A delicately violent celestial shell game

By Phil Plait | December 16, 2010 7:00 am

One of my favorite types of objects in space are the thin, ethereal shells of gas stars create when they die. So I was thrilled* to see this new image of one taken in exquisite detail by the Hubble Space Telescope:


[Click to supernovenate.]

I studied weird soap bubbles like this for quite some time for both my post-graduate degrees, and they still fascinate me. This one, called SNR 0509, is actually a very interesting example. There are lots of ring-shaped objects in the sky — the Helix nebula (seen below) may be the most canonical — but usually the ring itself is thick, the width of the band being a large fraction of the object diameter itself. Why does SNR 0509 have such a thin ring?

eso_helixI’m glad you asked! OK, so I asked, but if I were you I would’ve asked that. And I’m me and I did.

First off, we need to understand what happened here to form the object in the first place. When a relatively low-mass star like the Sun dies, it blows off a steady wind of subatomic particles, like the solar wind but much thicker. The physics is a bit complex, but basically as the star gets older that wind speeds up. The faster particles slam into the slower ones, and light up. You get what’s called a planetary nebula, like the Helix (pictured here on the right; click it to get a really big pretty version of it).

limbbrighteningIn many cases, the reason we see a ring is because of an effect called limb-brightening. If the star’s wind were steady, it would form a big, spherical nebula around the star. But the faster wind wind plows up material, hollowing out the sphere, forming a shell. The gas glows, and this makes it look like a soap bubble in space, because we see more gas near the edge than through the center — I drew the diagram here to show that (click it to make it bigger). Imagine you are off to the right, looking at a spherical shell of gas to the left. When you look through the center, you’re not seeing as much gas as you do near the edge. Since it’s the gas that’s glowing, the more you see, the brighter it gets. So the nebula is brighter in a circle around the edge (near the limb), and darker in the middle. A ring!

Oh– it’s important to know that the gas is transparent. If it were opaque, like a solid wall, you’d see a disk, and not a ring. So light passes right through this gas (you can see stars through it). That means that it doesn’t matter if an atom of gas is on the near side or far side of the shell; if it emits light, you’ll see it. It also doesn’t matter from what direction you look: since it’s a spherical shell, it looks like a ring no matter where you are!

The thing is, while limb-brightened objects may have a sharply defined outer edge, the inner edge is usually fuzzy. And they’re usually thick, as I said before. Look at SNR 0509: it’s got sharply defined edges, and is extremely thin. There’s some fuzziness on the inside, but not a whole lot. Why is it different?

The clue to this is in the name: SNR stand for SuperNova Remnant. So this wasn’t some star that passed gently into that good night. It exploded (you might say it raged against the dark).

When that happens, the physics is different. For one thing, the speed of the expanding material is far faster; a supernova unleashes with a titanic fury, blasting out matter at speeds of thousands of kilometers per second (the wind from a lower mass star’s death might be a few hundred). Also, in this case, there was lots of matter floating out there between the stars, and the expanding debris from the supernova crashed into it, plowing it up with a lot more force. This compresses the gas more violently, creating shock waves, and makes a much thinner ring. Variations in the density of the material being swept up would also explain the frayed edges at the upper left. Things are messy in space, so you don’t expect near-perfect spherical shells when all is said and done.

hstchandra_snr0509Confirming the idea that this is a supernova is this combined image from Hubble and the Chandra X-Ray Observatory. The matter inside the ring is glowing brightly in X-rays (shown in blue), which you’d expect from the über-violent collision going on. While the less-dramatic planetary nebulae do give off X-rays as well, the physics is different and the two can be distinguished by observations like this one.

Of course, the fact that this ring is over 20 light years (200 trillion kilometers!) across and expanding at 5000 km/sec (3000 miles per second) is also a pretty clear indicator we’re seeing the death-thrash of a supernova! Planetary nebulae, even large ones, are usually only a light year or two across. Only the power of the explosive demise of a star can drive such a huge object.

Still, a lot of the physics of the way supernovae and planetary nebulae sweep up material is similar… and there’s another connection too. Our Sun is far too low mass to explode. When it dies, it’ll go more along the planetary nebula route. But it’s thought that a nearby supernova explosion helped collapse the cloud of gas and dust that formed the Sun, the planets, and us. When we study objects like SNR 0509, we’re seeing both into the distant past and the distant future, and learning about the endpoints of the Sun’s personal timeline. That in itself is a worthy experience… but it certainly helps that these objects are also so beautiful.

SNR 0509 image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgement: J. Hughes (Rutgers University). Helix credit: ESO.

* Well, I feel I must be honest to you, BAbloggees: my first thought was "Dangit, they post this the same morning my Top 14 Pictures of the Year list goes up?!" I actually don’t think this image would make the list, but it’s still really cool and I wanted to write about it. With my list going up, I decided to wait a couple of days before talking about this object to prevent — haha! — shell shock.

CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (30)

Links to this Post

  1. Ne’er (Fe)rgit… | Sci-ənce! | December 17, 2010
  2. Astronews Daily (2455548) | December 17, 2010
  1. MarkW

    I’m forever blowing bubbles,
    Pretty bubbles in the sky.

