A flow of matter that is largely confined to a plane and that is spiralling in towards a central object.

The angle between two lines drawn from an observer’s eye to opposite sides of the object being observed.

A body made of mainly rocky materials (including metals) in orbit around the Sun, that is typically much less massive than a planet. Most asteroids are found in the asteroid belt between Mars and Jupiter. Asteroids (and comets) are often called minor planets. The largest known asteroid (Ceres) is classified as a dwarf planet.

A region between Mars and Jupiter in which most asteroids reside in stable orbits. Also called main belt.

The study of the movements and relative positions of celestial bodies claimed as having an influence on human affairs and the natural  world. Astrology is not a science.

Astronomy is the scientific study of stars, space, and the physical universe as a whole.

The building block of chemistry comprising a tiny, dense, positively charged atomic nucleus (which can contain both protons and neutrons) with circulating negatively charged electrons equal in number to the number of protons in the nucleus.

The event which is believed to mark the origin of time and space. Consequences of the Big Bang include the fact that space is expanding, that the temperature of the Universe is falling and that elements or isotopes such as helium, lithium and deuterium have certain abundances in the Universe.

An hypothesis that, if there were enough matter in the Universe, the expansion of the Universe would eventually become a contraction, leading to a big crunch. This hypothesis is no longer believed to be valid.