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In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables , sometimes called the Leavitt Law .
A classical Cepheid's luminosity is directly related to its period of variation. The longer the pulsation period, the more luminous the star. The period-luminosity relation for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds. [23]
Historically light was measured in the units of luminous intensity , luminance (candelas/m 2) and illuminance (lumen/m 2). After the discovery of ipRGCs in 2002 additional units of light measurement have been researched in order to better estimate the impact of different inputs of the spectrum of light on various photoreceptors .
The difference in bolometric magnitude between two objects is related to their luminosity ratio according to: [19] = where: M bol1 {\displaystyle M_{\text{bol1}}} is the bolometric magnitude of the first object
S Normae is a Classical Cepheid variable with a visual magnitude range of 6.12 to 6.77 and a period of 9.75411 days. The spectral type varies during the pulsation cycle from F8 to G0. Its mass has been measured at 6.3 M ☉ with reference to a close orbital companion, and it is over 3,000 times as luminous as the sun.
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RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than the Sun. [8] Their period is shorter, typically less than one day, sometimes ranging down to seven hours.