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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. There are an estimated 100 billion galaxies in all of the observable universe. [1] On the order of 100,000 galaxies make up the Local Supercluster, and about 51 galaxies are in the Local Group (see list of nearest galaxies for a complete list).
Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation).
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.
The eight planets of the Solar System with size to scale (up to down, left to right): Saturn, Jupiter, Uranus, Neptune (outer planets), Earth, Venus, Mars, and Mercury (inner planets)
True color image of the Trifid Nebula, showing complex gas and plasma structure. A nebula (Latin for 'cloud, fog'; [1] pl.: nebulae, or nebulas [2] [3] [4] [5]) is a ...
Mercury is the first planet from the Sun and the smallest in the Solar System.In English, it is named after the ancient Roman god Mercurius (), god of commerce and communication, and the messenger of the gods.
Euler diagram showing the IAU Executive Committee conception of the types of bodies in the Solar System.. The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, [1] a planet is a celestial body that: