Description: In astronomy, brightness is the term for the amount of electromagnetic radiation an object emits, or the amount of light we receive from an object.

It is not a formal scientific term but is often used to mean electromagnetic flux received from an object (energy received per unit time per unit area of the receiver in W/m²). The term "intrinsic brightness" is often used to mean the luminosity of an object (in watts) and "surface brightness" is used for extended objects as a measure of the energy received from an object per unit time per unit area of the receiver per unit area on the sky of the object (W/m²/steradian² or W/m²/arcsecond²). These different measures of brightness can be defined over the whole spectrum the object emits in or in specific areas of the electromagnetic spectrum.

For historical reasons, astronomers describe the brightness of an object using the so-called magnitude system which is a logarithmic system and gives lower numbered values to brighter stars.

Changes in brightness can allow us to reconstruct physical processes, e.g. when a star grows and becomes brighter, or a darker object passes in front of a brighter one.

Related Terms:

• Electromagnetic Radiation
• Luminosity
• Magnitude

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".