembly.
Sagittarius A*: Peering Into The Heart of Darkness
Jan 05, 2010 |
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(PhysOrg.com) -- The supermassive black hole at the center of the Milky Way galaxy.
Hubble Reaches the 'Undiscovered Country' of Primeval Galaxies
Jan 05, 2010 |
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(PhysOrg.com) -- NASA's Hubble Space Telescope has broken the distance limit for galaxies and uncovered a primordial population of compact and ultra-blue galaxies that have never been seen before.
