A galaxy is a massive, gravitationally bound system that consists of
stars, an interstellar medium of gas and dust, and dark matter.
Typical galaxies range from dwarfs with as few as ten million
(10^7) stars up to giants with one trillion
(10^12) stars, all orbiting a common center of gravity.
Galaxies can also contain a large number of multiple star systems and
star clusters as well as various types of interstellar clouds.
Historically, galaxies have been categorized according to their
apparent shape. Interactions between nearby galaxies, which may
ultimately result in a galaxy merger, may induce episodes of
significantly increased star formation, producing what is called a
starburst galaxy. There are probably more than a hundred billion
(10^11) galaxies in the observable universe. Most galaxies
are a thousand to a hundred thousand parsecs in diameter and are
usually separated from one another by distances on the order of
millions of parsecs. Intergalactic space, the space between galaxies,
is filled with a tenuous gas with an average density less than one
atom per cubic metre. There is some evidence that supermassive black
holes may exist at the center of many, if not all, galaxies. These
massive objects are believed 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.
Read the rest of this article:
http://en.wikipedia.org/wiki/Galaxy
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Today's selected anniversaries:
1776:
Samuel Nicholas and the Continental Marines successfully landed on New
Providence and captured Nassau in the Bahamas.
(
http://en.wikipedia.org/wiki/Samuel_Nicholas)
1878:
The signing of the Treaty of San Stefano established Bulgaria as an
autonomous principality in the Ottoman Empire.
(
http://en.wikipedia.org/wiki/Treaty_of_San_Stefano)
1918:
Bolshevist Russia signed the Treaty of Brest-Litovsk with the Central
Powers and exited from World War I.
(
http://en.wikipedia.org/wiki/Treaty_of_Brest-Litovsk)
1931:
"The Star-Spangled Banner" officially became the national anthem of
the United States.
(
http://en.wikipedia.org/wiki/The_Star-Spangled_Banner)
1958:
Nuri as-Said became the Prime Minister of Iraq for the 14th time.
(
http://en.wikipedia.org/wiki/Nuri_as-Said)
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Wikiquote of the day:
You cannot force ideas. Successful ideas are the result of slow
growth. Ideas do not reach perfection in a day, no matter how much
study is put upon them. -- Alexander Graham Bell
(
http://en.wikiquote.org/wiki/Alexander_Graham_Bell)