Classical Cepheid variable

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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 fraction of a day to 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, securing Cepheids as viable standard candles for establishing the galactic and extragalactic distance scales. 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. 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. Around 3,600 classical Cepheids are known in the Milky Way galaxy. Nearly ten thousand are known in the Magellanic Clouds, with hundreds discovered in other galaxies; the Hubble Space Telescope has identified some in NGC 4603, which is 100 million light years distant.