One good cosmic measure

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Science  29 Aug 2014:
Vol. 345, Issue 6200, pp. 1001-1002
DOI: 10.1126/science.1258425

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As Earth orbits around the Sun, nearby stars are seen to move against the background of more distant stars and galaxies. The angular amplitude of this motion is called parallax and provides a direct, trigonometric measurement of the stellar distances as compared to the size of Earth's orbit. A great achievement of the European Space Agency's satellite Hipparcos (1) was the measurement of parallaxes of over 100,000 stars simultaneously, making it widely recognized as having provided accurate distances to thousands of normal stars and to a few star clusters. Since the first release of the Hipparcos results in 1997, however, it was noted that the distance obtained for the Pleiades star cluster was abnormal (see the figure). Its hydrogen-burning stars, when compared with stars of similar effective temperature in other well-measured clusters like the Hyades, appeared to be systematically too faint, were the Hipparcos-derived distances adopted. The effect implied either that the Hipparcos distances were too short or that Pleiades hydrogen-burning stars had some very peculiar property, such as an extremely high fraction of helium in their interiors (2). On page 1029 of this issue, Melis et al. (3) present new observations of stars in the Pleiades cluster, demonstrating that the Hipparcos-derived distances were indeed off by 10%. Their radio-wave interferometry approach thus demonstrates a precise and accurate method to determine stellar distances.