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 11, 1996

Unexpected X-rays from Comet Hyakutake
Credit: C. Lisse, M. Mumma (NASA/GSFC), K. Dennerl, J. Schmidt, and J. Englhauser (MPE)

Explanation: The first X-rays ever detected from a comet were discovered from Comet Hyakutake with the ROSAT satellite on March 27th. The discovery is particularly surprising because there was little previous indication that comets emit any significant X-radiation. As the comet passed the Earth in late March, repeated observations with ROSAT also showed that the X-ray brightness changed over just a few hours. The crescent shape of the X-ray emission is also enigmatic. One possible explanation is that X-rays emitted from the Sun are absorbed by water in the comet's coma causing fluorescence. Another possible explanation involves interaction with the solar wind - fast moving particles streaming away from the Sun.

Information: The Scale of the Universe Debate in April 1996
Tomorrow's picture: Man Enters Space

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