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What is a Cepheid variable?

What is a Cepheid variable? I came across the term in a general interest news magazine and it piqued my curiosity.

Cepheid variables are a subgroup of a class of stars called variable stars. Other classes include RR Lyrae variables, Mira variables, W. Virginis stars etc. Variable stars are those stars whose brightness oscillates with a definite periodicity. The variation is caused because of the star puffing out and contracting back again. Cepheid variable stars have masses between five and twenty times the mass of the sun.

The period of oscillation has a relation to its luminosity (thus there is a period-luminosity relation for these stars), which is why they are so useful. If you have a method of determining the luminosity of stars, then you can calculate the distance to these stars easily. It was the observations of Cepheid variable stars which enabled astronomer Hubble (after whom the Hubble Space Telescope is named) to determine that spiral nebulae like the one in Andromeda were galaxies separate from the Milky Way.

November 2001, Jagadheep D. Pandian (more by Jagadheep D. Pandian) (Like this Answer)

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