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| supernova remnant | 10000 km s-1 | gas | radio source | radio source catalog | radio | SNR | synchrotron radiation |   | emission nebula | A gaseous nebula, the expanding shell ejected by a supernova, and deriving its energy (at least in some cases) from the conversion by the remanent neutron star of its rotational energy into a stream of high-energy particles being continually accelerated in the SNR. About 100 SNRs are known in our Galaxy. Supernova remnants are usually powerful radio sources. |   |   | enhanced with heavy elements relative to the interstellar medium | 
| X-ray source |   | celestial sphere | celestial body |   | X-ray |   |   |   |   | A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field. |   |   |   | 
| Puppis A | 10000 km s-1 | gas |   | radio source catalog | X-ray | SNR | synchrotron radiation | X-ray source | 2U 0821-42 | A supernova remnant. It is an extended nonthermal radio source, and also a source of soft X-rays. | 1 to 2 kpc | 104 to 105 years | enhanced with heavy elements relative to the interstellar medium |