We all have experienced sound echoes. Sound waves from an event (preferably loud and short) bounce off of distant walls and travel back to our ears. Because of the finite time it takes sound to travel, we can hear individual echos. Sound echos are used quite often by living beings -- The Navy and fishermen use sonar to find fish (or submarines) under water; bats and dolphins use echolocation to get their food.
Light also travels at a finite speed, though much, much faster than sound. Humans have learned to make use of "light echoes" for all sorts of clever things -- we call this radar (for radio waves) or lidar (for laser light).
There are some events in space that make nice, short bursts of light, such as supernova explosions or eruptions from the surface of a star. The neat thing (at least to me) is that we are now able to detect these "light echoes" from astronomical sources, as light from the event bounces off of dust or gas and toward the Earth.
A press release from the Chandra X-ray Observatory shows one cool example of a light echo detected in the Large Magellanic Cloud (LMC), one of the Milky Way's companion galaxies. About 400 years ago, a supernova exploded. The supernova was probably quite easy to see from the Earth, but I'm not aware of records from it. This wouldn't be too surprising, as the LMC is only visible south of the equator, and there just aren't a lot of written astronomy records from civilizations down under during that time.
Anyway, astronomers studying this supernova's remains (an X-ray picture of them is at the top of this blog post; I think it looks like a celestial pufferfish) also looked in optical (visible) light at the area surrounding the supernova, and they found an echo of light from the supernova itself! This page has movies showing the movement of the light echo over a period of five years (be warned, the movies are large. If they are too big for your internet connection, you can look at the individual pictures). Pretty neat!
What can we learn from the light echo? First, astronomers have been able to analyze the light from the echo and determine the type of supernova that made the explosion. There are two types of supernova explosions -- those that come from a massive star ending its life, and those that come from white dwarf stars. From the light echo, astronomers were able to confirm that the explosion was from a white dwarf. This knowledge helps us understand the X-ray light the Chandra observatory sees.
We can also use the light echos to get a three-dimensional picture of the dust and gas in the region surrounding the supernova. You can see the shape of the echo change slightly from picture to picture in the movie, which tells us that the gas and dust in the LMC galaxy are clumpy.
Perhaps astronomy's most famous light echo is from an eruption on the star V838 Monocerotis, shown in this video from the Hubble Space Telescope. Over a period of 4 years, the light from the eruption lit up swirling dust surrounding the star. While the light echo in the LMC pictures is not quite as dramatic, it serves the same purposes!
(Thanks to Jason Harris for pointing this story out)