An Etymological Dictionary of Astronomy and Astrophysics

The → electromagnetic radiation with → wavelengths shorter than that of
→ ultraviolet radiation and greater than that of → gamma rays.

Typical X-rays have a wavelength ranging from 0.1 to 100 Å (0.01 to 10 → nanometers), corresponding to frequencies in the range 3 × 10¹⁶ to 3 × 10¹⁹ Hz and energies in the range 100 eV to 100 → keV.

X-rays are produced artificially when high-speed → electrons collide with a heavy metal target such as tungsten. Astrophysical sources of X-rays include → plasmas with → temperatures in the range 10⁶-10⁸ K, and deceleration process of rapidly moving charges upon interaction with matter
(→ bremsstrahlung). X-rays were discovered in 1895 by Wilhelm Röntgen, a German physics professor (→ roentgen). For this discovery he won the first Nobel prize in physics in 1901. See also: → soft X-rays, → hard X-rays.

See also: X stand for “unknown,” since Röntgen was not sure what they were; → ray.