A Cepheid is a young star and a bright giant, but it is mostly a periodic variable, i.e. its brightness varies over time. Its mass is 4 to 20 times the mass of the Sun. The name comes from the Cepheid variable star discovered in 1784 (Delta Cephei) in the constellation of Cepheus. Cepheids are true standard candles because they vary not only in size but in brightness with great regularity (the period is between 1 and 135 days). Their brightness varies substantially from 0.1 to 2 magnitudes according to the period. These variable stars twinkling are visible in all the galaxies. These stars are very interesting because there is a strong correlation between the pulsation period, brightness and distance. By measuring the period of pulsation of a Cepheid, we deduce its luminosity and hence its distance. If you compress a variable star it bounces, if it stretches outwards, it falls back, but what are the causes and engine of these palpitations or these stellar pulsations? Indeed, it must an engine that continually provide pushes and pulls. The explanation was found in 1926 by the British astrophysicist Arthur Stanley Eddington (1882-1944). The stars are characterized by their mass, radius, luminosity and temperature. A newly born star is in its main sequence, it is a ball of gas that emits light because it is hot, it is in the phase where it burns its hydrogen, atoms at high temperature emit photons and it is these photons that we see. The gas in the star is in constant equilibrium under two opposing forces, the intrinsic pressure which tends to expand the star and the gravitational force which tends to compress. The star continuously loses energy as it emits a large amount of heat in the cold space, but the same amount of heat is generated by thermonuclear reactions in the heart of the star (transformation of hydrogen into helium), this compensates the energy lost by radiation. When there is no more hydrogen in its heart, it burns hydrogen that have in its intermediate layers, the gas pressure decreases and it swells. It becomes very bright then it is a red giant , then it will burn its helium in heart. It is in this particular state that the giant will begin to "flash". This phenomenon concerns the Cepheids but also the W Virginis very old low-mass stars (1 solar mass) that pulsate with a period between 1 and 60 days and RR Lyrae much less massive (0.5 M ☉) with a period of 0.5 days .
A Cepheid is a young star, but as it is a giant, it is more evolved than the Sun, it consumed a lot of hydrogen and light energy is now due to nuclear fusion reactions of helium, which turns into carbon. Normally when you compress a gas it heats and cools when it expands, but at certain temperatures, hydrogen or helium are forced to move from a state of ionization (electron stripping or addition) to another, then back to the initial state upon cooling. This is what happens in variable stars. Pulsations are actually swelling and contraction of different layers of the star, they fall over each other in contracting and bounce and swell again. The heart of the star it does not pulse, only the intermediate and outer layers pulsate, the outermost layer undergoes the most violent pulsation. The phenomenon of partial ionization of hydrogen and helium is located in one of the inner layers which drives the others. More gas is warm, less it absorbs light and becomes brighter, conversely, more a gas is colder and more it absorbs light and becomes opaque. The opacity of a gas is a measure of its ability to absorb light. When gas of this layer of ionization reaches a certain compression, ionization occurs, the opacity of the gas increases, the gas layer acts then as a valve that closes itself, the pressure increases and pushes up the others layers. Then, the different layers expand and cool, the ionization of hydrogen and helium will be reversed, the gas will return to its initial state, the opacity decreases suddenly and radiation floods the layers of the star dropping the gas pressure. The gas pressure is now too low to support the layers that fall on each others, waiting for the next cycle to bounce. This phenomenon is called the Eddington valve. All the stars do not pulsate, this only applies to certain stars, because it depends on the depth of the ionization zones. This feature variable stars allows astronomers to use as distance indicators. They can calculate by successive approaches, the distances to the large scale, that of galaxies and so back to the depths of the observable universe.
nota: The Eddington limit or Eddington luminosity, corresponding to the maximum brightness that can reach a star of a given mass. Beyond it begins to lose the upper layers of its atmosphere.
Image: This video montage was created from the Hubble Space Telescope NASA / ESA observations to show pulsations of variable star RS Puppis and thick dark clouds environment. RS Puppis is a Cepheid, a type of variable stars located in the constellation of Puppis. Dust surrounding RS Puppis allows to see a phenomenon known as a light echo around the star, with amazing clarity. This echo of light creates the illusion of gas cloud expanding from RS Puppis. The Hubble observations were taken over a period of five weeks in 2010 to capture the variable star at different stages of its life cycle, it varies in brightness by a factor of five or more every 40 days. The short video is repeated several times to more clearly show the light echo mechanism. These Hubble observations show the object in a dark sky filled with background galaxies. This is because RS Puppis is situated in a large nebula, that astronomers were able to measure the distance by light echoes, from particles of the nebula. They accurately determine the Cepheid stood at 6,500 ± 90 light years from Earth. The measurement accuracy is important because Cepheids are used as marker (standard candle) for distances within our galaxy and nearby galaxies. Credit: NASA, ESA, G. Bacon (STScI), team Hubble Heritage (STScI / AURA) Collaboration Hubble, et H. Bond (STScI and Pennsylvania State University).