April 16, 2001
Somewhere above you, light is arriving from places so distant their names feel like poetry.
The Hydra Cluster of Galaxies
Birthday · April 16, 2001
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APOD published April 16, 2001. Credit: AAO. A publication-date match, not a photo taken on your date. Independent of NASA.
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.
Image credit: AAO