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a model of the universe in which earth is at the center of the revolving planets and stars


a model of the solar system in which Earth and the other planets revolve around the sun


an oval shape, which may be elongated or nearly circular

solar system

system made up of the nine unique planets, including earth, and many smaller objects, that orbit the sun.

astronomical unit

A unit of length used for distances within the solar system


an object that orbits a star is large enough to have become rounded by its own gravity and has cleared the area of its orbit

dwarf planet

a large round object that revolves around the sun but has not cleared the region around its orbit


one of the small asteroid-like bodies that formed the building blocks of the planets


the central region of the sun where nuclear fusion takes place

nuclear fusion

the process in which two atomic nuclei combine to form a larger nucleus, forming a heavier element and releasing huge amounts of energy

radiation zone

a region of very tightly packed gas in the sun's interior where energy is transferred mainly in the form of light

convection zone

the outermost layer of the sun's interior


the inner layer of the suns atmosphere that gives off its visible light


the middle layer of the sun's atmosphere


the outer layer of the sun's atmosphere

solar wind

the blocking of gas from the sunlight to the earth that occurs when the moon is directly between the sun and earth


a dark area of gas on the sun that is cooler than surrounding gases


A huge, reddish loop of gas that protrudes from the sun's surface, linking parts of sunspot regions

solar flare

an eruption of gas from the sun's surface that occurs when the loops in sunspot regions suddenly connect

terrestrial planets

The name given to the four inner planets: Mercury, Venus, Earth, and Mars

greenhouse effect

the trapping of the heat near a planets surface by certain gases in the planets atmosphere

gas giant

The name often given to the first four outer planets: Jupiter, Saturn, Uranus, and Neptune.


a thin disk of small ice and rock particles surrounding a planet

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter, where many asteroids are found

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 spherical region of comets that surrounds the solar system


loose collections of ice, dust, and small particles whose orbits are usually very long, narrow ellipses


the fuzzy outer layer of a comet


the center of the atom which contains the protons and neutrons; in cells, structure that contains the cell's genetic material (DNA) and controls the cell's activities


Objects revolving around the sun that are too small and too numerous to be considered planets


A chunk of rock or dust in space.


a streak of light in the sky produced by the burning of a meteoroid in Earth's atmosphere


a meteoroid that strikes the surface of a moon or planet

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