AAS Report #3: Things that go boom!

The entire universe in blog form
Jan. 9 2007 11:54 PM

AAS Report #3: Things that go boom!

Note: My body was the target of a vicious but thankfully short-lived viral attack on Sunday. I was totally out of it for a full day... and the less you know about this the better. It did keep me offline -- literally -- but I'm all better now and ready to blog. Good thing, too: there is a lot of cool stuff coming out of this meeting!

It's a fact of life that some stars explode. Actually, it's a good thing: when stars explode they create and scatter the heavy elements that create us. The iron in your blood and the calcium in your bones were created in a supernova! So it's important to study these objects, so we can better understand our origins.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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But it's also fun! Stars explode! Bang! Cool!

Today there were three press releases about supernovae. All three were surprising to me, and pretty interesting.

1) Kepler's Supernova was a Type Ia

OK, so that title doesn't thrill you. But that simple statement is actually the answer to a long-standing mystery. Ready for this? OK, sit back...

You may already know that massive stars -- stars with something like 20 or more times the mass of the Sun -- explode when they end their lives. You may not know that for thousands of years before this, they blow off a sort of super-solar wind, dumping quite a bit of gas into space around them. So eventually the star explodes and the outer layers blow outwards at tremendous speed, while the core collapses. The outer parts slam into the gas previously ejected by the star's wind, form all kinds of weird and wonderful shapes, and become the supernova remnant (sometimes called the debris). The collapsed core turns into either an ultradense neutron star or a black hole. This is called a Type II (pronounced "two") supernova.

So duh, there must be a Type I. Originally, the distinction between a I and a II was that IIs had hydrogen in their spectrum (meaning there is a lot of hydrogen gas around the star), while Is didn't. Eventually, the classification was found to be more complicated (isn't everything?), and there had to be sub-classes, like Ia, Ib, and so on. It's now understood that a Ia is when a white dwarf explodes.

In fact, I've explained this before. Save me some writing, read the first part of that link, and then come back here. I'll wait.

<crickets>

OK, done? Now you understand how white dwarfs explode.

So, you'd think if you found a star that had exploded say, 400 years ago, it would be easy to tell if it was from a massive star or from a white dwarf. But it turns out it ain't. I give you Kepler's Supernova:

This is the expanding gas cloud from a star that blew up in 1604 (hey! That's my hotel room number at the AAS!). It was seen by many folks, including -- you guessed it -- Kepler, who observed it carefully, took notes, and got it named after him. Things were easier back then.

Anyway, it's always been an oddball supernova. There is no black hole or neutron star in the center, which sounds like it was from an exploding white dwarf, Yet the remnant itself appears to be expanding into a cloud of gas rich in heavy elements like nitrogen, which makes it sound like a massive star exploded. Which is it?

Astronomers using the Chandra X-ray observatory have finally solved this mystery. The different elements in the superheated supernova gas betray their presence by emitting X-rays. By carefully studying the ratios of these gases, astronomers have determined that the star that exploded was, indeed, a white dwarf -- the gas has a lot more iron than you expect from a massive star explosion, but the right amount as you'd expect from a white dwarf. The amount of oxygen in proportion to iron reveals the same thing.

So Kepler witnessed a Type Ia supernova.

But then where did that nitrogen that was seen come from? That's a fine question, the answer to which we still don't know. But there are some fun possibilities! One is that white dwarfs are sloppy eaters. As the white dwarf draws off matter from its companion, some of it might escape. That gas should have lots of nitrogen in it, and it's that escaped gas that the supernova is expanding into.

A weirder but more exciting possibility is that maybe the star that blew up was a lot more massive than the sun, like 7 times as massive. That's not big enough to explode, so it still died like the Sun eventually will. It blew the big wind, dumped the heavy elements into space around it, and then somehow sucked down enough matter from a nearby companion star to blow up... all within a hundred million or so years. In astronomy, that's pretty quick. A normal Type Ia might take billions of years to explode.

No one has ever positively IDed a prompt Type Ia before, but maybe what Kepler spotted 403 years ago would turn out to be the first of its kind (OK, the first that we could identify). In fact, it's pretty important that we figure this out. Type Ia supernovae can be seen from incredible distances, billions of light years, and are what astronomers are using to determine the overall shape and fate of the Universe. So there's a lot riding on this kind of stuff.

2) Supernova 1987A has some friends

(There's no picture with this part! I'm fretting over that, but I can't find any online. If I run across one I'll add it in. Bummer for now though.) Update: within minutes of posting this, I was packing my stuff up, and of course found a link to the image. I've corrected the text below.

A massive star blew up in 1987 (sigh, no, I'm not going to argue with anyone who says that the star really blew up 168,000 years ago and the light just reached us in 1987, since time flows like light in relativity, and so what I wrote is technically correct, and if you wanna argue, fine, comment away, but I'm right no matter what you say). It was the first one seen in that year, so it was named SN1987A. I studied it pretty intensely from 1990 to 1994, and eventually got my PhD with the results I found. My results were not exactly earth-shattering (the supernova wasn't close enough to us, haha) but if you have a mind to and want to torture yourself, you can read my thesis here. I suggest you read through my Bitesize pages about it instead if you want some background.

