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.

July 24, 1996

COMPTEL Explores The Radioactive Sky
Credit : The COMPTEL Collaboration, Compton Gamma Ray Observatory, NASA

Explanation: Diffuse gas clouds laced with radioactive aluminum atoms (Al26) line the plane of our Milky Way Galaxy! How do we see them? Relying on the Compton Effect, the COMPTEL instrument onboard NASA's immense orbiting Compton Gamma Ray Observatory can "see" the 1.8 million electron Volt gamma rays emitted by the radioactive decay. COMPTEL's first ever survey image of the entire sky in the light of gamma rays produced by this exotic radioactivity is shown above. The Galactic plane is horizontal, passing through the Galactic centre in the middle of the picture, as indicated by the superposed coordinate grid. The radioactive Al26 clouds are seen to lie in clumps near the plane, with some slightly above and below it. The brightest feature looks like a mysterious inverted "V", just to the left of centre. Where do they come from? Al26 decays to magnesium (Mg26) with a half-life of about a million years, a very short time compared to the age of the Galaxy -- so the clouds must have been produced relatively "recently". COMPTEL astronomers are exploring several origins for the radioactive clouds including nuclear processing (nucleosynthesis) by aging massive stars and supernova explosions. Because they are generally thought to be associated with short lived massive stars, the radioactive clouds are expected to be located near sites of recent star formation.
(Note added in press: Don't worry - the aluminum atoms in the foil in your kitchen are Al27 and are not radioactive!)

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
&: Michigan Tech. U.