    (With apologies for changing the lyrics.)

  2. Question: I’ve heard that a nearby supernova provided the shockwave that precipitated the collapse of the nebula that formed our solar system. Where is the white dwarf/neutron star/black hole that was left from that supernova? I guess if it was 4.5 billion years ago it’s had a long time to move on out and “nearby” in astronomical terms can be a pretty long ways so it could have had a good head start too but we should still be able to figure out which star it was.

  3. Messier Tidy Upper

    Magnificent supernova nebula image. Great write-up. Thanks BA. :-)

  4. Elias

    Phil-eidolia du jour:

    So that’s where my interstellar diaphragm went!

  5. thetentman

    Looks like a smoke ring.

  6. Messier Tidy Upper

    Just one very small nit to pick, here sorry BA, :


    Didn’t you mean “ensupernovenate?” there? 😉

    @1. MarkW : Forever? I could be mistaken but I’m pretty sure the star in question here could only blow that bubble once! 😉

    @2. Josh :

    Where is the white dwarf/neutron star/black hole that was left from that supernova?

    It would be astounding to find that I agree but, unfortunately, there’s a few big problems here – the vast spans of time and distance issue you’ve noted already but I’ll expand on just slightly with this quote here :

    “Since our Sun was formed more than 4 billion years ago, it has travelled around the Galaxy 16 times.”

    Source : “Two of the Milky Way’s Spiral Arms Go Missing” NASA e-newsletter news release 2008-June-4th.

    Another obstacle here is that the remains of the supernova – a black hole or neutron star – would have long since cooled and dimmed to a point where finding it is probably exceedingly unlikely. Chances are its long since spun down and it may also perhaps been ejected from our galaxy via an asymetric explosion giving it a kinetic kick away in one direction or, perhaps, gravitational encounters could have caused it to fall inwards and just possibly even have meant it has already merged with the central supermassive black hole at our Milky Way’s core.

    Futhermore, we don’t know what sort of supernova it was and not all supernovae leave “stellar corpse” remnants – if that precursor star was a type Ia “white dwarf” supernova (unlikely given the timescale but can’t be ruled out entirely I think) :


    or if it was a (more likely?) “metal poor supermassive” pair-instability supernova :


    Then the star in question here would have been totally and utterly destroyed leaving no remanant but dissipating gas and a shockwave for its cosmic tombstone.

    Plus, as far as I’m aware, we don’t really know how distant the star that triggered the proto-solar nebula to collapse was from our Sun’s natal coccoon nor, I think, exactly how long the process of nebular collapse and starbirth (*Sun* birth!) really took.

    So given all these uncertainties I very much doubt we’ll ever find a supernova remnant that can be definitively traced back as the trigger exploder that caused the nebula to collapse that led to the birth of our daytime star. (Sorry to be a wet blanket again here.)

    However, I would love to be proven wrong about this! 😉

    PS. Thinking Supernova types and remnants – do we know what type of supernova produced this one – SNR 0509 – & whether it is linked to any known central or nearby pulsar /magnetar / black hole?

  7. Aaron

    @2. Josh: Some time ago, I wondered the very same thing. I also wanted to know if the Sun’s current neighbors came from the same stellar nursery. Therefore a friend and I looked up some information and calculated that in the past 4.6 billion years, any remnants of the Sun’s parent and all of its sibling stars could have migrated to any corner of the galaxy! It is unlikely that we will ever know which few they are out of the 100-400 billion stars of the Milky Way.

  8. Tom

    Phil, I think the second-last paragraph should read ‘this ring is over 20 light years (200 trillion kilometers!) in diameter’

  9. rob


    Phil, http://www.treelobsters.com has a post with an appearance by a certain book you might recognize…

  10. Robert

    I came for the science, but I stay for quips like this:

    “I’m glad you asked! OK, so I asked, but if I were you I would’ve asked that. And I’m me and I did. “

  11. Michael

    I am wondering if perhaps the reason for the thin-ness of the shell, based on the diameter, might simply be age? Perhaps enough of the material has dispersed over time, rendering the perception of a thinner shell? I am still learning a lot about SNRs, especially neutron stars (a particular fascination for me), so maybe I’m looking at this wrong. Have we had a chance to undertake an effort to detect a remnant? Without that data, we’re all ‘crap-shooting,’ but the model I envision is that the supernova that made this shell (as well as being very old) happened really fast. A slower supernova would create a thicker shell, no? Or yes? On that note, am I being silly thinking that supernovas can occur at significantly different speeds for a given supernova type?

  12. John Paradox

    The physics is a bit complex, but basically as the star gets older that wind speeds up.

    Something similar can happen to humans, too…..

    “Hey, you kids! Get out of my stellar system!”

  13. One Eyed Jack

    Space bubbles…

    So space is really just a giant Lawrence Welk show?

    “And a one, and a two, and…”

  14. Matt B.

    Makes me wonder how likely it is that any intelligent life in the galaxy would be wiped out by a supernova. Those three stars in the corner look like they might be blinking “SOS – It’s comin’ right for us!” (Something to think about for the Drake Equation.)

