period-luminosity

Period-luminosity relationclick for a larger image
Period-luminosity relation

period-luminosity

(P-L) relation A relation showing graphically how the period of the light variation of a Cepheid variable depends on its mean luminosity: the longer the period the greater the luminosity and hence mean absolute magnitude of the star. The relation was discovered by Henrietta Leavitt in 1912 and much subsequent work was done in calibrating the graph. It was found, in the 1950s, that there are two types of Cepheid – classical Cepheids and the less luminous type II Cepheids – whose P-L relations are approximately parallel, as shown on the graph. The relation results from the dependence of both luminosity and period of light variation, i.e. of pulsation, on stellar radius (see pulsating variables).

The relation is an invaluable means of determining the distances of Cepheids and hence of their surroundings. To calibrate the two curves, the mean absolute magnitude of a small number of Cepheids is determined from the measured value of their periods, and the distances must then be found from an independent measure. The distance to other Cepheids can then be calculated from their periods and mean apparent magnitudes (see distance modulus). Cepheids fluctuate more markedly at blue and near-UV wavelengths than at longer red and near-IR wavelengths. The mean brightness is thus more easily and accurately measured at longer wavelengths. Cepheids occur in many star clusters within our own Galaxy and because of their great luminosity can be observed in nearby galaxies. They can therefore be used for measuring an immense range of distances. See distance determination.

References in periodicals archive ?
1908: Henrietta Leavitt, a "human computer" at the Harvard College Observatory, discovers the period-luminosity relationship (brighter stars appear to pulse more slowly).
This period-luminosity law, published in 1912, became crucial in measuring the distance to stars.
He identified the first Miras in the Magellanic Clouds, leading to the discovery that these variables obey a tight period-luminosity relation (Glass & Evans, 1981).
Henrietta Leavitt discovered the period-luminosity law for Cepheid type variable stars.
Systematic errors such as whether chemical composition differences among Cepheids might affect the period-luminosity relation, will be examined using the infrared data.
This period-luminosity relationship allows astronomers to use Cepheids as standard candles to judge distances.
Feast of the University of Cape Town in South Africa reexamined the period-luminosity relationship for Cepheids, using Hipparcos to measure the distances to 26 of the nearest such stars.
Since the same period-luminosity relationship likely applied to M31, but its Cepheids appeared so much dimmer and farther away, Hubble could announce the following year that M31 was definitely extragalactic--a spiral galaxy in its own right.
Henrietta Leavitt discovered this period-luminosity relationship in 1908 using a long run of old Harvard plates that covered wide fields.
Actually, there are two kinds of Cepheid variables, with different period-luminosity relationships.
But the Cepheid period-luminosity relationship is fraught with subtle complications (June issue, page 18).
In 1907 Harvard astronomer Henrietta Leavitt discovered the famous Cepheid period-luminosity relation.