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the branch of physics that studies celestial bodies and the universe as a whole


the branch of astrophysics that studies the origin and evolution and structure of the universe

light year

the distance that light travels in a vacuum in 1 year

astronomical unit

a unit of length used for distances within the solar system


a unit used in astronomy to describe large distances (1=3.26 light years)


space object made of dust and rock particles mixed with frozen water, methane, and ammonia that forms a bright coma as it approaches the Sun


any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter)


stony or metallic object that is the remains of a meteoroid that has reached the earth's surface


(astronomy) any of the small solid extraterrestrial bodies that hits the earth's atmosphere

Kuiper Belt

a region of the solar system that is just beyond the orbit of neptune and that contains small bodies made mostly of ice

Oort Cloud

a hypothetical huge collection of comets orbiting the sun far beyond the orbit of Pluto

Alpha Centuri

nearest star outside of our solar system


an immense cloud of gas (mainly hydrogen) and dust in interstellar space


a collection of star systems

Andromeda Galaxy

a spiral galaxy in the constellation of Andromeda that is visible to the naked eye

active galactic nucleus

a galaxy's core in which highly energetic objects or activities are located

black hole

final stage in the evolution of a very massive star, where the core's mass collapses to a point that its gravity is so strong that not even light can escape

Large Magellanic Cloud

nearest companion galaxy to the Milky Way

Local Group

a group of about 30 galaxies that includes the Milky Way


a gravitationally bound collection of many clusters of galaxies

globular cluster

a large, round, densely-packed grouping of older stars

gamma ray bursts

a burst of gamma ray photons


a spinning neutron star that emits rapid pulses of light


a "quasi-stellar" object; a starlike source of light that is extremely far away; one of the most powerful sources of energy in the universe


a star that explodes and becomes extremely luminous in the process


a hypothetical connecting passage between black holes and other places in the universe

white dwarf

late stage in the life cycle of a comparatively low-mass main sequence star; formed when its core depletes its helium and its outer layers escape into space, leaving behind a hot, dense core

red giant

a very large old reddish star that is greatly expanded and cooled as its fuel has begun to run out;

dark energy

A mysterious force that appears to be causing the expansion of the universe to accelerate

dark matter

Unseen matter that may make up more than ninety percent of the universe. As the name implies, it does not interact with light or other electromagnetic radiation, so it cannot be seen directly, but can be detected by measuring its gravitational effects. It is believed that it was instrumental in forming galaxies in the Big Bang.


Brightest star in the night sky; known as the "dog star".

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