| A | B |
|---|
| geocentric | a model of the universe in which Earth is at the center of the revolving planets and stars |
| heliocentric | a model of the solar system in which Earth and the other planets revolve around the sun |
| ellipse | an oval shape, which may be elongated or nearly circular; the shape of the planets' orbit |
| core | 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 transgerred mainly in the form of light |
| convection zone | the outermost layer of the sun's interior |
| photoshere | the inner layer of the sun's atomosphere that gives off its visible light |
| chromosphere | the middle layer of the sun's atomosphere |
| corona | the outer layer of the sun's atmosphere |
| solar wind | a stream of electrically charged partticles |
| sunspot | a dark area of gas on the sun's surface that is cooler than surrounding gases |
| prominence | 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 suddnely 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 giants | the name often given to the first four outer planets: Jupitar, Saturn, Uranus, Neptune |
| ring | a thin diskof small ice and rock particles surrounding a planet |
| comet | a loose collection of ice, dust, and small rocky particles, typically with a long, narrow orbit |
| coma | the fuzzy outer layer of a comet |
| nucleus | 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 |
| asteriods | rocky objects recolcing around the susn that are too small numerious to be considered planets |
| asteroid belt | the region of the solar system between the orbits of Mars and Jupitar, where many ansteriods are found |
| meteoroid | a chunk of rock or dust in space |
| meteor | a streak of light in the sky produced by the curning of a meteoroid in Eath's atmosphere |
| meteorite | a meteorite that passes through the atmosphere and hits Earth's surface |
| extraterrestrial life | life that exists other than that on Earth |
