[flux](https://learn2.open.ac.uk/mod/glossary/showentry.php?eid=1146990&displayformat=dictionary "S284 Glossary: flux") is a measure of how much energy in the form of light (or other electromagnetic radiation) we _observe_ from a star or galaxy in a given time interval. It therefore depends both on the luminosity of the star or galaxy in question _and_ on its distance away from us. It is measured in the SI unit of power per unit area, which is watts per square metre (W m−2). It can be related to apparent magnitude (m) [[../Chapters/Part 2 Photometry measuring brightness]] [[luminosity]] [[Docent/S284 Astronomy]]