Astronomy Picture of the Day

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

December 30, 1999
See Explanation.  Clicking on the picture will download 
 the highest resolution version available.

The Century that Defined Galaxy
Credit: FORS1, 8.2-meter VLT, ESO

Explanation: As the twentieth century dawned a debate raged over whether the Milky Way was the entire universe. The 1920s brought observations of spiral-nebulae that housed familiar but faint variable stars, and hence placed these nebulae much farther than anything else ever found. The Milky Way, apparently, is itself a spiral nebula seen from the inside. Soon interstellar dust was identified as the opaque material that blocks our view of distant spiral arms, confirming that our Sun was well away from the center. As the twentieth century closes, galaxies are seen forming across the universe, and our Milky Way is referred to as a spiral galaxy quite similar to NGC 1288, as pictured above.

Tomorrow's picture: The Millennium that Defined Earth

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
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