Cas A blowout

The entire universe in blog form
Aug. 29 2006 2:24 PM

Cas A blowout

Earlier, I blogged about the Cas A supernova explosion. I noticed an interesting thing in the images just released by Hubble, but the original post was getting too long, and my "discovery" deserves its own post.

Looking at the image, you can see that Cas A isn't perfectly spherical (hardly anything really is). Look at the upper left side: it looks a little like a freeze frame of a popping balloon, doesn't it? Here's a closeup:

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Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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It may have been that the explosion was not perfectly spherical, that the actual supernova may have been a little off-center. It's also possible that the explosion was fairly symmetric, but the gas outside it was slightly less dense in one direction, so the expanding supernova blast wave could move more quickly in that direction. However it happened, that upper-left structure is a blow-out, where the blast wave burst through the surrounding material (you can see it better in the super high-res version which is 4000x2900 pixels). It left behind those long fingers, those tendrils of gas. They stretch all the way to the edge of the frame.

You can see that structure in radio waves and X-rays, but I've never seen it so clearly in optical light before. That's part of the power of Hubble, to be able to trace faint fuzzy stuff due to its higher resolution. Even more interesting is the separation of color in the "jet": in the high-res version you can see how the material closer in to the center of the explosion is green, and the material farther out is purple. In this image, purple is from sulphur, while green is from oxygen. Sulphur is heavier than oxygen, which is funny; you'd expect the lighter stuff to be outside the heavier stuff (lighter stuff would be moving faster). But here it's the opposite. Why?

On Earth, heavy stuff sinks, right? Gravity is a downward force, and the heavy stuff feels a greater force in that direction than light stuff does, so it sinks. A similar thing is happening in the Cas A blowout. The material ejected from the supernova is accelerated outward by forces in the explosion. Heavy stuff (like sulphur) flows toward the direction of the force, so the heavy stuff "sinks" outward, passing the lighter stuff. Another way to think of it is that the acceleration is outward, opposite the direction of gravity, so the heavy stuff will sink in the direction opposite the way it does on Earth. Heavier stuff here sinks downward, so heavier stuff in the explosion sank outward. Weird, huh? It's the same thing as when a helium balloon moves backward when you hit the brakes in a car.

Sometimes things happen the opposite way than you expect. The Universe is subtle, but there's always a reason for things to happen the way they do. It's like there is a higher force at work here... and there is. It's called science.

Image credit: Robert A. Fesen (Dartmouth College, USA) and James Long (ESA/Hubble), NASA, ESA, and the Hubble Heritage (STScI/AURA)- ESA/Hubble Collaboration

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