Tuesday, December 02, 2008

Light Echo Mind Warp

NASA's APOD site featured a four-frame animated GIF of this sequence of shots back in 2003. Yesterday, the Boston Globe's Big Shot feed featured a larger, five-frame version at the top of their growing Hubble Space Telescope Advent Calendar 2008.

What appears to be a spectacular explosion, complete with ejected matter, is actually a "light echo".

(By the way, I snagged the pictures in this post from this HubbleSite news release and included them here for better flow.)

Light echo? What seemed, at first, to be a pretty simple concept, got more complicated the more I thought about it. Eventually I just let it go, but now I think I've got it.

OK, the basic idea of a light echo is pretty simple. Shine a flashlight on something and see the light echo off of it. Look at the night sky and see the sun's light echo off of the moon. Simple, so what was my problem?

The star V838 Mons had been observed producing a tremendous burst of light, like a strobe flashing light in all directions. Once. Think of an old-time flash bulb.

I could easily envision a spherical shell of light expanding outwards from the central source. Obviously, when that thin, expanding shell of light encounters something, it illuminates it for the duration of the original flash, and then the shell moves on. Whatever we see in these dramatic images, we see thanks to the original, central flash of light echoing off of the matter it encounters as its thin, sperical shell expands at the speed of light.

Once I accepted the notion that what we see is not ejecta from an explosion, but rather, illumination of matter that was already there, things got a little bit clearer.

But wait! How could it be that, whereas we'd already seen the original light pass the Earth's position some time back, now we're seeing that same light bounced off of matter surrounding the star from whence said light came?

Well, light that bounced off of something before it reached you had to take a longer path, so it took longer to get here.

Yes, but look at the image. We're seeing light supposedly bounced off a shell of matter surrounding the flash star, which is presumably at the center of the illuminated matter. The distance from Earth to the star is much, much greater than the apparent distance from one edge of the illuminated matter back to the central flash point. How could the expanding shell of light have illuminated that surrounding matter that we see in the images after we've long since seen the original flash from the star? Something didn't seem to make sense to me.

What I had failed to assimilate is the concept illustrated below:

The matter off of which the light from the flash is echoing (the reason we can see it) is actually only coming from a small part of the spherical shell of light from the original flash, the part behind the star that flashed. The distance from edge to edge of the image we see is defined by an almost flat, circular disc within the thickness of the spherical shell (which thickness is the duration of the original flash times the speed of light). The reason it's an almost flat disk is that all of the light in the image had to arrive at the Hubble at about the same time, which means it must have originated at the intersection of the two spheres defined by the flash sphere and the sphere defined by the distance from the Hubble to the back edge of the flash sphere (which is essentially a plain within the tiny piece of that much larger sphere).

Maybe I can draw a picture to illustrate this, but it'll have to be later.

I feel SO much better now that I think I have a decent understanding of this "light echo" business. The picture is not of traveling ejecta, and it is not of interstellar matter surrounding the flash star. The picture is of matter behind the flash star that was illuminated by the original flash. The distance from the flash star to the back surface of the spherical shell of flashed light, and then back to the flash star (on its way to the Hubble), defines the difference in time between the original flash and its echo as seen here on Earth by the Hubble Space Telescope.

I'll have to try to wordsmith this a bit more, but later. Stuff to take care of now.

Amazing stuff.

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