Space Telescope Science
Voices reverberating off mountains and the sound of footsteps bouncing off walls are examples of an echo. Echoes happen when sound waves ricochet off surfaces and return to the listener.
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 has just captured one of these cosmic echoes, called a “light echo,” in the nearby starburst galaxy M82, located 11.4 million light-years away. A movie assembled from more than two years’ worth of Hubble images reveals an expanding shell of light from a supernova explosion sweeping through interstellar space three years after the stellar blast was discovered. The “echoing” light looks like a ripple expanding on a pond. The supernova, called SN 2014J, was discovered on Jan. 21, 2014.
A light echo occurs because light from the stellar blast travels different distances to arrive at Earth. Some light comes to Earth directly from the supernova blast. Other light is delayed because it travels indirectly. In this case, the light is bouncing off a huge dust cloud that extends 300 to 1,600 light-years around the supernova and is being reflected toward Earth.
So far, astronomers have spotted only 15 light echoes around supernovae outside our Milky Way galaxy. Light echo detections from supernovae are rarely seen because they must be nearby for a telescope to resolve them.