Moon Outshines the Sun

When could the Moon possibly be brighter than the Sun? The Sun is much bigger, produces energy, and gives all the energy needed for life on Earth.  But if you look at the sky in gamma rays, the highest energy photons on the electromagnetic spectrum, you’ll see the Moon more easily than the Sun.  Why?

Gamma Moon. Image Credit: NASA, DOE, International Fermi LAT Collaboration

The Moon is the brightest gamma ray source in the sky, because it has no atmosphere or magnetic field.  Essentially it has no protection from the dangerous cosmic rays that are constantly zipping through space.  When they hit the Earth’s atmosphere they create a cosmic shower of thousands of particles that do hit people, but have such a low energy they pass through us without incident.

For the Moon, the high energy cosmic rays that hit the surface produce reactions only found in particle colliders on Earth, and gamma ray photons are a common product of these energetic reactions.  The other fascinating part of this is that there is no difference whether you look at the illuminated or night side of the Moon.  Because the cosmic rays come from all directions, they make the entire surface of the moon glow in gamma, with little variation.

Using technology to look at the sky in all wavelengths across the electromagnetic spectrum has given astronomers a deeper understanding of the universe around us.  What we can’t see in one wavelength may be visible in another, and the different wavelengths of light reveal different properties of the source.

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