Youngest galactic supernova (not aliens) found

By Phil Plait | May 14, 2008 11:00 am

If you’re wondering what all the buzz has been about the past few days over a NASA discovery, then wait no longer. No, it’s not aliens or an incoming asteroid. Instead, it’s still very cool: astronomers have found the youngest supernova in the Milky Way.

First, before I explain, here’s the photo of the newest galactic family member:

It kind of looks like a baby head swaddled in a blanket. Or a really bad drawing of Caesar. Anyway, seriously, this is a big deal. Why?

When a star like the Sun dies, it blows off a lot of its outer layers, leaving behind a dense hot object called a white dwarf (FYI, I have a more detailed description of all this here). If the star is binary — it has a companion — then the immense gravity of the white dwarf can draw material off its mate, and that matter will pile up on the surface of the dwarf. If enough piles up at just the right rate, it can ignite in a thermonuclear fire. This sets off a chain reaction, and the entire star self-destructs. This creates an immense amount of energy — as much energy is released every second as the Sun emits for billions of years — and an octillion tons of gas is launched violently into space at a large fraction of the speed of light.

The event is so titanic that it can be seen clear across the Universe, and of course you don’t want one to happen too close*. But somewhat close is good: we can study them better.

We know how many stars like this there are in our galaxy (as well as massive stars which can also explode, although using a different mechanism), and we know roughly how long they live, so we should be able to predict how often one should go off. The answer is, about three per century, more or less.

But observationally, it’s been more less than more. That is, the last one we know of that blew up in the galaxy was over 400 years ago. That’s been a major pain for astronomers; statistically speaking, it’s a little weird that we haven’t seen one since the 1600s.

But that’s changed. After searching for literally decades, astronomers have found a supernova in our galaxy! It’s official name is G1.9+0.3, which doesn’t exactly make your heart sing, I know. But it’s very cool. It’s a remnant, the expanding gaseous debris from a supernova blast. It’s located very near the center of the galaxy, about 28,000 light years away, and it’s only at most about 140 years old.

The false-color image above shows the remnant as seen by the orbiting Chandra X-Ray Observatory, and the ground-based Very Large (radio) Array in New Mexico. To give you a sense of scale, the object is about 13 light years across, or 80 trillion miles end-to-end. The orange crinkly stuff is extremely hot — millions of degrees hot — X-ray emitting gas, generated by vast magnetic fields in the gas. The bluer material is smoother radio wave emission also dominated by magnetic forces.

Together, they paint an interesting picture of this explosion. For one thing, it looks like a ring, or a smoke bubble. That’s a clear sign that it’s actually a shell of material, and not a solid sphere. A filled sphere of gas would be brightest in the middle and fainter near the edges (because we’re seeing more bright material when we look through the center of a sphere as opposed to near the edge), but a shell has the opposite behavior.

For another, it’s asymmetric: the gas is not expanding in a perfect sphere. Either it’s slamming into gas that existed outside the star before it blew up, or the explosion wasn’t perfectly spherical. That tells astronomers quite a bit about the physics of the explosion mechanism.

How do we know it’s young? Ah, an excellent question! I love this part: we’ve seen this sucker expand!

Here are two radio images of the remnant taken 23 years apart:

See how it’s gotten bigger over time? By measuring that expansion and knowing the time elapsed between the two pictures, we can extrapolate backwards to see how old the object is. If you do the math, all that gas was in one point about 140 +/- 30 years ago. That’s actually an upper limit to the age: it may have been less than that, if the expansion is slowing over time due to the material slamming into gas floating in space. That’s likely; that region of the galaxy is pretty thick with dust and gas.

In other words, this thing went off around the time of the American Civil War.

So that’s how we know it’s the youngest we’ve ever seen. But there’s more! We know the distance to the remnant as well. The amount of dust and gas between us and it can be measured and compared to known maps of the galaxy, kind of like knowing how far away distant mountains are by the amount of haziness you see between you and them. Combining the distance with the expansion measured means we can get a real velocity for the gas, and it’s a whopper: 14,000 kilometers per second, or 5% the speed of light! That’s fast. The amount of energy released in a supernova is numbing.

So you may also ask, why didn’t anyone see this thing when it went off? All things being equal, at that distance it should have been as bright as Venus in our skies, visible even in daylight! But all things are not equal: all the gas and dust between us and it absorb visible light, making this object almost totally invisible. It might have been visible to someone using a good telescope a century or more ago when the explosion took place, but that astronomer would have had to have been looking at just the right spot, and noticed a very faint star that wasn’t there a few weeks before — and this object sits in a part of the sky loaded with faint stars. It would be like noticing a new grain of sand on the beach. Unlikely, and in fact no one did notice.

It can be seen now because we have more advanced instruments these days. X-rays and radio waves are not as affected by intervening glop in the galaxy, and pass right on through. That’s why we can see it at all; even in big optical telescopes G1.9+0.3 is totally invisible.

So there you go. This object will be heavily studied now, I’m sure, because it’s the youngest such explosion we can see up close. It may help us understand how white dwarfs explode, and what the environment is like near the center of the galaxy, and how gas behaves when it violently expands in such a place.

And, well, it’s just cool. It’s been a mystery for a long time why we haven’t seen any young remnants — we expect there to be 60 of them younger than 2000 years, but only 10 are known — and now that we’ve seen this one we know they’re out there, but really just a pain to detect. You can bet that astronomers will look even harder for more of them now that we know they exist.

*Why not? you ask. Ah, because if it’s closer than about 20 or 30 light years it can destroy our ozone layer and do serious damage to us. I have more than you ever want to know about that in gory detail in my upcoming book, Death from the Skies!, which will be out in October.


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