Decrease in intensity of radiation, when it crosses a material medium, as a consequence of an interaction between the radiation and the material medium.

A gaseous suspension of ultramicroscopic particles of a liquid or a solid.

Layer of gases surrounding a star or planet.

Icy body which orbits the Sun. Thought to be leftover planetesimals from the formation of planets in the outer Solar System. The small, solid nucleus consists of water and other ices coated with dark organic compounds. As the nucleus approaches the Sun, it vapourises, creating a coma and two main tails. These tails - one made of gas and one of dust [..]

An unstable orbit from which the orbiting object will gradually spiral into the body it is orbiting.

The unrestricted collecting area of a telescope after obstructions in the optical path have been taken into account.

An orbit which describes an ellipse or oval shape.

A mirror whose surface is figured to the shape of a paraboloid, a particular form of open curve.

The first time that nuclear atomic particles, neutrons and protons, could combine to make atomic nuclei. This happened during the first thousand seconds of existence of the Universe. The first atomic nuclei made were those of the light elements.

The process by which solids are transformed directly into vapor without passing through the liquid state.