Classical Cepheid variable

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Hertzsprung–Russell diagram showing the location of several types of variable stars superimposed on a display of the different luminosity classes.

Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cepheid variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes from a few tenths of a magnitude to about 2 magnitudes.

There exists a well-defined relationship between a classical Cepheid variable's luminosity and pulsation period,^[1][2] securing Cepheids as viable standard candles for establishing the galactic and extragalactic distance scales.^[3][4][5][6] HST observations of classical Cepheid variables have enabled firmer constraints on Hubble's law.^{[3][4][6][7][8]} Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.^[5]

Around 800 classical Cepheids are known in the Milky Way Galaxy, out of an expected total of over 6,000. Several thousand more are known in the Magellanic Clouds, with more known in other galaxies.^[9] The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant.^[10]