Word: cepheids
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Dates: during 1940-1949
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...Harrison Joy plotted the rotation of the Milky Way-the great star galaxy, six hundred thousand trillion miles across, to which the sun and all other visible stars belong. The regions near the centre of the galaxy are rotating fastest, the outermost regions slowest. By measuring the speeds of Cepheid variable stars, Professor Joy found that the region of the sun, two-thirds of the distance from the centre, rotates once every 207,000,000 years...