Astronomy Basics

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star

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A ball of gas in space that produces its own light and heat. It has a large gravity because it has a large mass.

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solar system

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The sun and all of the planets and other objects that travel around it due to gravity.

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gravity

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A force of attraction between objects that is due to their mass and distance (more mass = strong gravity, close distance = strong gravity).

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galaxy

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A huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity.

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planet

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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.

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natural satellite

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Smaller objects that orbit larger objects and are not man-made; Earth's moon is an example.

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planet revolution

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(Orbit) a regular, repeating path that one object in space takes around another object due to gravity and inertia.

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planet rotation

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The spinning of a planet on its axis; one rotation equals one day/night for that planet.

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universe

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All matter and energy that exists in space (galaxies, stars, solar systems, dust, gas, and empty space).

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inertia

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The tendency of an object to resist a change in motion (speed and direction); objects in motion/at rest will stay in motion/at rest unless acted on by an outside force.