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The Question

(Submitted October 30, 2003)

Are some cosmic rays nuclei of iron atoms, and are they from sources in our own galaxy? I read that the ultrahigh-energy cosmic rays that arrive to our atmosphere at 1020 electron volts (eV) or more, are mainly protons? Have any cosmic rays nuclei of iron ever been detected at this range of energy?

The Answer

Thank you for your question. The cosmic rays that have been identified as iron are believed to originate from supernova explosion shock waves in our galaxy.

We don't yet know the composition of the ultra high energy cosmic rays, since they are not directly detected. Although protons seem the likeliest candidate, there is no real physical evidence to support this.

For more on cosmic rays, see

-- Michael Loewenstein and Amy Fredericks
for "Ask an Astrophysicist"

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