Cas A blowout

By Phil Plait | August 29, 2006 2:24 pm

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:

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 4000×2900 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

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (15)

  1. idlemind

    Just an astonishing image. However, do hubble images really have such pronounced diffraction spikes? Or did someone add those? (I really prefer my stars round, thank you.)

  2. Hubble is a reflector, so it wouldn’t be too surprising but saying that I don’t know how the secondary mirror is held in place, you’d expect spikes especially when imaging something like a supernova with many relatively bright objects in the field of view.

    I think its pretty unlikely NASA would go around adding diffraction spikes to its images.

  3. The spikes are indeed caused by the “spider”, the metal frame holding the secondary mirror in place. You see it in most images. In the ACS images, like this one, they are up/down/left/right, but in WFPC and STIS images they are tilted by 45 degrees. It depends on the orientation of the camera.

  4. kingnor

    “It’s like there is a higher force at work here… and there is. It’s called science.”


  5. Dan Gerhards

    I don’t get it. I thought helium balloons moved backward because the air has more inertia. Being harder to stop, it moves past the balloon and piles up there, pushing the balloon the other way. That doesn’t seem like just a different way to say the same thing. Also, the different amounts of gravity acting on heavy and light objects is exactly cancelled out by their different inertia. They fall the same speed in vacuum, so only “sink” because they’re surrounded by air.

    Both effects seem to be bacause the objects are surrounded by air, which isn’t the case here.

    I suppose my problem is not really understanding how type II supernovae work. Does the neutron degeneracy pressure *accelerate* matter outward? I thought it just hit escape velocity instantly and was deccelerating as it flew outward.

  6. bassmanpete

    Regarding bubbles flowing downwards, this happens every day in British pubs! Pint glasses, the ones without handles, have a bulge about two thirds of the way up (or used to, I haven’t lived there for over 20 years). When beer is poured into them, the bubbles above the bulge flow up & below the bulge they flow down. I often wondered why this happened – anyone have an explanation?

  7. JackC

    Dan, you bat me to it. I read the BAs article on the bubbles thinking “He has GOT to be kidding…” the whole time. But then, I remind myself, he is an astronomer, not a physicist.

    You are absolutely correct – it is the air continuing forward (or backward under positive acceleration) that the helium balloon is responding to – thereby again seeking it’s equilibrium by moving in the opposite direction. It should be noted that it is not going to move quite as FAR as it would in a non-moving frame though!

    Under negative acceleration (braking), the air mass continues forward, forcing the balloon to the back.

    As for the bubbles in a Guiness glass (or any other fine, dense brew for that matter!) we can refer here:



  8. Actually, I expect that the differentiation in color has more to do with which species are ionized and excited at various places along the shock than where the Oxygen and Sulfer are….

    You often see this kind of differentiation in nebulae. It’s not because of mass segregation, it’s because different radiation penetrates to different depths. Look at the Ring Nebula. It’s bluer towards the inside, redder towards the outside. The inside blueness is from OIII (doubly ionized Oxygen) excitation, the outside redness is mostly from Hydrogen (H-alpha). The ionization structure is dominated mainly by Hydrogen and Helium. Towards the outside, there aren’t enough high-energy photons left to keep Oxygen doubly ionized, thus you don’t see any lines from doubly-ionized Oxygen– all the higher energy photons originally emitted by the central star have been absorbed by Hydrogen and Helium in the nebula. Meanwhile, there’s still more lower energy photons farther out left to keep Hydrogen ionized, thus the reddish H-alpha is still there. This will happen even if the Oxygen/Hydrogen ratio is constant throughout the nebula.



  9. icemith

    bassmanpete – are you referring to the first pot or the fifth? Not being a drinking man that much I would not know, but is the effect observeable in a Bacardi and Coke? I guess I can check that anyway.

    Refering to the ‘bulge’, I’m guessing it bulges out, as do our schooner glasses, and I think, so does any other large glass.


  10. Greg Fuchs

    Phil said:

    “Like the bubbles in a soda bottle, the helium in the balloon is lighter than air and wants to move opposite the direction of gravity.”

    Not exactly.

