Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. In this century, the discovery that the Universe is expanding has produced a revolution in human thought about the Cosmos. This single number describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance. One group, led by astronomer Allan Sandage, measures distances to galaxies using pulsating Cepheid variable stars and supernovae observed in galaxies like the Virgo Cluster spiral galaxy, NGC4639, shown above.
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