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.

April 19, 1996

The Virgo Cluster: Hot Plasma and Dark Matter
Credit: R. Mushotzky and S. Snowden LHEA/ GSFC, ROSAT

Explanation: This ROSAT image of the Virgo cluster of galaxies reveals a hot X-ray emitting plasma or gas with a temperature of 10-100 million degrees pervading the cluster. False colors have been used to represent the intensity of X-ray emission. The large area of X-ray emission, just below and left of center, is about 1 million light-years across. The giant elliptical galaxy M87, the biggest member of the cluster, is centered in that area while other cluster members are scattered around it. By adding up the amount of X-ray emitting gas astronomers have found that its total mass is up to 5 times the total mass of the cluster galaxies themselves - yet all this matter still does not produce nearly enough gravity to keep the cluster from flying apart! Where is the unseen mass? Because galaxy clusters are the largest structures in the Universe, this mysterious Dark Matter must dominate the cosmos but its nature is still an open question.

Information: The Scale of the Universe Debate in April 1996
Tomorrow's picture: A Magnificent Desolation

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