Institute of research into the fundamental laws of the Universe

A major international collaboration [1], involving researchers from the CEA-IRFU Astrophysics Department, CEA-IRAMIS and CEA-DAM, has succeeded in measuring for the first time the effects of light absorption by nickel in high temperature plasmas similar to those found around Cepheid-type variable stars. These stars are important indicators of distance in the Universe. They exhibit a periodic pulsing behavior caused by sudden increases in the absorption of light by the hot gas surrounding the star. These variations result from interactions between partially ionized elements including helium, oxygen, iron and nickel. Until now, these absorptions could only be evaluated using complex models of atomic physics and plasmas. Using a pulsed laser, the researchers have succeeded in recreating a nickel plasma in the laboratory at temperatures of between 116 000 and 440 000 degrees and with densities of around a few milligrams per cubic centimeter, similar in magnitude to the ionic distributions found in stellar envelopes. A direct measurement of the opacity of nickel is an essential step in the verification of current models of star structures.