A céphéide is a giant star or Supergéante yellow, from 4 to 15 times more massive than the Sun and of 100 with 30 000 times more luminous, whose glare varies from 0.1 to 2 magnitude S according to a period well defined, ranging between 1 and 100 days, from where it draw its name from variable star. They were named according to the prototype of the star δ of the constellation of Céphée.
The céphéides played a big role in the years 1910-1920 when Henrietta Leawitt, working with the Université Harvard, notices the presence of several céphéides in the Nuages of Magellan. She realizes that the periods of these céphéides are all the more large as those are brilliant. She thus will formulate a relation binding the period of variation (length of time between two locatable modifications) to the apparent Luminosité of these star S very particular. Blow, it would be enough to measure the distance from one to these céphéides (with the Earth or rather with the Sun) to obtain a general relation binding their period and them absolute Luminosité, and still to better determine the distance any other céphéide, where that she is. This measurement was carried out for the first time in 1916, still at the Harvard university, by Harlow Shapley which, by doing this, supplemented the discovery of Henrietta Leawitt. Starting from this date, the céphéides became the reference to measure the " distance" of Star S increasingly distant in the Universe.
Young person but of structure more advanced than the Sun, a céphéide owes his luminous energy with the reactions of nuclear Fusion which in its central area transforms Hélium into Carbone. The external part of star contracts and dilates alternatively, because of a self-sustained imbalance of the forces related on the pressure of gas and the Gravité. These movements are accompanied by changes of temperature responsible for the periodic variation for the luminosity. The period of variation of glare of a céphéide approximately twice represents the time put by a Onde of pressure to be propagated center of star on its surface; it depends on the state of the medium crossed by the wave and constitutes of this fact an invaluable source of information on the internal structure of star.
Role in the calculation of the distances
The céphéides play a very important part as standards of the scales of distance in the Universe thanks to the Relation period-luminosity which characterizes them: more a céphéide is luminous more its period of variation of glare is long, because more the star is bulky plus the way which the light waves must traverse is long.
Since the period of a céphéide is known, easily measurable, the relation period-luminosity makes it possible to determine the intrinsic glare of this star. By a simple comparison with its point brilliance, one from of deduced his distance, and thus that of the Galaxy which shelters it.
Very brilliant, therefore visible by far, the céphéides are detected now in other galaxies that ours until distances from 80 million light-years approximately thanks to the Space telescope Hubble. These determinations of distances are essential with the calculation of the value of the Constante of Hubble, which measures the rate/rhythm of expansion of the Universe. The delicate point lies in the absolute calibration of the relation period-luminosity, which requires independently to determine in a precise way the distance from at least some céphéides located in our galaxy.
In addition, when one determines the luminosity of a céphéide starting from the relation period-luminosity, it should be known that the galaxies, and thus the céphéides that they contain, are not identical but different by their chemical composition. It is what appeared during these last years with the analysis of the very great number of céphéides detected in two close galaxies, the Nuages of Magellan.
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