Galaxies 
The Universe: Galaxies
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.
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Featured Article
Galaxies - Detail
Galaxies are the lighthouses that plumb the Universe – constituents of the largest-scale texture we know. They span a vast range of properties, from dwarf galaxies with a few million stars barely...
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Featured Blog Post
Notes from the Astronomy Underground- Astropalooza
According to the tagline in Ridley Scott’s 1979 blockbuster Alien, “In space, no one can hear you scream.” It’s true that sound waves, unlike light, need a medium- some kind of...
Latest News
Dense Gas in Ultraluminous Infrared Galaxies
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...
