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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; the shape of the planets' orbits


the central region of the sun, where nuclear fusion takes place

nuclear fusion

the process by which hydrogen atoms join together in the sun's core to form helium

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 sun's 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

a stream of electrically charged particles that eminate from the sun's corona


a dark area of gas on the sun's surface 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 often given to the four inner planets: Mercury, Venus, Earth, and Mars

greenhouse effect

the trapping of heat by a planet's 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


a loose collection of ice, dust, and small rocky particles, typically with a long, narrow orbit


the fuzzy outer layer of a comet


the solid inner core of a comet

Kuiper belt

a doughnut shaped region that stretches from around Pluto's orbit to about 100 times Earth's distance from the sun

Oort Cloud

a spherical region of comets that surrounds the solar system


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

asteroid belt

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


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 passes through the atmosphere and hits Earth's surface

extraterrestrial life

life that exists other than on Earth

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