A class of highly luminous yellow stars that vary periodically in brightness. The importance of Cepheids to astronomy comes both in their application to practical problems of distance determination (within the Milky Way Galaxy itself, and far beyond) and in their acting as critical tests of both stellar evolution and pulsation theory. From observations it is known that the luminosity of a Cepheid is closely predicted by the period of oscillation, a relation known as the period-luminosity relation, discovered by Henrietta Leavitt in 1912. This relation provides a powerful tool for estimating distances, since the period can be determined without prior knowledge of the distance. Using the period to predict how bright a given star would appear at various distances, it is possible to calculate the distance of a Cepheid from the observer, given its apparent luminosity.
In the 1920s, Edwin Hubble conclusively ended the debate as to whether other galaxies existed in addition to the Milky Way when he discovered Cepheids in nebulae now considered to make up the Local Group of galaxies. Hubble went on to show that the distances of galaxies correlated with their apparent recession velocities, consistent with an expanding universe. See also Galaxy, external; Hubble constant; Local Group.
Cepheids are generally identified by the distinctive and periodic optical variations in their light output. The time period over which a complete cycle is executed ranges from a few days to a few hundred days. Analysis of spectroscopic properties, including the time variation of the radial velocities of Cepheids, led to the identification of the mechanism behind the changing light. The total light variation is the result of temperature changes in the stellar atmosphere, induced by, and combined with, an inward and outward motion of the surface of the star. Measurements of the color variation, interpreted as surface temperature variations, suggest that the surface temperature is changing by a few hundred kelvins during the pulsation cycle. The surface temperature changes the surface brightness and, especially at visible wavelengths, this is the primary cause of the large observed periodic light variation of a Cepheid. See also Astronomical spectroscopy; Variable star.
The second helium ionization layer is the main driver of pulsation in Cepheid variables. If slightly perturbed in temperature, this zone can either add considerable energy to the flow of radiation from the center of the star, by recombining and releasing the ionization energy, or it can extract energy from the flow by ionizing new material. This situation is unstable to slight perturbations in temperature because there is an opportunity for a cycling between these two states of ionization to occur.
With the aid of the Hubble Space Telescope, Cepheids have been found in galaxies as far away as the Virgo cluster, more than 20 times farther away than the Andromeda galaxy, M31, in the Local Group. At these distances, the general expansion of the universe begins to dominate the radial velocities of the galaxies. (For nearby galaxies, the motions of galaxies can be perturbed by the interaction with neighbors, or motions due to bulk flows can be a significant component of the observed velocity.) Cepheids currently provide the most accurate zero point for the calibration of other (secondary) distance methods (for example, bright supernovae) which extend the range of distance measurements by over a factor of 10. At such distances, the observed velocities are representative of the overall expansion of the universe. See also