    I’ve been reading the helium balloon explanations and everyone seems to be all around it with derivatives but here’s what they teach in pressure instrument calibration school –

    The reason a helium balloon rises is because of the pressure differential around the balloon (Bernoulli). To simplify, air presses down on the top, up on the bottom, to the left from the right and to the right from the left (you can add front an rear if you wish). In calm air the force of the air pushing from the left and right are equal, so the balloon will not move in those directions. There is a pressure differential between the air pushing on the bottom of the balloon and the air pushing on the top of the balloon. Because air is more dense closer to the earth there will be a net force upward. If this force is greater than the force of gravity acting on the mass of the helium and its container the balloon will rise.

    Likewise with the balloon in the car. When you hit the brakes the air in the passenger compartment tries to maintain its initial velocity. This results in a high pressure area at the front and a low pressure area (relatively) at the rear. It’s the force caused by this difference in pressure that pushes the balloon to the rear. The lower inertia of the helium filled balloon allows the very small force of the pressure differential to cause the rearward movement. If I remember correctly, the inertia of an air filled balloon is greater than the pressure differential force so it would continue to move forward, albeit somewhat slower.

    The difference in pressure is demonstrated in pressure instrument calibration school using a high accuracy, high resolution barometer. Each student, in turn, takes the barometer places it on the floor and records the reading. He/she then moves the barometer to the top of another student’s head and records the reading. The subject’s height can then be calculated by the difference in air pressure between the two levels. Accuracy is typically within 2 – 3%. Pretty good considering the uncontrolled classroom atmosphere.

    If you had a pressurized air vessel (closed system) with a helium balloon in it the balloon would maintain its initial position. It would neither rise nor fall no matter what its initial position. If a helium balloon rose due to being lighter than air, a helium balloon in a highly pressurized closed vessel should rocket to the top as the air is very dense (heavy). This is not the case. It does not move because the air pressure is equal all around the balloon (no pressure differential to move the balloon).

    Sorry to all who knew this but the ‘lighter than air’ thing doesn’t really explain the particulars of the effect.

    As an unrelated aside –

    When dealing with a high quality pressure calibration device the biggest source of error is ……


    The equipment is so accurate, the gravity at the use site can cause an error in measurement as much 10x that of the equipment itself. This is corrected by getting a gravity report and using software to apply a correction factor.

    It’s a pretty interesting field.


  11. In the original essay, I took a shortcut that obscured what I meant.

    It’s not that the balloon wants to move in the opposite direction of gravity, it’s that it moves along the gradient of air pressure (high to low), and that is due to gravity. In an accelerating car, the acceleration is the substitute for gravity, creating a horizontal pressure gradient. The balloon therefore moves along that gradient, again from high to low.

    I could have been more clear and written “acceleration” and not “gravity” but what I wrote is still essentially correct.

    Rob Knop: the gas in the nebula is optically thin to optical light, so any visible photon generated will just pop right out of the gas. I expect that given the age and low density of the nebula it’s a pressure-driven ionization and not photoionization, and therefore the segregation of the light emitted is due to a real mass distribution, and is not an ionization effect.

  12. Rob Knop: the gas in the nebula is optically thin to optical light, so any visible photon generated will just pop right out of the gas. I expect that given the age and low density of the nebula it’s a pressure-driven ionization and not photoionization, and therefore the segregation of the light emitted is due to a real mass distribution, and is not an ionization effect.

    I don’t know enough about SNR, so you could be right….

    I would note that the Planetary Nebulae that I was talking about are also optically thin to optical light, but it’s optical depth at the ionization edges that matter.


  13. Ah, true. I may have oversimplified. I do know that Cas A is ferociously complex, and you have to take care if you want to understand what its doing. I learned that lesson pretty well putting together the activity.

  14. bassmanpete

    JackC, thanks for the Guiness connection site, it was nice to find out why it happens at last. I first noticed this back in the 60s when I was in a rock group playing pubs & clubs. It did happen with other beers but was more obvious & pronounced with Guiness. Obviously the bulge in the glass (yes, icemith, it is an outward bulge) has nothing to do with the effect, it just happens to be in the place where the up bubbles & down bubbles go their separate ways.

  15. Gary Ansorge

    Always nice to see a simple reality connection to the esoteric. Beer bubbles, balloons and supernova, they’re all related. Much better than, ” ,,,and then here, a miracle occurs,,,”.

    Gary 7


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar