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
Astronomers Discover Most Dark Matter-Dominated Galaxy in Universe
Sep 18, 2008 |
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(PhysOrg.com) -- A team led by a Yale University astronomer has discovered the least luminous, most dark matter-filled galaxy known to exist.
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 ...
Study plunges standard Theory of Cosmology into Crisis
May 05, 2009 |
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As modern cosmologists rely more and more on the ominous “dark matter” to explain otherwise inexplicable observations, much effort has gone into the detection of this mysterious substance in the last two decades, yet no direct ...
Hubble scores a perfect ten
Oct 30, 2008 |
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(PhysOrg.com) -- The Hubble Space Telescope is back in business after a one-month breakdown with a snapshot of the fascinating galaxy pair Arp 147. Scientists made two repair attempts, and last week's effort ...
Study shows clumps and streams of dark matter in inner regions of the Milky Way
Aug 06, 2008 |
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(PhysOrg.com) -- Using one of the most powerful supercomputers in the world to simulate the halo of dark matter that envelopes our galaxy, researchers found dense clumps and streams of the mysterious stuff ...
A pool of distant galaxies -- the deepest ultraviolet image of the universe yet
Nov 07, 2008 |
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This uniquely beautiful patchwork image, with its myriad of brightly coloured galaxies, shows the Chandra Deep Field South (CDF-S), arguably the most observed and best studied region in the entire sky. The ...
Colossal black holes common in early universe
Oct 16, 2008 |
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Astronomers think that many - perhaps all - galaxies in the universe contain massive black holes at their centers. New observations with the Submillimeter Array now suggest that such colossal black holes were ...
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."
'Missing link' galaxies discovered
Nov 25, 2008 |
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Astronomers at The University of Nottingham have identified a type of galaxy that could be the missing link in our understanding of galaxy evolution.
Physicists on the prowl for dark matter
Jul 23, 2009 |
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(PhysOrg.com) -- 95%. That is the percentage of the known Universe that is missing. As in it is not there. Or at least if it is there, we can't see it. We call this unseen stuff "dark matter". That has been ...
Ancient Galactic Magnetic Fields Stronger than Expected
Jul 23, 2008 |
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(PhysOrg.com) -- Mining the far reaches of the universe for clues about its past, a team of scientists including Philipp Kronberg of Los Alamos National Laboratory has proposed that magnetic fields of ancient galaxies like ...
Scientists discover minimum mass for galaxies
Aug 27, 2008 |
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(PhysOrg.com) -- By analyzing light from small, faint galaxies that orbit the Milky Way, UC Irvine scientists believe they have discovered the minimum mass for galaxies in the universe – 10 million times the ...
Herschel's daring test: A glimpse of things to come
Jun 19, 2009 |
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Herschel opened its 'eyes' on 14 June and the Photoconductor Array Camera and Spectrometer obtained images of M51, 'the whirlpool galaxy' for a first test observation. Scientists obtained images in three colours ...
Herschel first images promise bright future
Jul 10, 2009 |
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Herschel has carried out the first test observations with all its instruments, with spectacular results. Galaxies, star-forming regions and dying stars comprised the telescope's first targets. The instruments ...
Refined Hubble Constant narrows explanations for dark energy
May 07, 2009 |
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Whatever dark energy is, explanations for it have less wiggle room following a Hubble Space Telescope observation that has refined the measurement of the universe's present expansion rate to a precision where ...
