An example of K-alpha lines are those seen for iron as iron atoms radiating X-rays spiraling into a black hole at the center of a galaxy. For such purposes, the energy of the line is adequately calculated to 2-digit accuracy by the use of Moseley's law: , where "Z" is the atomic number and Ry=Rydberg energy=13,6 eV. For example, K-alpha for iron () is calculated in this fashion . For astrophysical purposes, Doppler and other effects (such as gravitational broadening) show the iron K-alpha line to better accuracy than 6.4 keV. Additionally, the K-alpha line in Copper is frequently used as the primary source of x-ray radiation in lab based x-ray diffraction spectrometry (XRD) instruments.