One Ring to Rule Them All

By Phil Plait | June 6, 2005 10:56 am

It’s a mystery. I don’t like mysteries! They give me a bellyache, and I’ve got a beauty right now.

-Captain James T Kirk, “Man Trap”

Starship captains may not like mysteries, but scientists do.

Back in the 1990s I had my teeth sunk into a big one. I was studying a ring of gas a light year across, centered on a star that had exploded in 1987 [note: that link will take you to a series of essays I wrote on this star and the ring, and is a good preliminary read to get more details on what I'm talking about here]. I was a graduate student working on a team led by Bob Kirshner, and we had many observations of this object using Hubble Space Telescope. The exploded star, called Supernova 1987a, had been studied for years by people all over the globe, but the ring was seen best in our Hubble images. My job was to analyze the data and figure out what was going on with it.

I was able to find out quite a bit about it: how big it was, how much gas was in it, how dense it was, and by really pushing the data to its limit I was able to get an educated guess at the cross-sectional shape of it (like slicing a ball in half shows it has a cross-section of a circle, or cutting a piece of wood shows it has a rectangular cross-section; bizarrely, the ring appeared to have the cross section of a crescent, so it was shaped like the metal rim of a bicycle wheel).

What we couldn’t figure out, to be blunt, was what it was doing there. It didn’t make sense that such a dense doughnut of gas would encircle a star like that. We had ideas, and we knew they were the right track, but the devil is in the details, and the details are what kept us up late at night. Worse, the star had two other rings around it, offset from it, like the two bulbs in an hourglass. What the heck were they? We’d seen them around other stars, but again, the specifics of this case made it hard to understand what they were doing there.

Things got weirder. In the middle of the ring, the exploding star was expanding (you can see that debris as the elongated nearly-vertical stuff in the middle of the ring in the images above) . Eventually, after more than 10 years, that debris started hitting that inner ring. We expected to see the ring light up again starting in the lower part then moving around in both directions, like a circular fuse lit at one spot. That didn’t happen; we got a single bright blob for a while, with no other action. Finally, months later, we started seeing more blobs.

And yet, after all that, this weird structure had (at least!) one more surprise waiting for us.

When a star like this detonates, only the outer layers explode outward. The inner part, the core, collapses down, and you get either a black hole or a weird dense object called a neutron star. Everyone expects this event to have formed a neutron star, but no one can find it. There were early reports it had been seen in the late 1980s, but they turned out to be false alarms.

But now, 18 years after the event, we really should be seeing it. But recent observations show it just ain’t there, or, if it is, it’s much, much fainter than expected. The team tried everything they could to find it, and came up blank. Generally, a young neutron star is bright because gas and junk left over from the explosion fall onto it. So maybe there is some reason that the junk was cleared away by the blast. Or maybe there is more dust blocking the view than expected. Or maybe it formed a black hole, or maybe the magnetic field of the neutron star is too weak, or maybe maybe maybe. There are lots of maybes to go around.

But no matter how you slice it, Supernova 1987a is a weird object, and probably always will be. There may be other objects similar to it out there, but 87a was the first we ever discovered, and is the best studied. And yet, for all we know about it, there are still mysteries to be found.

Personally, I like things that way. I enjoy finding things out. If we knew everything, what fun would that be?


Comments (22)

  1. Why I like the Bad Astronomy Blog:

    Generally, a young neutron star is bright because gas and junk left over from the explosion fall onto it.

  2. Tobin Dax

    Phil, it looks like there’s a typo in the third paragraph from the end. In the sentence that says, “or maybe the magnetic field of the neutron is too weak,” shouldn’t that read “neutron star” instead?

    Other than that, great article. Your blog entries are wonderful to read.

  3. Thanks, I fixed it. And thanks for the compliment!

  4. anonymous

    I think it must be aliens. Since science can’t explain it, aliens are the next best explanation! :)

  5. Jason

    If you look at the picture just right….You can see god. Ha.

  6. You have intrigued me. I was aware of a supernova where a neutron star couldn’t be found but I must have read the reports that it had been found and lost interest. Now I’m all amazed again.

    This will be something else to bore my friends with.

    [Thanks for "Bad Astronomy", always a good, stimulating read]

  7. George

    Thanks for your illumination of one of the great heavenly bright spots of our times. It’s a treat to be so mystified by such dynamics.

