| A | B |
|---|
| Geocentric | Term describing a model of the universe in which Earth is at the center of the revolving planets and stars. |
| Heliocentric | Term describing a model of the solar system in which Earth and the other planets revolve around the sun. |
| Astronomical unit | A unit of distance equal to the average distance between Earth and the sun, about 150 million kilometers. |
| Planet | 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 | An object that orbits the sun and is spherical, but has not cleared the area of its orbit. |
| Plantesimal | One of the small asteroid-like bodies that formed the building blocks of the planets. |
| 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; the process by which energy is produced in stars. |
| Radiation zone | A region of very tightly packed gas in the sun’s interior where energy is transferred mainly in the form of electromagnetic radiation. |
| Convection zone | The outermost layer of the sun’s interior. |
| Photosphere | The inner layer of the sun’s atmosphere that gives off its visible light; the sun’s surface. |
| Chromosphere | The middle layer of the sun’s atmosphere. |
| Corona | The outer layer of the sun’s atmosphere. |
| Solar wind | A stream of electrically charged particles that emanate from the sun’s corona. |
| 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 suddenly connect. |
| Greenhouse effect | The trapping of heat near a planet’s surface by certain gases in the planet’s atmosphere. |
| Ring | 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 where many small objects orbit the sun and that stretches from beyond the orbit of Neptune to about 100 times Earth’s distance from the sun. |
| Oort cloud | A spherical region of comets that surrounds the solar system. |
| Comet | A loose collection of ice and dust that orbits the sun, typically in a long, narrow orbit. |
| Coma | The fuzzy outer layer of a comet. |
| Asteroid | One of the rocky objects revolving around the sun that are too small and numerous to be considered planets. |
| Meteoroid | A chunk of rock or dust in space, generally smaller than an asteroid. |
| Meteor | A streak of light in the sky produced by the burning of a meteoroid in Earth’s atmosphere. |
| Meteorite | A meteoroid that passes through the atmosphere and hits Earth’s surface. |
