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The Question

(Submitted April 25, 1998)

Does anyone actually know what exactly Cepheid variables are and what causes the fluctuation in absolute magnitude? Also, why is the period of Population I Cepheid variables proportional to their distance from our solar system - is this a relativistic effect?

The Answer

The Cepheids are pulsating variables: The radius and the surface temperature of a Cepheid change periodically so the overall brightness varies.

The temperatures of the Cepheids are in the 6000-8000K range, which is when hydrogen atoms become ionized, and this causes the opacity of the atmosphere to change. When the opacity is high, meaning that the radiation has a hard time getting out, in a particular region of the atmosphere, the supply of energy from stellar interior gets trapped there. So the temperature and the pressure increase in this region. This eventually causes expansion, which causes atmosphere to become more transparent...

The periods of Cepheids are not related to their distances. The are linked with the true brightness (and hence called the Period-Luminosity relationship). We can easily measure the apparent brightness and the period of the pulsation of a Cepheid variable. Since the apparent brightness depends on the true brightness and distance, we can use the measurements to infer the distance to this Cepheid.

Hope this helps. Best wishes,

Koji Mukai
for Ask an Astrophysicist

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