(Box) 1. Glossary of terms
Asteroid a rocky planetesimal (tiny planet) that orbits in a belt between Mars and Jupiter; probably the remnant of a larger object that was destroyed as a result of multiple collisions.
Astronomical unit (AU) a unit of distance equal to that from the sun to the Earth.
Big Bang the theoretical beginning of the universe, in which hydrogen and helium were synthesised.
Comet a 'snowball-like' planetesimal that formed in the outer solar system from an agglomeration of icy grains.
Dense molecular clouds a mixture of hydrogen, helium and the ejecta from supernovae and stellar winds. They are between 10 and 100 light-years in diameter and the densest clouds contain millions of particles per cm3.
Galaxy an autonomous collection of stars, gas and dust. There are thought to be about a thousand million galaxies in the observable universe.
Interstellar dust a component of the interstellar medium (see left), includes polycyclic aromatic hydrocarbon (PAH) molecules, silicates, graphite and microdiamonds.
Interstellar medium (ISM) the gaseous and dusty matter permeating interstellar space, which forms one-tenth of the mass of our galaxy.
Stellar wind outflow of material from the atmosphere of a star.
Supernova the final explosion of a massive star, which for several months shines brighter than a whole galaxy. As a result of this process most of the elements in the Periodic Table are synthesised and dispersed into the interstellar medium.