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Cepheids are pulsating stars of high luminosity, named so after one of the first discovered variable star - δ Cepheus. The Cepheids are yellow supergiants of F and G spectral classes. Their masses are several times larger than the mass of the Sun.

Cepheids assume a peculiar structure during their evolution. The layer that accumulates energy from the star core and later donates it emerges at a certain depth from the surface. The Cepheids shrink from time to time. The temperature of Cepheids grows against a decrease of their radius. Afterwards, the surface area begins to grow in unison with the temperature, causing an overall change in brightness.

Analysis of Cepheid spectrums shows that radial velocities also change from time to time. Photospheres of these stars approach us at the highest speed near at the maximum brightness period, and they retract in the opposite direction during the period of minimal brightness. This becomes apparent if one studies the spectrums of Cepheids in the context of Doppler effect. The Cepheids' radius is subject to change. The greater is the temporal span of the Cepheid brightness fluctuation, the higher is the Cepheid's luminosity.

You may observe the described effects in this model's right-side window. Press "Run" button and watch the Cepheid pulsate. Surface temperature of the star will change in inverse proportion to the increase in its size, and the star will grow red in color. If a star's diameter becomes smaller, it will grow yellow. "Stop" button suspends the animation, and pressing "Reset" button will return the model to its initial state. Windows on the model's left-hand side reflect the dependence of brightness and radius of the stars on the passage of time.

Cepheids played an important role in the development of modern astronomy. In 1908, Henrietta Lewitt studied Cepheids in Minor Magellan Cloud and noticed that the smaller the visible magnitude of a Cepheid is, the greater is the temporal span of its brightness fluctuation. All the stars of Minor Magellan Cloud are situated at approximately the same distance from us, and therefore, visible magnitude m of Cepheid reflects its luminosity L. Since supergiants are clearly visible at large distances, this dependence may be used to calculate distances from the Earth to other galaxies. The Hubble constant was reaffirmed by 1999, owing to measurements of 800 Cepheids in 18 different galaxies. At present, Hubble constant it is considered to be 70 km/s per 1 Mpc with precision of 10%.

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