Photograph courtesy NASA/CXC/OCIW/P. Martini et al.
In 2000, astronomers studying the A2104 galaxy cluster (in blue) discovered powerful x-rays emanating from several black holes in regions previously thought too old and devoid of gas to create such radiation. They had expected to find perhaps one such x-ray source in the area, but instead found six. The discovery, made using the Chandra X-ray Observatory, changed many of the assumptions scientists had made about the life cycles of galaxies and black holes.