Classical Cepheid variable

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

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] Hubble Space Telescope (HST) observations of classical Cepheid variables have enabled firmer constraints on Hubble's law, which describes the expansion rate of the observable Universe.^{[3][4][6][7][8]} Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the local spiral arm structure and the Sun's distance from the galactic plane.^[5]

Around 3,600 classical Cepheids are known in the Milky Way galaxy^[9]. Nearly ten thousand are known in the Magellanic Clouds, with hundreds discovered in other galaxies;^[10] the Hubble Space Telescope has identified some in NGC 4603, which is 100 million light years distant.^[11]