Astronomy dictionary – Q

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.



When a planet is 90 degrees from the Sun in the sky.

Quantum theory

This is the study and respective theories on how the Universe is put together on a tiny sub-atomic scale. It’s the dream of many scientists to combine quantum theory with the theories that explain the large scale Universe in one unified theory.

Quark star

A theoretical type of massive neutron star that is so dense that matter is broken down to its constituent particles. Quarks are the hypothetical constituent particles of subatomic particles, like neutrons.


A very distant active galaxy that emits large amounts of energy from its centre as electromagnetic radiation. The word is a contraction of ‘quasi-stellar radio source’.