File:Chandra X-ray Image of G21 5-0 9 with Scale Bar (1999-snrg-more-2).jpg

English: Chandra X-ray Image of G21.5-0.9 with Scale Bar Scale bar = 1 arcmin. The identification of G21.5-0.9 as the remnant of a supernova explosion is based on indirect evidence from radio and X-ray observations. At both radio and X-ray wavelengths, it appears as round patch in the sky. Detailed observations with radio telescopes confirm that the radio waves are produced by high energy electrons spiraling around magnetic field lines (synchrotron radiation). The X-rays are probably produced by the same process, but the electrons involved have energies many thousands times higher than those that produce the radio waves. The favored theory is that the high-energy electrons responsible for both the radio and X-ray emission are produced by a rapidly rotating, highly magnetized neutron star left behind when a massive star exploded some 40,000 years ago.