A galaxy is a principal component of our universe. It is made up of stars, an interstellar medium of gas and dust and a large number of stars, ranging from a few tens of million to as many as a trillion. Stars may congregate in clusters, associations and in spiral arms. Galaxies are also thought to contain vast amounts of dark matter, extending beyond the distribution of stars.  Galaxies are held together by gravity including that due to the dark matter. Most galaxies are 1,000 to 100,000 parsecs (3000 to 300,000 light-years) in diameter and are usually separated by distances on the order of millions of parsecs. There are about 100 billion galaxies in the visible Universe. When capitalized, Galaxy refers to our own Milky Way Galaxy.


  • Galaxies Demand a Stellar Recount Featured News Article Galaxies Demand a Stellar Recount Galaxies Demand a Stellar Recount

    NASA JPL, Pasadena, CA (Aug. 20, 2009) – For decades, astronomers have gone about their business of studying the cosmos with the assumption that stars of certain sizes form in... More »

  • 'Dropouts' Pinpoint Earliest Galaxies Featured News Article 'Dropouts' Pinpoint Earliest Galaxies 'Dropouts' Pinpoint Earliest Galaxies

    PASADENA, CA (Nov. 9, 2009) – Astronomers, conducting the broadest survey to date of galaxies from about 800 million years after the Big Bang, have found 22 early galaxies... More »

  • X-ray Jets from Galaxies Featured News Article X-ray Jets from Galaxies X-ray Jets from Galaxies

    Harvard-Smithsonian Center for Astrophysics (Oct.  22, 2009) – Some dramatic galaxies eject gigantic, collimated jets of ionized gas millions of light-years long, powered... More »

  • Baffling Galactic Bulge Featured News Article Baffling Galactic Bulge Baffling Galactic Bulge

    GARCHING, GERMANY (Nov. 18, 2009) – Just as many people are surprised to find themselves packing on unexplained weight around the middle, astronomers find the evolution of... More »

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ESO Very Large Telescope Shows Spectacular Spiral Galaxy in Leo Last Updated on 2011-08-10 00:00:00 ESO, Garching, Germany (10 Aug. 2011) – This new picture from ESO’s Very Large Telescope shows NGC 3521, a spiral galaxy located about 35 million light years away in the constellation of Leo (The Lion). Spanning about 50 000 light-years, this spectacular object has a bright and compact nucleus, surrounded by richly detailed spiral structure. The most distinctive features of the bright galaxy NGC 3521 are its long spiral arms that are dotted with star-forming regions and interspersed with veins of dust. The arms are rather irregular and patchy, making NGC 3521 a typical example of a flocculent spiral galaxy. These galaxies have “fluffy” spiral arms that contrast with the sweeping arms of grand-design spirals such as the famous Whirlpool galaxy or M 51, discovered by Charles Messier. NGC 3521 is bright and relatively close-by, and can easily be seen ... More »
Dense Gas in Ultraluminous Infrared Galaxies Last Updated on 2010-01-18 00:00:00 CAMBRIDGE, MA (Jan. 18, 2010) – Ultraluminous infrared galaxies have luminosities that exceed a trillion suns. (For comparison, the Milky Way's luminosity is only that of about ten billion suns.) Extreme infrared activity is known to be associated with interacting galaxies, and optical imaging indeed shows that many ultraluminous systems are in collision. The physical mechanism(s) that actually power the luminosity, however, are still not understood. Might the same process(es) be underway at a low level in our galaxy? One of the primary sources of global energy production in galaxies is star formation, and ultraluminous galaxies show all the diagnostic signs of having vigorous star formation. In a new paper by CfA astronomer Desika Narayanan and six colleagues, the case is made that this activity is the result of a higher proportion of dense clouds of gas in these objects, and... More »
Black hole zapping a galaxy into existence Last Updated on 2009-11-30 00:00:00 GARCHING, GERMANY (Nov. 30, 2009) – Which comes first, the supermassive black holes that frantically devour matter or the enormous galaxies where they reside? A brand new scenario has emerged from a recent set of outstanding observations of a black hole without a home: black holes may be “building” their own host galaxy. This could be the long-sought missing link to understanding why the masses of black holes are larger in galaxies that contain more stars. FIGURE CAPTION – This artist’s impression shows how jets from supermassive black holes could form galaxies, thereby explaining why the mass of black holes is larger in galaxies that contain more stars. “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today,” says lead author David Elbaz.... More »
Baffling Galactic Bulge Last Updated on 2009-11-19 17:01:52 GARCHING, GERMANY (Nov. 18, 2009) – 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 recent NASA/ESA Hubble Space Telescope image of NGC 4710 is part of a survey that astronomers have conducted to learn more about the formation of bulges, which are a substantial component of most spiral galaxies. When targeting spiral galaxy bulges, astronomers often seek edge-on galaxies, as their bulges are more easily distinguishable from the disc. This exceptionally detailed edge-on view of NGC 4710 taken by the Advanced Camera for Surveys (ACS) aboard Hubble reveals the galaxy's bulge in the brightly coloured centre. The luminous, elongated white plane that runs through the bulge is the galaxy disc. The disc and bulge are surrounded by eerie-looking dust... More »
Radio Galaxies: Early Universe Last Updated on 2009-11-11 17:09:13 Radio galaxies are cosmic beacons, with the brightest ones beaming nearly a trillion solar-luminosities of radiation into space at radio wavelengths. The origin of this intense emission is the environment of the massive black hole at the galaxy's nucleus -- a so-called active galactic nucleus (AGN). It is thought that electrons moving rapidly in strong magnetic fields produce the radio emission, but at the same time high temperature regions near the AGN also radiate intensely at other wavelengths. Astronomers are interested in understanding the physics underway in these extreme cosmic radio sources, and whether they are cosmic oddities or a normal evolutionary stage of galaxies like our own Milky Way. SAO astronomers Giovanni Fazio and Steve Willner, along with nine of their colleagues, used the Spitzer Space Telescope to study two well known radio galaxies whose light has been... More »