IB Physics Option D Astrophysics Definitions

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Binary Star
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Two stars orbiting a common centre
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Black dwarf
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The remnant of a white dwarf after it has cooled down. It has very low luminosity
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Black hole
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A singularity that is the end result of the evolution of a very massive star
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Cepheid variable
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A star of variable luminosity. The luminosity increases sharply and falls off gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star
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Cluster of galaxies
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Galaxies close to one another and affecting one another gravitationally, behaving as one unit
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Constellation
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A group of stars in a recognizable pattern that appear to be near each other in space
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Dark matter
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Matter in galaxies that is too cold to radiate. Its existence is inferred from techniques other than direct visual observation
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Galaxy
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A collection of very large number of stars held together by gravitational forces
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Main sequence star
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A normal star that is undergoing nuclear fusion of hydrogen into helium.
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Neutron star
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The end result of the explosion of a red super giant. A very dense star consisting of neutrons
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Planet
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A celestial body that orbits a star
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Planetary system
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Gravitationally bounded non-stellar objects in orbit around a star or star system.
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Planetary nebula
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The ejected envelope of a red giant star
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Nebula
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A cloud of dust, hydrogen, helium and other ionized gases.

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