Astronomy Picture of the Day

Gamma-Ray Moon

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Gamma-Ray Moon


If you could see gamma rays - photons
with a million or more times the energy of visible light - the
Moon would appear brighter than the Sun!

The startling notion is demonstrated by this
image of the Moon from the Energetic Gamma Ray Experiment Telescope
in orbit on NASA's
Gamma Ray Observatory
from April 1991 to June 2000.

Then, the most sensitive instrument of its kind, even EGRET could
not see the quiet Sun which is extremely faint at gamma-ray energies.

So href="">why is the Moon bright?

High energy charged particles, known as
cosmic rays,
constantly bombard the unprotected lunar surface generating gamma-ray

EGRET's gamma-ray vision
was not sharp enough to resolve a lunar
disk or any surface features, but its sensitivity reveals the
induced gamma-ray moonglow.

So far unique,
the image was generated from eight exposures
made during 1991-1994 and
covers a roughly 40 degree wide field of view with
gamma-ray intensity represented in false color.

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