The left image, taken by the Hubble Space Telescope, shows a ring of light from a distant galaxy surrounding a closer or foreground galaxy (galaxy SDSS J1631 + 1854), which is at the center of the left image. The closer galaxy is called a "gravitational lens" because its gravity bends light from the more distant galaxy to form the ring of light, named an Einstein ring, as seen from the telescope. The image on the right has been adjusted to remove the lens galaxy and show the ring more clearly. In a new study, University of Utah astronomer Adam Bolton and colleagues measured these Einstein rings to determine the mass of 79 lens galaxies that are massive elliptical galaxies, the largest kind of galaxy with 100 billion stars. The study found the centers of these big galaxies are getting denser over time, evidence of repeated collisions between massive galaxies.