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The physical universe is defined as all of space and time [a] (collectively referred to as spacetime) and their contents. [10] Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space.
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. [1] [2] The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System.
Size (left) and distance (right) of a few well-known galaxies put to scale. The following is a list of notable galaxies.. There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in the Local Supercluster, and an estimated 100 billion in all of the observable universe.
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The transition between Earth's atmosphere and outer space lacks a well-defined physical boundary, with the air pressure steadily decreasing with altitude until it mixes with the solar wind. Various definitions for a practical boundary have been proposed, ranging from 30 km (19 mi) out to 1,600,000 km (990,000 mi). [16]
The surface brightness radial profile of the galactic disc of a typical disc galaxy (viewed face-on) roughly follows an exponential function: = [],where is the galaxy's central brightness and is the scale length. [2]
Composite image of five galaxies clustered together just 600 million years after the Universe's birth [1]. A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, [1] with typical masses ranging from 10 14 to 10 15 solar masses.
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