Tiger, tiger, burning bright/In the forests of the night

August 22, 2005 3:30 AM

Surviving a supernova...

• What would happen if superheated gas from a supernova hit our solar system?
• Would we know if a supernova explosion happened near enough for us to know it is coming?
• How far away would a supernova need to be for us to survive its effects?

posted by Rothko to science & nature (11 comments total)

I have no idea how widely accepted it is but there is a theory that a supernova 120 light years away from earth destroyed the ozone layer and caused a die off of marine life.
posted by rdr at 4:53 AM on August 22, 2005

Apparently, 30 light years is the bare minimum before mass extinction. Thanks, rdr.
posted by Rothko at 5:03 AM on August 22, 2005

BTW, we would see the light from the supernova before the gas hit us, but the interval depends on how far away the supernova is and what happens to be between us and it.
posted by bshort at 5:13 AM on August 22, 2005

Apparently our sun is not large enough to supernova!

All is not lost though as the luminosity change in the next billions years will be more than enough to do us in.
posted by Wong Fei-hung at 7:30 AM on August 22, 2005

Let's try that again:

Apparently our sun is not large enough to supernova!

Link

All is not lost though as the luminosity change in the next billions years will be more than enough to do us in.
posted by Wong Fei-hung at 7:31 AM on August 22, 2005

Would we know? Yes.

Why? The neutrio flux. Back in 1987, three large neutrino detectors picked up an abnormally high number of neutrino detections. Note, by "very high", we are talking single digits, but it was a very significant burst, happening in less than 13 seconds.

What had happened? A "nearby" supernova, Supernova 1987A. But, by nearby, we really mean "In a nearby galaxy" -- SN1987A happned in the Large Magellanic Cloud. In terms of linear distance, 50 kiloparsecs, or 164,000 light years away.

If a supernova occured with 100LY of the earth, the neutrino flux would be detected hours before any visual change. The fact that neutrino can sail through most mass [1] means that most of the neutrino flux from the core collapse escapes instantly, while the other effects take some time to work through the enourmous mass of the star.

The neutrino flux generated in a Type II Supernova (a collapsing star) is immense. Most of the neutrinos generated by the core collapse leave the star in a straight line. Despite this, the energy transfered to the outer layers of the star by the neutrino flux is enough to blow the star apart -- and the entire neutrino flux, 10^46 joules, is generated in 10 seconds.

The energy is so large that they've coined a unit to deal with it, the Foe. One Foe=10^51 ergs, thus, the name, Fifty One Ergs. The core collapse generates 100 Foe, about one of which is captured by the star (and which destroys the star.) The other 99 Foe escape as neutrinos (well, a very very smal amount escapes as other radiation and physical mass, but to three significant digits, this is 0.)

To give you an idea of the scale -- the entire output of our sun, from fusion ignition to today, is about .03 Foe, and the total output, from ignition to white dwarf, will be about .1 Foe.

The neutrino flux from a close supernova would be very very noticible -- we'd see hundred of neutrinos, all grouped in a short burst. At first, I'm sure the operators will wonder what's wrong with the detector....

[1] Look at the sun. Every second, billions of neutrinos shoot through your eyes. Now, wait 12 hours. Look at the ground, in the direction of the Sun, now hidden from you by the mass of the planet. Every second, billions of neutrions will be shooting through your eyes. The number of neutrinos stopped by the mass of the Earth will be in the single digits per second. The usual measure of sheilding is the amount of a substance needed to block 1/2 the incoming particles. For high energy particles, the gold standard (ahem) is lead.

It would take a wall of lead almost one lightyear thick to stop half of the solar neutrion flux.
posted by eriko at 8:55 AM on August 22, 2005 [11 favorites]

Eriko, that was awesome!
posted by furtive at 10:09 AM on August 22, 2005

So, basically, once we get independent verification of about 15 neutrinos a minute for about 10 minutes, we know we're screwed?

I say break out the bubbly and listen to some Oasis.

