1. Photo-electric Absorption
The energy of the X-ray goes tearing into one of the electrons away from its orbit around the nucleus of a nitrogen or an oxygen atom. (Credit: NASA/CXC/S.Lee)

2. X-ray Absorption
Absorption by the Earth's atmosphere restricts ground-based observations to radio, near infrared, and visible wavelengths. X-rays are absorbed high above the Earth. (Credit: NASA/CXC/M.Weiss)

3. Fluorescence
The absorption of a photon of energy, or wavelength, can lead to re-emission of one or more photons at lower energies, or longer wavelengths. (Credit: NASA/CXC/M.Weiss)