  8. sophia8

    Thanks for telling us about this. This isn’t Bad Astronomy – this is Weird Astronomy!

  9. aiabx

    Pardon the question if the answer is obvious, but would we see anything if the neutron star was a pulsar pulsing in the wrong direction? I couldn’t help noticing that the ring’s polar axis (if the ring is circular) doesn’t seem to point at us.
    -Andy B

  10. In the paper linked from the press release, they say that if the neutron star were a pulsar (unlikely, actually, since it takes many years for the pulsar to “turn on”), it would be dumping a lot of energy into the surrounding gas, lighting it up. That is not seen, so it’s unlikely that there is a pulsar in there.

    There are lots of weird things about the supernova and its environs. Read the essays to which I linked in the entry and you’ll get a taste of them. Even after all this time, 87a is one of the weirdest and most exciting beasties in the sky.

  11. Kebsis

    Have we seen other supernova turn into neutron stars in the past? Or is the first time we’ve ever observed a supernova, and it isn’t doing what we assumed it should be doing?

  12. There are lots of neutron stars in supernova remnants, like the Crab, Vela, and Cas A. Check out the Chandra X-ray observatory website.

  13. Kebsis

    Well, what I mean is, have we ever seen the whole process from beginning to end like this before. Watched the star nova and then the aftermath.

  14. JPax2003

    Are there any planets orbiting this star remnant? Would a large planet induce enough wobble in the neutron star to make it optically indistinct. Would a large planet’s gravity and orbit create the observed anomalies in the debris clouds?

  15. Kebsis– no, this is the first time we’ve ever known the star that blew up before it blew up. It was identified later, from older images.

    Jpax, I suspected early on that a planet orbiting the original star may be behind the formation of the rings; I studied a similar (but not terribly similar) object for my Masters degree, and my advisor was very big on wondering if planets got engulfed by their parent stars, made them spin faster, and caused the formation of weird structures like rings. I think he was right; it explains a lot of things we see.

  16. Hey dR bAdastro, I think that should be supernova 1987A….

  17. JPax2003

    Would it be possible to locate a planet orbiting 1987A in current or old images using reflected x-rays similar to the x-ray reflection analysis Chandra used on Saturn (which was announced by NASA on May 25, 2005)? Would such images reveal an x-ray “source” orbitting the remnants, or would it be completely obscured by x-ray absorbing or emitting debris? If it is visible would it appear to go through “phases” like the moon and could it ecplipse some of the debris in transit?

  18. Kebsis

    How did that play out? Was it like, one day some unspectacular star just exploded?

  19. Kebsis

    …or did we know beforehand that it was gonna blow?

    (sorry for the double post)

  20. CR

    I remember when it happened, and my initial reaction thinking “how cool!” But I don’t recall if anyone had actually expected it to happen. Admittedly, I’d forgotten about it over the years, so this BA bolg entry was a nifty reminder about that event, and as usual a reminder about what an amazing & mysterious universe we live in.

  21. Nigel Depledge

    I was just re-reading this blog entry and associated comments, and I suddenly noticed something that I had not noticed before.

    From whence does the magnetic field of a neutron star come?

    I’d never really thought about it before, but I have just done so. If the gravitational field is strong enough to overcome electron degeneracy pressure and force electrons and protons to combine into neutrons, there will, I assume, be very few charged particles in a neutron star. Since magnetic fileds are related to current flowing (or at least the movement of an electrically conductive fluid), doesn’t this require charged particles that are free to move around? As I understand it, the surface of a neutron star is coated in a crust of iron. I had assumed this was solid (and so the mechanism that generates Earth’s magnetic field would not apply) and, since lumps of iron on Earth do not generate magnetic fields unless current flows through them, I just wondered what generates the strong magnetic fields around neutron stars.

    Can anyone out there could provide me with an answer?

  22. Blondin

    I had the pleasure of meeting Dr Ian Shelton last year when he was the keynote speaker at our annual Gateway to the Universe star party. He explained how he discovered supernova 1987A purely by accident. He had been taking photos of the LMC with an older, neglected telescope using up some old discarded film at Las Campanas, Chile. He noticed a 5th magnitude star in one of his photos that had been only 12th magnitude only 25 hours previous. It was just luck that he was the first to recognize what he was seeing.


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