In a sequence of images captured over the course of two years, Hubble scientists tracked a ring of light as it appeared to expand outward into space. This light came from an initial stellar explosion — which was first identified three years ago — reflecting off a nearby gas cloud, according to a statement from the Space Telescope Science Institute's Hubble site.
"Voices reverberating off mountains and the sound of footsteps bouncing off walls are examples of an echo," the statement said. "Space has its own version of an echo. It's not made with sound, but with light and occurs when light bounces off dust clouds." [The Hubble Telescope's 10 Most Amazing Discoveries]
The initial explosion was a supernova called SN 2014J, discovered on Jan. 21, 2014, in the nearby galaxy M82, located 11.4 million light-years away from Earth. Stars larger than the sun turn into supernovas when they run out of fuel, leading to a chain reaction of energetic events including the explosion of material.
"So far, astronomers have spotted only 15 light echoes around supernovae outside our Milky Way galaxy," according to the statement. "Light-echo detections from supernovae are rarely seen, because they must be nearby for a telescope to resolve them."