A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. The namesake and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784. http://en.wikipedia.org/wiki/Cepheid_variables Cepheid stars oscillate between two states: In one of the states, the star is compact and large temperature and pressure gradients build up in the star. These large pressures cause the star to expand. When the star is in its expanded state, there is a much weaker pressure gradient in the star. Without the pressure gradient to support the star against gravity, the star contracts and the star returns to its compressed state. Cepheid variable stars have masses between five and twenty times the mass of our Sun. The more massive stars are more luminous and have more extended envelopes (the outer layers of gas in a star are sometimes called its "envelope"). Because these envelopes are more extended and the density in their envelopes is lower, their variability period, which is proportional to the inverse square root of the density in the layer, is longer. http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/cepheid.html

A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. The namesake and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784. http://en.wikipedia.org/wiki/Cepheid_variables Cepheid stars oscillate between two states: In one of the states, the star is compact and large temperature and pressure gradients build up in the star. These large pressures cause the star to expand. When the star is in its expanded state, there is a much weaker pressure gradient in the star. Without the pressure gradient to support the star against gravity, the star contracts and the star returns to its compressed state. Cepheid variable stars have masses between five and twenty times the mass of our Sun. The more massive stars are more luminous and have more extended envelopes (the outer layers of gas in a star are sometimes called its "envelope"). Because these envelopes are more extended and the density in their envelopes is lower, their variability period, which is proportional to the inverse square root of the density in the layer, is longer. http://imagine.gsfc.nasa.gov/.../cepheid.html