A Colorful Cloud Hints at a Very Violent Origin

By Phil Plait

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Posted Tuesday, Feb. 26, 2013, at 8:00 AM

Halfway across the galaxy sits a most unusual object. Given the mundane name W49B, you might not think much of it, but once you see its portrait, you’ll change your mind.

The supernova remnant W49B, formed a thousand years ago in the titanic explosion of a massive star. Click to chandrasekharenate.
Image credit: X-ray: NASA/CXC/MIT/L.Lopez et al.; Infrared: Palomar; Radio: NSF/NRAO/VLA

This image, a combination of pictures taken in X-rays, infrared, and radio waves, is, obviously, very pretty. But it tells an interesting tale, one I haven’t been seeing in the press release or write-ups so far.

W49B is a supernova remnant, the expanding gas blasted out from an exploding star. Ignoring how long it took the light to reach us, the remnant is only about a thousand years old, making it roughly the same age as the more famous Crab nebula. The structure of W49B, though, is very odd.

The star that exploded probably had a mass of 25 times that of the Sun, which is pretty hefty, putting it in the top tier of stars in the galaxy. Not many get that big. As it neared the end of its life, though, it shed a lot of its mass (though not all) through a super-dense wind of material, like a solar wind turned up to 11 (or 11 million). Over the next few hundred thousand years, it actually lost a majority of its mass this way.

Eventually, though, the end came. The core of the star ran out of fuel, which it was using to generate energy, which in turn was what was holding the star up. When the fuel ran out, it was like a stool with the legs kicked out form under it: The core collapsed, plunging down into itself at huge speeds. As it dropped down, the material started to rotate rapidly, like (to use a cliché) an ice skater bringing her arms in and increasing her spin.

At this point we’re not precisely sure what happens, but the thinking goes like this. The material in the very center of the collapsed core formed a black hole. But material just outside the hole probably formed a dense disk of material whirling around the black hole at nearly the speed of light. This whipped up huge amounts of heat and magnetism, and through methods not entirely understood formed a pair of beams, like lighthouse beams, blasting outward from the poles of the disk. These screamed out, boring right through the material still falling inward from the star.

Now I want to take a moment to let that sink in. Imagine you are just above the core of the star. Beneath you, in a millisecond, poof! The core is gone, collapsed down into a tiny point a million kilometers below you. Looking up, you see an octillion tons of superheated matter crashing down onto your head.

Got that apocalyptic picture clear? Yes? Now look down again: Those two beams of matter and energy come screaming out of the collapsed core at nearly the speed of light with enough power behind them to bore through that infalling matter like a megawatt laser through a warm patty of butter.

So we’re talking fairly serious events here.

And this is when what was left of the star exploded. All that remained after that was a black hole in the center, and material moving violently outward at high speed. That material, though, was not expanding in a sphere, but instead was moving preferentially along the direction of those beams, up and down, if you will.

Now, a thousand years later, we see the effects. Along the middle of the remnant you can see a blue streak. In the false color image, those are X-rays from iron, created in the blast itself. You can see how elongated that structure is, not spherical at all. That’s a dead giveaway this explosion was asymmetric, that is, not spherical.

This type of supernova explosion is called a Type Ic. Technically, that means it doesn’t appear to have any hydrogen or helium in it, which is rare. But it happens when a very massive star sheds its outer layers shortly before exploding; all the hydrogen and helium were blown away before the explosion. By the time the star explodes, that material has moved well out from the star.

The three-ring circus of Supernova 1987A. Click to embiggen.
Image credit: Dr. Christopher Burrows, ESA/STScI and NASA

But it’s still there. Eventually, the matter blasting out from the explosion slams into the previously-shed outer layers of the star. We see that in the above image as well. The yellow and pink material is where the expanding debris is colliding with the slower moving gas, hitting it so violently that powerful shock waves are formed.

I’m fascinated by the shape of that outer region of this object. You can see that it’s barrel-shaped, tilted lower left to upper right. But there also appear to be rings of material there, perpendicular to the barrel. I’ve seen that before: Supernova 1987A was a very well-studied exploding star; I got my PhD examining it with Hubble. It has a three-ring system around it like an hourglass. How those rings formed is still debated, but we know they were created from a powerful wind of gas from the star millennia before it exploded. When the star did explode, it lit them up like a flashbulb, making the gas glow.

Was W49B once a multiply-ringed structure similar to SN1987A? I suspect it’s possible, judging from the image. There are several rings of material visible in the Chandra image. And as it happens, the expanding debris from SN1987A is also highly elongated, suggesting a similar explosion as W49B. You can see that in this sequence of Hubble observation over a decade, showing the debris expanding inside the dense ring of gas (the middle of the three rings):

The debris from the supernova 1987A expands inside a ring of older material, seen in observations by Hubble taken over nearly a decade.
Image credit: NASA/ESA, P. Challis, R. Kirshner (Harvard-Smithsonian Center for Astrophysics) and B. Sugerman (STScI)

There are differences, mostly that the debris from the SN1987A explosion is apparently expanding preferentially in the same plane as the ring, which is not at all what I would expect. But SN1987A was always a weirdo, and you have to be careful when comparing one supernova to another in this case.

Also, very intriguingly, there is no clear leftover object in the center of W49B, no obvious highly dense neutron star (which would be very obvious in X-rays, even after a millennium). We think therefore it formed a black hole, which fits with the scenario I described above. But the same thing is true for SN1987A! We’ve been searching for years, but no neutron star is evident. It’s possible a black hole formed there as well. It seems unlikely, because the effects of a black hole forming should have been seen when SN1987A went off, but in my opinion that’s not nearly as weird as forming a neutron star that we can’t seem to find. Either way, like I said, SN1987A was bizarre no matter how you slice it.

But so is W49B. If it formed a black hole, it’s one of the youngest in the Milky Way, a whippersnapper at a thousand years young. But to me, that’s not the interesting story. What gets me is how the supernova exploded in the first place, with the spinning and the disk and the jets and the rings. That’s the real story here.

Plus one other thing, that is. When a black hole forms from an exploding star, and you get that spinning disk and high-energy pair of jets, you also get a tremendous flare of gamma rays, super-high-energy light. We call these events gamma-ray bursts, and they are the most energetic explosions in the Universe second only to the Big Bang itself. Was W490B such a burst? It turns out I’m not the only one to wonder about that. While we see GRBs all over the Universe, they tend to be very far away, which may mean they happened more when the Universe was young, billions of years ago. Yet there is some evidence they still occur today.

Perhaps that’s the biggest story here. As usual, with astronomy, when you observe a single object there is more than one tale to tell. But they’re all amazing, and all well worth hearing.