Planets are rocky bodies, sometimes enveloped in an atmosphere, which may be very thick indeed. They are encircled by rings (made of small bits of rock), natural satellites (called moons) and artificial satellites (communications, GPS, etc). The Sun's planets are, in order, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Stars are entirely gaseous. They radiate enormous quantities of energy, which has its origin in nuclear fusion reactions taking place in the star's core. The Sun is a medium-sized star. Stars are encircled by planets (see above) and rings, consisting of quite large lumps of rock (the asteroid belt and Kuyper belt) or icy rock (the Oort cloud, from which comets occasionally detach themselves into an elongated elliptical orbits). Lumps of rock finding their way to Earth are called meteors (and 'meteorites' after they have landed).

Galaxies are collections of stars. Sometimes they are just that, and sometimes they are spiral in form with a central core (often harbouring a super-massive black hole) surrounded by a disc broken up into spiral arms (corresponding structurally to the rings surrounding stars and planets). A biggish galaxy will contain some 10¹¹ stars

The galaxies are organised into clusters, which in turn are organised into super-clusters. The complete assembly of super-clusters is what constitutes the Universe. The Universe contains some 10¹¹ galaxies.