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Glossary term: Magnitude

Description: In astronomy, the magnitude is a measure of how bright a celestial object is. The magnitude system used in astronomy originated in antiquity as a ranking of stars from the brightest to the least bright. That is why a smaller (or more negative) magnitude value means that the object is brighter, and a larger number means a fainter object. Hence a star with magnitude -1 is brighter than a star with magnitude 0 which in turn is brighter than a star of magnitude 1.

Magnitude has a logarithmic scale, in which a magnitude difference of five corresponds to a factor of 100 difference in the amount of energy received: a star with magnitude 10 is a hundred times dimmer than a star with magnitude 5.

There are different types of magnitude: apparent magnitude measures the apparent brightness of an object, which depends both on the object's luminosity – how much light the object emits – and on its distance from Earth.

In contrast, the absolute magnitude is the value we would obtain if the object were at a standard distance of 10 parsecs (32.6 light years) from Earth. (For reflecting objects such as asteroids, there is a different definition.)

In practice, magnitude is specified for observations through a specific filter, corresponding to an object's brightness in a given wavelength range of light. Numerous "photometric systems" for specifying filters, and corresponding magnitudes, exist. In contrast, the bolometric magnitude is a direct measure of the luminosity of an object: the total electromagnetic energy emitted in unit time. Visual magnitudes correspond to the brightness as perceived by the human eye.

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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".

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How Light Pollution Affects the Stars: Magnitude Readers

How Light Pollution Affects the Stars: Magnitude Readers

astroEDU educational activity (links to astroEDU website)
Description: Build a Magnitude Reader to explore the magnitude of stars.

License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons

Tags: Hands-on , Dark skies , ecology
Age Ranges: 10-12 , 12-14 , 14-16 , 16-19
Education Level: Middle School
Areas of Learning: Modelling , Observation based
Costs: Medium Cost
Duration: 1 hour
Group Size: Group
Skills: Asking questions , Planning and carrying out investigations

Let's play with powers of 10

Let's play with powers of 10

astroEDU educational activity (links to astroEDU website)
Description: A game of cards to understand powers of 10 and the very diverse scales in Nature

License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons

Tags: Scales , Game , Distances , size , Power , length
Age Ranges: 12-14 , 14-16 , 16-19 , 19+
Education Level: Informal , Secondary , University
Areas of Learning: Guided-discovery learning , Social Research
Costs: Low Cost
Duration: 2 hours
Group Size: Group
Skills: Analysing and interpreting data , Asking questions