Ammonia

A poisonous gas, with a strong unpleasant smell, or the gas dissolved in water.

Andromeda Galaxy

A spiral galaxy (=group of stard and planets) about two million light years away and about twice as large as our galaxy.

Asteroid

A mass of rock that moves around in space. Most asteroids are found in the region of space beween the planets Jupiter and Mars, a region know as the asteroid belt.

Atmosphere

The air around the Earth or around another planet. It consists of three main layers, the lowest being the tropposphere, the middle one the stratosphere, and the highest the ionosphere.

Axis

An imaginary line through the middle of an object such as a planet, around wich it seems to spin.

Big Bang

The explosion of a very large mass of matter that is believed to have caused the Universe to begin to exist. The Big Bang is believed to have happened about 15 billion years ago, and this theory explains why the Universe is still increasing in size.

Black hole

An object in outer space that has such strong gravity that nothing near it can escape from it, not even light. Black holes are thought to be formed when a very large star stops existing.