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If you could see gamma-rays, the night sky would look strange and unfamiliar. The gamma-ray moon just looks like a round blob - lunar features are not visible. In high-energy gamma rays, the Moon is actually brighter than the quiet Sun. This image was taken by EGRET.

Credit: D.J. Thompson, D.L. Bertsch (NASA/GSFC), D.J. Morris (UNH), R. Mukherjee (NASA/GSFC/USRA)

If you could see gamma-rays, these two spinning neutron stars or pulsars would be among the brightest objects in the sky. This computer processed image shows the Crab Nebula pulsar (below and right of center) and the Geminga pulsar (above and left of center) in the "light" of gamma-rays.

The Crab nebula, shown also in the visible light image, was created by a supernova that brightened the night sky in 1054 A.D. In 1967, astronomers detected the remnant core of that star; a rapidly rotating, magnetic pulsar flashing every 0.33 second in radio waves.

Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s. Detectors on board the Vela satellite series, originally military satellites, began to record bursts of gamma-rays -- not from Earth, but from deep space!

Today, these gamma-ray bursts, which happen at least once a day, are seen to last for fractions of a second to minutes, popping off like cosmic flashbulbs from unexpected directions, flickering, and then fading after briefly dominating the gamma-ray sky.

Gamma-ray bursts can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime! So far, it appears that all of the bursts we have observed have come from outside the Milky Way Galaxy. Scientists believe that a gamma-ray burst will occur once every few million years here in the Milky Way, and in fact may occur once every several hundred million years within a few thousand light-years of Earth.