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The formation of planets and stars is said to begin with one large event. Scientists believe the process begins when a large cloud of dust and gas collapses. The material that collapses to the center forms the star and the rest of the material is left to rotate as a cloud around the star. Eventually, after tens of thousands of years, the dust in the cloud clumps together, forming planetesimals. These bodies collide and become larger bodies known as planets. Stars are sources of energy; they emit light and heat created through nuclear fusion of hydrogen and helium in their cores. In contrast, planets have no renewable source of energy and cool off slowly through time.
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==Galaxy==
A galaxy or star cluster is a large collection of stars to each other by their combined gravity. This gravitationally bound system is composed of of stars, stellar remnants, interstellar gas, dust, and dark matter.
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===Stellar system===
A stellar system or star system is a small number of stars bound by gravitational attraction that orbit each other.

====Star====
What distinguishes a star from the other celestial bodies is its size and temperature. A star is a giant ball of very high temperature gas. The temperature at its center is high enough to enable 
nuclear (fusion) reactions that convert hydrogen into helium and generates light. <br>A star is a body of gas which is large enough and dense enough that the heat and crushing pressure at its center produces nuclear fusion.

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===Planetary system===
A planetary system is a set of gravitationally bound non-stellar objects in orbit around a star or star system. It is a system made of one or more planets.

====Planet====
A planet is a celestial body that has formed around a star from a collection of a combination of solids, liquids, and or gases. A planet orbits a star. <br>
Because it is smaller than a star, a planet never reaches an internal temperature high enough to ignite the thermonuclear reaction. Planets do not have the necessary mass, when sufficiently compressed by its own gravity, to support the hydrogen-to-helium fusion reactions. <br>

====Moons====
A moon, also called a natural satellite, is a celestial body that orbits another (its primary) of greater mass.

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