This episode of Astronomy Cast is brought to you by Swinburn Astronomy Online, the world's longest running online astronomy degree program.

Visit astronomy.swin.edu.au for more information.

Astronomy Cast Episode 215 from Monday, January 10, 2011. Light Echoes.

Welcome to Astronomy Cast, our weekly facts based journey through the cosmos, where we help you understand not only what we know but how we know what we know.

My name is Fraser Cain on the publisher of University and with me is Dr. Pamela Gaye, a professor at Southern Illinois University, Everett's Film.

At Pamela, how are you doing?

I'm doing well. How are you doing, Fraser?

Doing really well. So one quick announcement, which is you've been confirmed as a guest for Dragoncon 2011?

Is that right? Yes. I have applied as a guest. I have not been confirmed yet, but you know, make a big stink if it's only one.

So expect a Astronomy Cast live at Dragoncon, which will be in Labor Day weekend 2011.

And we will be looking for volunteers to help us mount a kick-ass booth with all sorts of NASA stuff. So be thinking ahead.

There you go. Okay. So if you were on the fence, should I go to Dragoncon? Yes, you should.

We'll be down. So, yes.

All right. So just as sound can echo off of distant objects, light can echo too.

And the echoes of light bouncing off stellar remnants, black hole accretion disks, and clouds of gas and dust provide astronomers with another method of probing the distant cosmos.

Light echoes.

All right. So then like I know what a regular echo is, you know, echo, echo, echo.

So what is the difference between a regular echo and a light echo? What are we looking at here?

The only difference between the two of them is in one case you have sound waves bouncing off of a distant object and taking a non-linear path to get to whoever's doing the hearing.

And that's the two sounds, the first echo and the second echo get heard at different times.

So we've got a we've got a sound source and it, you know, like a siren or an explosion or whatever or, you know, someone yelling echo.

About quacking.

...