Spectroscopy is simply the science of measuring and graphing the intensity of light at different energies. X-ray light contains a range of energies just like visible light, with its range from reds to blues. Satellites like the Rossi X-ray Timing Explorer (RXTE) have spectrometers on them. RXTE's spectrometer counts the number of photons of differing energies hitting it. A graph of the number of photons over an energy range can tell us how the source is producing its X-rays, and give us clues as to what kind of object it is! Spectra of stars can give us information about their composition, since particular elements emit light at characteristic energies evident as peaks in the spectrum. Spectra of black holes can reveal orbiting disks of matter.

Here is an example of a spectrum: