Astronomers in the US have succeeded in observing changes in the diameter of a Cepheid variable star for the first time. Measurements of Cepheids are important because they restrict the range of possible values of the Hubble constant, which is a measure of how fast the universe is expanding. From Earth, the diameters of even very large stars appear too small to be measured using conventional telescopes. Observing small changes in a star's diameter therefore required a new method of observation.
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