| A | B |
|---|
| gravity | The force that attracts one object to another. Gravity holds galaxies together and it holds planets, moons, and spacecraft in orbit. It also holds you on the ground. |
| light-year | The distance that light travels in one year. One light-year equals about 6 trillion miles or 10 trillion km. |
| universe | Everything that exists – all the galaxies, black holes, stars, nebulas, moons, planets, comets, asteroids, meteoroids, and dust scattered through space. This includes you! |
| galaxy | A collection of millions or billions of stars plus lots of gas and dust, held together by gravity. There are spiral, elliptical, and irregular types of galaxies. |
| Milky Way | The galaxy that contains our solar system and all the stars you can see in the night sky with the naked eye. Also, the softly glowing band of light made from faint stars, that arcs across the night sky. |
| solar system | A star (or group of stars) and the planets, moons, comets, and asteroids that orbit it (them). |
| star | A large ball of hydrogen gas that produces light and heat by means of a nuclear reaction (fusion) in its core. |
| Sun | The star at the center of our solar system. It is a huge ball of hot gas, mostly hydrogen (92.1%) and helium (7.8%). In its core, extremely high temperatures and pressure fuse hydrogen into helium. This nuclear fusion releases energy that slowly travels to the surface and makes the Sun shine. |
| sun spots | Temporary areas of concentrated magnetic fields on the Sun. In these areas, the Sun is cooler and darker than other regions of the Sun. |
| geomagnetic storms | Disturbances in the geomagnetic field caused by gusts in the solar wind that blows by Earth. |
| solar radiation storms | Elevated levels of radiation that occur when the numbers of energetic particles increase. |
| radio blackouts | Disturbances of the ionosphere caused by X-ray emissions from the Sun. |
| planet | A large object that orbits a star. Planets shine by reflecting sunlight – they do not produce their own light like a star does. |
| rocky planet | A planet that is made mostly of rock. These are sometimes called terrestrial planets. In our solar system, Mercury, Venus, Earth, and Mars are rocky planets. |
| gas giant | A large planet made up primarily of hydrogen. In our solar system, Jupiter, Saturn, Neptune, and Uranus are gas giants. |
| year | The time it takes a planet to orbit the Sun. |
| orbit | The path of an object around another object. (Examples: the path a planet takes around the sun; the path a moon takes around a planet; the path asteroids or comets take around the sun.) |
| rotation | The spin of a planet, moon, or star on its axis. |
| axis | The imaginary line through the center of a planet, moon, star, or galaxy around which it rotates. |
| sidereal day | The time it takes a planet or moon to make one full turn on its axis. |
| solar day | The time it takes a planet or moon from one noon to the next moon. |
| moon | A natural object orbiting a planet. Also called a natural satellite. |
| asteroid | A small rocky or metallic object orbiting the Sun, sometimes called a minor planet. Most asteroids in our solar system orbit in the main asteroid belt between Mars and Jupiter. |
| Kuiper Belt | A region beyond the orbit of Neptune where multitudes of icy comets orbit the sun. |
| comet | – A small body made of ices and dust that orbits the Sun on an elongated path. When near the Sun, its ices grow warm, and the comet develops a large coma and streams off long tails of dust and gas. |
| coma | The gaseous atmosphere that surrounds the icy nucleus of a comet. It forms as the comet’s ices evaporate in the warmth of sunlight and can be thousands of miles wide. |
| Oord Cloud | A swarm of trillions of comets believed to extend from the Kuiper Belt out roughly halfway to the nearest star, or about 2 light-years. |
| meteor | The bright streak of light produced by a bit of space debris burning up as it enters the atmosphere at high speed. Meteors are also called shooting stars. |
| meteorite | A piece of solid debris that lands on the surface of a planet or moon. It may be stony or metallic or a mixture of the two. Most are pieces broken off from asteroids. |
| meteoroid | Any small debris traveling through space. Larger meteoroids are usually pieces of shattered asteroids. Smaller meteoroids are mostly dust particles shed by comets. |
| meteor shower | A large number of meteors that appear to come from one small area of sky. A meteor shower happens when Earth runs into the debris thrown off by a comet. Meteor showers are named for the constellation from which they appear to come. |
| crater | A dish- or bowl-shaped depression in the surface of a planet, moon, or asteroid. Most are geological scars caused by the high-speed impact of a meteorite. |
| atmosphere | A layer of gas surrounding a planet, moon, or star. |
| astronaut | A person trained for spaceflight. |
| astronomy | The scientific study of the universe, including the solar system, stars, and galaxies. |
| astronomer | One who studies the universe, including the solar system, stars, and galaxies. |
| new moon | Moon in which NONE of the side we see is lit up. |
| full moon | A moon in which the side we see is ALL lit up. |
| waning | A moon in which the amount lit up gets less each night. |
| waxing | A moon in which the amount lit up gets more each night. |
| cresent moon | A moon with less than half of the side we see lit up. |
| gibbous moon | A moon with more than half of the side we see lit up. |
| quarter moon | A moon with exactly half of the side we see lit up. |
| first quarter moon | A quarter moon when the moon is waxing. |
| third quarter moon | A quarter moon when the moon is waning. |
| Mercury | The first planet from the Sun. |
| Venus | The second planet from the Sun. |
| Earth | The third planet from the Sun. |
| Mars | The fourth planet from the Sun. |
| Jupiter | The fifth planet from the Sun. |
| Saturn | The sixth planet from the Sun. |
| Uranus | The seventh planet from the Sun. |
| Neptune | The eighth planet from the Sun. |
| Pluto | The ninth planet from the Sun. |
| Planet X | The tenth planet from the Sun. |
