Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

January 9, 1996
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M100 and the Expanding Universe
Credit: NASA, HST, W. Freedman (CIW), R. Kennicutt (U. Arizona), J. Mould (ANU)

Explanation: The distance to the swirling grand design spiral M100 is causing quite a stir among astronomers. Many believe that the Hubble Space Telescope's recent distance measurement to this galaxy accurately calibrates the expansion rate of the universe. Others believe this distance measurement is misleading. The universe's expansion rate is usually given as a quantity called "Hubble's constant", a factor dividing well-measured recession velocity of a galaxy to give actual distance. Scientific debate over the value of Hubble's constant has been ongoing since it was first measured by Edwin Hubble in 1929. A real live debate involving the value of Hubble's constant titled "The Scale of the Universe" will occur in April 1996 in Washington, DC.

Tomorrow's picture: The Cepheids of M100

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Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
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