  15. No. 6

    Messier Tidy Upper @6:

    IIRC, B^2FH estimates that the Earth and Sun formed from the remnants of a supernova that blew up about 6.4^10^9 years ago. ( http://en.wikipedia.org/wiki/B%C2%B2FH ) The paper itself is available online for free as well.

    I’m pretty sure there have been substantial efforts recently to locate/identify/ the Sun’s natal cluster, as well as ‘sister’ stars formed from the same cloud. I thought they had had some small successes, but don’t remember details. Maybe the BA knows…?

  16. Back me up here. I’m claiming that when things blow up in space in movies that they should look something like this. Sure, a ring might come off of the Death Star, but it’s always gonna be a circle radiating perpendicular to the line of sight. Never at a wonky angle like planetary rings might look. Never in such a way that the ring would hit you. The blastwave, sure, but not the ring.

    I’m making this claim in opposition to a friend who says that some movie would try to get it right and since all movies show the ring at a dramatic or dangerous angle that’s how they must go.

  17. Jon Hanford

    SNR 0509 is one of three former supernovae in the LMC found to display light echoes (light from the original blast seen reflecting off dust as the “light sphere” expands and moves through the interstellar medium).

    The discovery was made using the 4m Blanco telescope at Cerro Tololo back in 2005. Here’s a link to a video showing the position of SNR 0509 in the LMC (this is an X-ray image of the SNR) and a short movie of the moving light echoes: http://chandra.harvard.edu/photo/2008/snr0509/E509_sm_web.mov

    Pretty amazing stuff!

  18. Dr Jeremy Greenwood

    @6. Messier Tidy Upper. …any known central or nearby pulsar…
    What’s that pink spidery thing in the middle of the supernovenated image? Is that the stellar remnant?

  19. DLC

    hm. looks like a star-size soap bubble to me.

  20. JB of Brisbane

    @OneEyedJack #13 – Wunnerful, wunnerful!

  21. Messier Tidy Upper

    @15. No. 6 & 17. Jon Hanford : Thanks. :-)

    @18. Dr Jeremy Greenwood :

    @6. Messier Tidy Upper. …any known central or nearby pulsar…
    What’s that pink spidery thing in the middle of the supernovenated image? Is that the stellar remnant?

    Quite possibly so. It could very well be that, but then, alternatively, it might potentially only be an unrelated line-of-sight co-incidence foreground or background object. So I don’t know for sure & would merely be guessing.

    Can anyone further enlighten us all on that central nebulosity’s exact nature, please?

  22. Yeebok Shu'in

    @21 / All – purely visually, the reddish faint nebulosity looks off-centre (slightly up and left in the image) – however given that the bubble seems as if it is more compressed in the same quadrant (the wavy edges etc) I get the feeling that it may be related in some way, but that’s counter-intuitive : if the star blew itself apart I can’t think of a reason there’d be a visual remnant anywhere. I was reading the comments and had noticed the smudge before I read comments related to it and that’s the track my brain followed. So it looks right visually but logically it doesn’t fit. I, like most of us here would love to know what Phil thinks :)

  23. Jon Hanford

    @ 18, 21, 22 re the “central nebulosity”

    A quick search of the literature turned up no known pulsars (or neutron stars) related to SNR 0509. Also, I found no mention of a central plerion (such as that seen in the Crab Nebula). It seems likely that the “central nebulosity” is indeed a foreground-background object seen in projection as MTU noted. Given the number of background galaxies visible in the frame, I’d go with a background galaxy (also note the sharp, reddish nucleus (AGN?) and the “bluer” amorphous region around it). Just a guess.

    WRT the progenitor of SNR 0509, formation of a black hole seems plausible.

  24. Isaac

    @Ibid, #16: Your friend has an unrealistically optimistic idea of movie science. In fact, movie makers tend always toward what is visually interesting over what is scientifically accurate. However, there are many factors which can shape an explosion. You are correct that an unshaped explosion would be a sphere, but other factors might come into play to make the explosion take another shape, especially at first. Not likely that it would take the form of a ring at a dramatic angle, though.

    You and your friend might find this site interesting: http://www.intuitor.com/moviephysics/

  25. Matt B.

    @16. Ibid. 2010: The Year We Make Contact does it right. A ring whose axis is on the line of sight is easily perceived as a sphere when done like that.

    If anything the Death Star’s ring should have been equatorial, since that’s the only plane of asymmetry, and Alderaan’s should have been perpendicular to the Death Star’s ray, since the planet’s rotation would be a negligible factor. So ILM basically managed to get things backward.

  26. Messier Tidy Upper

    @25. Jon Hanford : Thanks. :-)

  27. Prefer physics to astronomy but this is superb. Not seen this before. Iagine that the study of this is in its infancy. You wouldn’t be doing a lot of geology in it I’ll assume.

  28. jeremy greenwood

    Pink spidery thing in the middle… Thank you for the feedback. Sorry I’m late to respond – forgot my email address.


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