This composite image contains the first picture of the Earth in X-rays, taken in March 1996, with the orbiting Polar satellite. The area of brightest X-ray emission is red. Such X-rays are not dangerous because they are absorbed by lower parts of the Earth's atmosphere. Credit: Polar, PIXIE, NASA. (The caption and image are from the Astronomy Picture of the Day for December 30, 1996.)
X-ray generation is producing X-radiation by a variety of phenomena. For example, when high-energy X-rays, gamma-rays, electrons, or protons bombard materials, the excited atoms within emit characteristic "secondary" (or fluorescent) X-rays. Alternately, whenever charged particles pass within certain distances of each other without being in fixed orbits, the accelerations (or decelerations) can give off X-rays.
