| A | B |
|---|
| solar system | The system composed of the sun (a star) and the planets and other bodies that travel around the sun. |
| nebula | A large cloud of dust and gas in interstellar space; the location of star formation. |
| solar nebula | The nebula that formed into the solar system. |
| planetesimal | The tiny building blocks of the planets that formed as dust particles stuck together and grew in size. |
| rotation | The spinning motion of a body on its axis. |
| orbit | The elliptical path a body takes as it travels around another body in space; the motion itself. |
| revolution | The elliptical motion of a body as it orbits another body in space. |
| period of revolution | The time it takes for one body to make one complete orbit, or revolution, around another body in space. |
| ellipse | A closed curve in which the sum of the distances from the edge of the curve to two points inside the curve is always the same. |
| astronomical unit (AU) | The average distance between the Earth and the sun, or approximately 150,000,000 km. |
| corona | The sun's outer atmosphere, which can extend outward a distance equal to 10 - 12 times the diameter of the sun. |
| chromosphere | A thin region of the sun's atmosphere between the corona and the photosphere; too faint to see unless there is a total solar eclipse. |
| photosphere | The layer of the sun at which point the gases get thick enough to see; the surface of the sun. |
| convective zone | A region of the sun where gases circulate in convection currents, bringing the sun's energy to the surface. |
| radiative zone | A very dense region of the sun in which the atoms are so closely packed that light can take millions of years to pass through. |
| core | The center of the sun where the sun's energy is produced. |
| nuclear fusion | The process by which two or more nuclei with small masses join together, or fuse, to form a larger, more massive nucleus, along with the production of energy. |
| sunspot | An area on the photosphere of the sun that is cooler that surrounding areas, showing up as a dark spot. |
| crust | The thin, outermost layer of the Earth, or the uppermost part of the lithosphere. |
| mantle | The layer of the Earth between the crust and the core. |
| core | The central, spherical part of the Earth below the mantle. |
