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galaxy - n. any one of the very large groups of stars that make up the universe
theory - n. an idea or set of ideas that is intended to explain facts or events
astrophysicist - n. a scientist who studies the physical and chemical properties and structures of stars, planets and other objects in outer space
gravity - n. the natural force that tends to cause physical things to move towards each other
collision - n. a crash in which two or more things or people hit each other
pulsational - adj. a periodically recurring increase and decrease of a quantity, such as pressure, volume or voltage
astronomy - n. the scientific study of stars, planets, and other objects in outer space
