Physics - Astronomy

Physics - Astronomy

Definitions:

Star: Massive, luminous ball of plasma; held together by gravity; shining due to thermonuclear fusion. (EG: the Sun)

Planet: A body large enough to be rounded by its own gravitational pull and held in orbit by the gravitational attraction between the planet and its star. (EG: Jupiter)

Moon: A natural satellite of a planet; held by the gravitational attraction between the Moon and its planet. (EG: Enceladus, one of Saturn's moons)

Asteroid: Lumps of rock from several metres up to a 100km. (EG: The asteroid belt between Mars & Jupiter)

Comet: Has a nucleus of ice/dust/rock; from a few hundred metres up to tens of km; usually have an elliptical orbit, with the Sun not at its centre. (EG: Halley's comet)

Meteoroid: From sand to boulder sized debris; meteor = shooting star (EG: The Perseid shower); meteorite = solid matter that lands (EG: Hoba, the largest known meteorite)

• The Solar System is around 4.6 billion years old
• We know this through radiodating, using the half life of uranium
• An accretion disc is a spinning cloud flattened into a disc
• An accretion disc contains H, He (+water, ice, methane, ammonia, and rocky material)
• Accretion discs grew via collisions with other fragments
• Jupiter and Saturn were the first planets formed