In the section above I talked about the wind that blows from a massive star. 87A was pretty massive, and it blew a wind, and it formed some pretty weird shapes. Here's what it looks like:

Trust me, that's weird. When we first saw those three rings, we were baffled. Nothing like that had ever been seen before! We'd seen some similar objects, but nothing with such a bright inner ring and faint outer rings, and nothing that well-defined. They defied explanation.

I searched high and low to find a similar object that we could study to compare with it. I found nothing. There was one object I was hopeful about but never pursued for various reasons (it was Mayall-Cannon 18, which would eventually be observed by a different guy, and would become a famous picture and be on the cover of a Pearl Jam album, not that I am extremely bitter about losing out on having a famous image from Hubble with my name on it and everything), and in the end it wasn't all that close enough in shape to 87A. I eventually gave up.

Years later, a new object was found that was similar. Called Sher 25, it showed a nebula pretty similar to 87A's, and even had a massive star in the center. It was a good analogue, but still not quite close enough.

But that's changed. Astronomer Nathan Smith from UC Berkeley has found two more stars that look quite a bit like 1987A. They are massive blue stars, surrounded by rings. One of them looks a lot like 87A's ring system, in fact. Here's an image of it:

OK, the image is cool and all, but you may ask yourself, so what?

Well, for one, SN1987A was a really oddball supernova. It blew up while it was a blue supergiant, when astronomers thought only red supergiants could blow up. This really shook the foundations of supernova physics, which is always pretty cool. And now we know that there are at least three other stars in the sky just like it!

I find this fascinating. How did we miss these all these years? For one thing, the stars are bright and the gas around them is faint. From the ground, the stars swamp the rings. Dr. Smith used the Spitzer Space Telescope, which can see faint objects more easily and distinguish faint and bright objects as well. What's funny to me is that one of the stars is pretty close to the Omega Nebula, a bright cloud of gas easily visible in binoculars on a summer night. Yet it had been missed all these years! Another is near the Eta Carina nebula, another extremely well-studied area of the sky. Yet people missed it.

These were truly the purloined supernovae.

I'll add there was some science involved with all this as well, but since Pamela already covered it, I'll leave you with that.

3) The Pillars of Destruction

So there was no new picture in the last section. Big deal. I'll make it up to you, and you will forgive me:

Check this image out:

Click it for the way-fab hi-res version.

What you're seeing there is almost certainly familiar to you, but shown in an unfamiliar way. This nebula is M16, also called the Eagle Nebula. If you look just below center and to the right, you'll see three green fingerlike objects poking into the pink part. Recognize them? They are the famous Pillars of Creation!

That's right, the most famous of all Hubble images. In this case, though, the new image is from Spitzer, the orbiting infrared telescope. The original Pillars image was taken using visible light, which shows gas as bright, and dust as dark, pretty much the way our eyes do. But the Spitzer image is infrared, so it shows warm dust as glowing!

What's interesting about the Spitzer image is the pink stuff. It's not really pink, of course, it's a wavelength of light our eyes can't detect, but Spitzer can. It shows dust inside the nebula glowing. The green shows cooler dust. Note how the green stuff looks like a cavity with the pink stuff filling it. That's not an illusion: that's really what's going on. Something has carved an enormous hole in the middle of the nebula. It seems logical to conclude that it's also responsible for filling the resulting cavity with hotter dust, or at least heating up dust that was previously there.

Here's where the fun is. Up until now, it's been thought that sitting in the center of the nebula are bright, massive stars still in their youth, and they are what formed the Pillars of Creation. Ultraviolet light floods out from the stars, as well as fast solar winds. When this hits a dense blob of gas and dust, it blows around them and forms these long fingers.

But there's a problem, and it's that hot dust. According to Nicholas Flagey, a PhD student at the Institut dAstrophysique Spatiale in France, the dust is way too hot to have been heated just by stars. There must be another cause. His idea: a star blew up in the middle of the nebula.

Wow! That's a pretty bold claim. But it does seem to fit a lot of the clues. A big blast could easily carve out that cavity, and the blast wave could certainly heat up the dust. Using various techniques, Flagey was able to estimate the explosion as happening between 1000 to 2000 years ago. He found some interesting clues in Chinese literature that might be possible eyewitness reports, but nothing firm. Amazingly, the dust is so thick in the nebula that it could hide such a colossal event from our eyes. Cas A was another explosion just a few thousand light years away from us (close for a supernova), and it went totally unnoticed because it's buried in thick dust. So there's precedent.

If this supernova idea is true, it means some interesting things. For one, the new stars forming in the Pillars of Creation were triggered by the supernova shock, and not stellar winds. But it also means that the pillars are not long for this (or any) world: the blast wave will eventually obliterate them. Flagey estimates that will take about a millennium. A thousand years from now, astronomers will no longer see the same structures in the Eagle as we do today.

But such is the Universe. Things change. Stars blow up. People get norovirus and then can't blog for a day or two. Both are important events, but I guess how important they seem depends on your point of view. I'd rate them about even right now.

OK, I hope all that verbiage makes up for my going missing for a day or two. There was a lot more going on here in Seattle, and I hope to have more to say tomorrow. But I do leave here, and there is still much to be done. Stay Tuned!

Previous AAS blog entries: AAS Report #1

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