x-ray astronomy. The study of objects lying beyond the solar system in the photon energy band 100 to 500 000 electronvolts (corresponding to wavelengths from 12 nanometres (nm) to 0.0024 nm). X-ray observations are now an integral part of astronomy, relating particularly to galactic and extragalactic systems where violent or energetic phenomena give rise to copious high-energy particles or super-hot gas.
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