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Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei 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,〔 securing Cepheids as viable standard candles for establishing the Galactic and extragalactic distance scales.〔〔〔 HST observations of classical Cepheid variables have enabled firmer constraints on Hubble's law.〔〔〔〔 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. 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. The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant. ==Properties== Classical Cepheid variables are 4–20 times more massive than the Sun, and around 1,000 to 50,000 (over 200,000 for the unusual V810 Centauri) times more luminous. Spectroscopically they are bright giants or low luminosity supergiants of spectral class F6 – K2, although the temperature and spectral type is variable. Their radii are a few tens to a few hundred times that of the sun. More luminous stars are cooler and larger. Along with the temperature changes their radii also change during each pulsation (e.g. by ~25% for the longer-period l Car), resulting in brightness variations up to two magnitudes. The brightness changes are more pronounced at shorter wavelengths. Cepheid variables may pulsate in a fundamental mode, the first overtone, or rarely a mixed mode. Pulsations in an overtone higher than first are rare but interesting.〔 The majority of classical Cepheids are thought to be fundamental mode pulsators, although it is not easy to distinguish the mode from the shape of the light curve. Stars pulsating in an overtone are more luminous and larger than a fundamental mode pulsator with the same period. Classical Cepheid variables were once B type main sequence stars earlier than about B7, possibly late O stars. More massive and hotter stars develop into more luminous Cepheids with longer periods, although it is expected that young stars within our own galaxy, at near solar metallicity, will generally lose sufficient mass by the time they first reach the instability strip that they will have periods of 50 days or less. Population I stars more massive than 20- are not thought to ever reach the instability strip and do not become Cepheids. At lower metallicity, for example in the Magellanic Clouds, stars can retain more mass and become more luminous Cepheids with longer periods.〔 When an intermediate mass star (IMS) first evolves away from the main sequence, it crosses the instability strip very rapidly while hydrogen shell burning. When the helium core ignites in an IMS, it executes a blue loop and crosses the instability strip again, once while evolving to high temperatures and again evolving back towards the asymptotic giant branch. In some cases, stars may cross the instability strip for a fourth and fifth time when helium shell burning starts. The rate of change of the period of a Cepheid variable, along with chemical abundances detectable in the spectrum, can be used to deduce which crossing a particular star is making. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Classical Cepheid variable」の詳細全文を読む スポンサード リンク
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