A very interesting puzzle about the ring of gas around Supernova 1987A may finally be solved after 34 years! Or, at least, a new avenue of investigation has been opened that does seem to answer a persistent question about it (note: I recently wrote about the neutron star in the center of the explosion having finally been found, but this new bit is different).

In April of 1990, astronomers got a big shock from Hubble Space Telescope. Yes, it was launched with a flawed mirror, and we soon discovered all the images were out of focus. But that’s not the shock I mean; using Hubble, despite the fuzzy images, we discovered the exploding star Supernova 1987A was surrounded by a ring of gas (I wrote about this many years ago; that article is part of a series about Hubble and those observations of the supernova).

To be fair, everything about SN87A (as its known to its friends) was shocking and weird; it really threw a monkey in the wrench of supernova astrophysics. We know that massive stars (ones with 8 or more times the mass of the Sun) expand and cool as they age, becoming red supergiants, and then explode when their cores collapse. But Sanduleak -69 202 (the star that exploded to become SN87A) was found to be a blue supergiant when it exploded, which had not been predicted at all. Something bizarre happened to it shortly before it exploded to turn it from red to blue.

We knew the star was surrounded by material before it blew. The International Ultraviolet Experiment was an orbiting observatory that detected a flash of UV light from the explosion, but it also found the light didn’t just turn on and off, it faded over time in a peculiar way. The thought was that the star was surrounded by a shell of gas that was lit up by the explosion, which then glowed due to the atoms in it being excited by the flash. This then faded slowly over time, in much the same way a glow-in-the-dark toy will fade slowly after you “charge” it with light.

But the way it faded didn’t make sense; it was thought the gas was a spherical or ellipsoidal (football or flying saucer shaped) shell, but the way the gas actually faded didn’t seem to correspond well to those shapes.

Then we launched Hubble in April 1990 and got the first high-resolution images of the gas, and saw it was indeed a ring. Regular readers may remember that I was involved with the program that got those images, and it was my task to process and analyze them, which was — not to put too fine a point on it — a nightmare due to the misshapen mirror blurring everything. But after a time I was able to show the structure really was a flattish ring and not the edge of a shell (like a balloon filled with gas). We also saw — and this is very important, so pay attention! — that instead of the ring being a continuous structure like a bagel, it was instead made up of a series of dozens of clumps of gas arranged in a ring, like a pearl necklace. This string of pearls structure was assumed to form as wind from the star slammed into the slower gas in the ring, causing it to break up. This sort of thing is called a Rayleigh-Taylor instability, and was well understood, though it was difficult to get the details right of how it would work for this particular situation. It was suspected something else was at play here, though just what was a mystery (oooo, foreshadowing). 

Not only that, but at the time I even had marginal evidence the ring wasn’t shaped like an inner tube with a circular cross-section, but instead had a parabolic cross-section. That evidence was pretty thin, I admitted, but it kinda made sense at the time…

We knew that the star blew off a wind of gas long before it exploded, probably 20,000 years or so earlier. The size of the gas structures and their expansion speed made that clear; it would’ve taken that long for them to grow to that size. The early Hubble observations I worked on (and later ones far more clearly revealed) an hourglass-shaped nebula around the star, with the flat ring at its narrow waist, and two much larger rings at the equators of the two hourglass lobes, above and below the ring. The most likely scenario to form them was that the star that exploded had a companion star in a tight orbit. As the massive star expanded over time it merged with the smaller one. This would have created a single star that spun rapidly, making it easier to blow material away (it also turns a red supergiant blue, so that may be the solution to that weirdness, too).

Now imagine a dense ring of gas surrounding the star before it exploded. A wind of gas from the star is blowing past it. What happens? When wind hits a small structure it blows around it and can create little vortices, spinning whirlwinds of air. These are similar to circulating eddies of water as it flows around a rock in a creek, for example.

As the star’s wind blew past the ring, some of it hit the ring at the top, and some at the bottom. The wind at the top would then curl around the top edge of the ring, flowing downward, and creating spinning vortices. Wind hitting the bottom would curl upward, creating a vortex that spun in the opposite direction. This would create a pair of counter-rotating structures, one at the top of the ring and one at the bottom.

And this is where the new research comes in [link to research paper]. Remember, the Rayleigh-Taylor instability didn’t seem to do a good job of explaining all the clumps, the pearls, we saw in the ring. But the scientists looking in to this realized a different action may have come into play: the Crow instability, a complicated physical process where the vortices can break apart and form lots of smaller clumps.

You may have seen this sort of thing before without noticing it. The Crow instability happens a lot in jet contrails. The air flowing past the wing tips of a plane create oppositely spinning vortices that interact with the contrail, causing it to break apart into clumps (and create all sorts of fantastic shapes as well). 

We have a similar situation with the ring. A wind of gas from the star flowing past the ring creates those counter-rotating vortices, very much like in a contrail. They interact and break up, creating clumps of material. Cool! Even better, the mathematics of the Crow instability show that the number of clumps you’d expect to see in the ring to be about 30 – 40, which is just what’s seen! That’s a reassuring conclusion that indicates the scientists who published this study may be on the right track here. 

Now, I can’t say for sure whether this is what’s happening in the ring, but Crow instabilities are to be expected, and they explain the existence of and the number of clumps. It’s funny: As soon as I saw the press release mention airplane contrails I figuratively slapped my forehead and literally said “Oh, Crow instabilities!” out loud, because I’m a dork. I first read about them years ago and meant to write something about them, and now here I am being able to link them directly to my own research from decades ago.

That work I did was so painful; just days and weeks and then years of trudging through very difficult math and coding complex image processing software to try to figure out how to make the Hubble images sharper. So now it’s amazing and fun to see it paying off, at least a little… and of course being able to crow about it. 

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