Astronomy

A Cepheid variable star is class of variable stars that brighten and dim in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, although they range from 1 to 50 days.

These stars are important because the period of a Cepheid depends on its intrinsic brightness, or absolute magnitude, in a known way: the brighter the star, the longer its period. All Cepheid variables with the same period have nearly the same intrinsic brightness, but their apparent brightnesses differ because they are at different distances. By observing a Cepheid’s period, one can determine how bright it actually is. By comparing this intrinsic brightness to how bright it appears to be, one can determine the star’s distance. Thus Cepheids are important indicators of interstellar and intergalactic distances, and they have been called the “yardsticks of the universe.”

Physically, Cepheids are yellow supergiant stars, and their fluctuations in luminosity result from an actual physical pulsation, with attendant changes in surface temperature and size. The stars are hottest and brightest when expanding at maximum rate midway between their largest and smallest size. The Cepheid class takes its name from Delta Cephei, the first such star discovered by John Goodricke in 1784.

The period-luminosity relation was discovered on early 20th century by Henrietta Leavitt and Harlow Shapley from Harvard College Observatory when they studying many Cepheids in the Magellanic Clouds, the two closest galaxies. While these stars are all almost equally distant, it was found that the brighter variables had the longer periods. The absolute magnitude of a few Cepheids is required to infer absolute, rather than merely relative, distances. These absolute magnitudes were measured by a statistical study of the proper motions of Cepheids within our own galaxy.

Cepheids are now considered to fall into two distinct classes. The classical Cepheids, which are dependable in their period-luminosity relationship, all have periods from about 1.5 days to more than 50 days and belong to the class of relatively young stars found largely in the spiral arms of galaxies and called Population I. Short-period Cepheids, also called cluster-type variables, or RR Lyrae variables, all have periods of less than one day and show no dependable relationship between period and luminosity; this last fact caused considerable confusion among astronomers before it was recognized. RR Lyrae variables can still be used as distance indicators because their absolute magnitudes tend to be alike. Short-period Cepheids belong to Population II, a class of older stars found in the core and in the halo of the Milky Way Galaxy.

Classical Cepheids exhibit a relation between period and luminosity in the sense that the longer the period of the star, the greater its intrinsic brightness; this period-luminosity relationship has been used to establish the distance of remote stellar systems. The absolute magnitude of a classical Cepheid can be estimated from its period. Once this is known, the distance of the star can be deduced from a comparison of absolute and apparent (measured) magnitudes.

In the 1950s a second class of Cepheids with different period-luminosity relations was found, leading to a dramatic doubling of estimated cosmological distances. The Hubble Space Telescope will permit the observation of Cepheids in more distant galaxies, giving a more accurate picture of the size and age of the universe.

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