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.

2001 April 16
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The Hydra Cluster of Galaxies
Credit & Copyright: David Malin (AAO), AATB

Explanation: The Hydra Cluster of Galaxies contains well over 100 bright galaxies - but perhaps fewer galaxies than might be expected from its mass. Clusters of galaxies are the largest gravitationally bound objects in the universe. Most of a cluster's mass, however, appears to be in a form too dark to see, as analyses of the distribution of X-ray light, gravitational lensing, and internal motions indicate. Abell 1060, as the above cluster is also known, appears to have an even higher fraction of dark matter than seen in a similar cluster, a situation astronomers cannot easily reconcile with both clusters forming solely from gravitational attraction. The Hydra Cluster of Galaxies, named for its home constellation, spans about ten million light years.

Tomorrow's picture: Colorful Clouds Over Mars

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