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The results of a very interesting study on Cepheids, that confirm that the period-luminosity relation of this class of variable star can sometimes be unreliable in the calculation of cosmological distances, have been published in The Astronomical Journal.
The range of luminosity variation of a Cepheid is proportional to the period, so measuring the period via straightforward observation, one can determine the absolute luminosity, and by comparing this with the apparent magnitude, determine the distance.
For Cepheids in nearby galaxies it is therefore relatively simple to estimate their distance, and therefore also the distance to the supernovae that occur within them. By calibrating the luminosity of supernovae of different types it therefore becomes feasible to measure the distances to more distant galaxies (where Cepheids are too faint to see) and get some idea of the dimensions of the Universe.
At least that was what was believed during the last century. But recently there have been doubts about the reliability of Cepheids as standard candles for the calibration of other cosmological distance estimators.
The study recently carried out by Massimo Marengo (Iowa State University), Pauline Barmby (University of Western Ontario, Canada) and other researchers, in fact clearly shows various cases where the luminosity of the Cepheids is reduced by gas and dust that make them seem more distant than they really are.
The discovery is based on observations of a number of Cepheids made with NASA's Spitzer Space Telescope, amongst them Delta Cephei, the prototype of the class.
Around this star (illustrated above) a kind of envelope was detected, similar to a comet's coma, compressed in the direction of travel of the star in space.
Its rapid motion, and above all, its extremely intense stellar winds (up to a million times more powerful than those of the Sun) blow material away from the surface of the star and compress the interstellar gas and dust, which is heated, thus increasing the infrared emission.
Although the infrared luminosity is increased, the envelope around Delta Cephei (the detection of which was aided by the presence of a weak companion star), and other similar stars, attenuates the light in the visible part of the electromagnetic spectrum.
By determining the structure and dimensions of the compressed front of the envelope, Marengo and colleagues reached the conclusion that a significant part of the gas in the envelope comes from Delta Cephei itself, that is therefore constantly losing mass, and slowly shrinking. The same phenomenon effects about 25% of the Cepheids studied by Spitzer.
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