BTW, I nominate that as official song of the extinction-by-supernova.
posted by chota at 2:56 PM on August 22, 2005

If a supernova occured with 100LY of the earth, the neutrino flux would be detected hours before any visual change.

How would the neutrons travel faster than light? Or is this hypethetical supernova one that occurred 100 years ago?
posted by sleslie at 5:11 PM on August 22, 2005

Neutrinos, not neutrons, which are a different beast (and stopped by inches, not lightyears, of lead.)

Neutrinos, now apparently having mass [1], aren't supraliminal. The reason they reach first is simple. Light and other particles have to fight through the star to become visible. Neutrinos, with their incredibly low mass interaction rate, don't.

When the core collapses, there's a whole bunch of star heading back into the core. Before, the tremendous gravity was held back by the tremendous energy of the fusion reactions in the core -- Hydrogen to Helium, then Helium to Carbon, Carbon and Helium fuse into Oxygen, then Oxygen to Neon, Neon into Magnesium, Magnesium into Silicon and, finally Silicon into Iron. All these fusion reactions release energy. But Iron is the end of the line -- each one releases less energy, and Iron doesn't release any. It doesn't matter if you fuse it, or break it apart -- either reaction takes energy to accomplish, and one thing that's in plentiful supply in the core of a star is energy.

So, the last dregs of silicon fusion at the very core start to fail, and instead of iron fusing into something else, it starts to fission, into, IRRC, 14 helium and some neutrons. But this reaction just sucks in more energy. The trigger point happens when the iron fission eats enough energy that the silicon fusion can't create enough energy to hold back the outer layers of the star.

Blink. Core goes "out" -- in our terms, it's still generating gobs of energy, but to a star, needing tremendous energy to hold itself up, the core just turns off.

Gravity wins, and the star starts rushing in. Eventually, though, it all meets in the middle, and the core turns into a neutron star, and the shockwave blows the star apart. It's suspected that extremely massive stars would "hypernova" into a black hole, but that's theory (This is one possible candidate for a gamma ray burst.)

The hard part to grasp is not only how much energy is released by this, but how quickly it all happens. In 15 seconds, the main event is over. The iron core is now a neutron star, a good part of the star is heading into that neutron star, and the rest of it is heading outwards. 100 Foe=1000 times the total output of our sun, over the entire 5 billion year lifetime, released in 10 or so seconds. Boom.

All that infalling matter presents quite the traffic jam, though, and stars large enough to Supernova are very large indeed. Despite the fact that the energy release takes such a short time, it takes hours for most of that energy to manifest to the outer limits of the star.

Or, to put in another way, Neutrinos don't travel faster than light. They just beat the traffic jam.

I and some friends have wondered, but don't have the numbers, but if you were *real* close to the star when it blew, what would happen when the Neutrino flux hit you?

A supernova within 100ly would saturate every neutrino detector on the planet -- 10 in 15 seconds wouldn't be enough. That's SN1987A levels, and we'd just be stoked. Given that we don't see "nearby" supernovas that often, and that even detecting neutrinos is hard, we don't have a very good calibrated scale. At the time, it took some hours for everyone to piece together the fact that the neutrions had come from SN1987A. Now, however, a sudden spike of detections causes a few quick phonecalls to other detectors, and if they saw the same thing, a quick heads up goes out to the astronomical community. I think the collaboration is called SNEWS (google...yes, the SuperNova Early Warning System.)

Wow. Talk about an info dump. Well, it was that or play Nethack. (This is known as "Geek Answer Syndrome.")

[1] ObJoke: Neutrinos have mass? I didn't even know they were Catholic. Of course, at least one of the Vatican Astronomers[2] has that on a T-shirt.

[2] Yes, the Vatican has astronomers. The one I've met is one hell of a Guy.
posted by eriko at 6:39 PM on August 22, 2005 [14 favorites]

Wild answer eriko!
posted by bystander at 3:54 AM on August 23, 2005