A Supernova Could Nuke Us
Edward M. Sion is a Professor of Astronomy and Astrophysics at Villanova University. He received a B.A. in Astronomy from the University of Kansas in 1968, an M.A. in Astronomy from the University of Kansas in 1969, and a PhD in Astronomy from the University of Pennsylvania in 1975. His primary research interests include the formation and evolution of white dwarf stars, the physics and evolution of cataclysmic variable stars, and theoretical studies of accretion physics.
Question: What is a Type 1A supernova?
Ed Sion: A Type 1A Supernova is thought to be a white dwarf that undergoes total thermonuclear detonation. It explodes and completely obliterates itself leaving no remnant. For example, we have yet to detect a neutron star or a black hole remnant of a Type 1A supernova explosion. It appears that the stellar explosion complete destroys the star. This explosion is extremely energetic. The amount of energy release is approximately 10 to the 51 – 10 to the 52 ergs of energy. In other words, a type 1A supernova can outshine the galaxy it’s in. The entire galaxy for a short time. And so, they are extremely energetic.
Question: What properties of the T Pyxidis binary system make it a likely candidate for going supernova?
Ed Sion: Yes. T Pyxidis is a recurrent nova, not a supernova, but a classical nova, but it recurs. Most classical novae we see only once. T Pyxidis has a classical nova explosion every roughly 20 years. And this continued from the early 1890s until 1967. So, there were five nova explosions interspersed every roughly 20 years up until the 1967 explosion. Since 1967, this 20 year cycle has disappeared. Nothing has happened yet. In other words, it’s 44 years overdue for the next thermonuclear explosion.
What happens in a recurrent nova, and the reason I distinguish it from the supernova is the recurrent nova is a white dwarf. This is a star about the size of Earth, but it has an extremely high density. The density is a hundred million grams in a cubic centimeter. Except for a massive white dwarf, a hundred million grams in a cubic centimeter, which means a thimbleful of this material, would be hundreds of tons if we weighed it here on earth.
What happens is, the white dwarf is so dense that the electrons surrounding the nuclei have all been stripped away from the nuclei and the electrons are actually forced to forma separate gas. We call it a generate electron gas. What happens is, when you squeeze matter tighter and tighter, there’s a principle in physics called the Pauli Exclusion Principle that states that no more than two electrons with opposite spins can occupy the same energy state. So, what happens is, when you squeeze matter tighter and tighter to higher density, there are fewer and fewer energy states available for the electrons to occupy because all the lowest lying energy states are filled first. So the electron is forced to move very, very fast and it can’t slow down. It can’t de-excite because the Pauli Principle prevents it. And so what happens is, the electrons move extremely fast and as the density goes up, they move faster and faster and that exerts pressure. Well, it’s the pressure of these degeneral electrons, the degeneral electron gas, that prevents gravity from pulling this stuff in. But what happens is that as the white dwarf mass increases, there’s an ultimate mass limit called the Chandrasekhar Limit beyond which a white dwarf cannot exist. Because at the Chandrasekhar Limit the degenerate electron gas pressure can no longer withstand the pull of gravity. It can no longer balance gravity and prevent the collapse of the white dwarf.
Type 1A supernovae occur very close to the Chandrasekhar Limit. When the white dwarf mass is very close to the Chandrasekhar Limit, T Pyxidis has a white dwarf which is very close to the Chandrasekhar Limit. Its presently determined mass, fairly reliably well-determined, is 1.37 solar masses. The Chandrasekhar Limit for carbon oxygen white dwarf is 1.44 solar masses. So, it doesn’t have much more material to accumulate until it reaches the Chandrasekhar Limit in which case you would have instantaneous collapse. But just before that happens, as the white dwarf grows in mass, the compressional heating, the weight of the material from the neighboring star from its companion presses down and raises the temperature high enough to detonate carbon. That is to cause the carbon nuclei inside the white dwarf to fuse together, releasing energy.
And so, that is what we call a detonation and that’s, the T Pyxidis we believe is very close to that point where a little bit more mass accreted onto the white dwarf will lead to so much heating that the temperature will rise to a few billion degrees detonating carbon. But the carbon, when it ignites, through fusion it ignites, it’s explosive because this weird gas, this degenerate electron gas, unlike an ideal gas like what the sun is made, or the air we breathe where it heat it and it expands, the degenerate electron gas has no sensitivity to temperature, so it doesn’t know it’s being heated. In other words, the heat builds up, the nuclear reactions occur faster and faster and faster, but there’s no compensating expansion to cool off. And so it’s like a time bomb. Energy builds up very rapidly and you get a tremendous thermonuclear explosion. That’s a type 1A supernova.
That is, a class – a classical nova takes place when the accreted hydrogen burns. But it burns in a degenerate region and you get a classical nova explosion. That’s about 10 to the 45 ergs of energy. It’s a million times less energetic than a supernova. The supernova I’m talking about is the carbon nuclei, not the hydrogen, accreted hydrogen, but the carbon nuclei detonate and fuse together and that’s a type 1A supernova. That will happen to T Pyxidis when it reaches the – close to the Chandrasekhar Limit.
Question: How soon do you predict that this could happen?
Ed Sion: At the present rate of accretion that our modeling indicates for T Pyxidis, it will take another 10 million years, roughly 10 millions years to reach 1.4 solar masses. If the present mass of the white dwarf is 1.37 solar masses and if the accretion rate that we’ve estimated from our accretion disk model fitting, from theoretical models of accretion disk, there’s a pancake of matter surrounding the white dwarf. A pancake-shaped disk of gas we call an accretion disk. And that accretion disk is adding material to the white dwarf. But at the rate at which it is adding material to the white dwarf, it’s going to be another 10 million years, roughly before the Chandrasekhar Limit is reached.
Question: What would happen to Earth if a nearby star went supernova?
Ed Sion: Yes. What will happen is that as the interior – the core of the white dwarf becomes hotter and hotter due to the compression and the temperature gets up into a few billion degrees, the ignition temperature of carbon will be reached. That is, carbon will be able to undergo carbon on carbon fusion with the release of energy. This will start out, according to supernova models that have been carried out in the last few years; this will start out as what we call a deathlagration. A burning front will propagate, will move outward at subsonic speeds, but as this burning front moves out, very soon it will turn into a supersonic burning front. You’ll have a breakout of the shock wave, or blast wave from this thermonuclear explosion. That breakout will, should – will produce a burst of gamma rays and hard x-rays – very high energy radiation for a few seconds. This radiation, if the supernova is close enough. This radiation could then affect earth. In other words, you would have input of hard x-rays and gamma rays into our atmosphere. This could introduce chemical reactions producing nitrous oxides which could then, eventually destroy the ozone layer. That’s the first thing you think about is if the ozone layer is destroyed, then very high energy radiation is very lethal to DNA, it would destroy the biosphere. But the supernova has to be close enough.
Now, Type 1A supernovae, they’re more common than the Type 2. The Type 2 supernova, when the ordinary person thinks of a Type 2 supernova, when a public thinks of a normal supernova, they think of a huge massive star that collapses in on itself and produces a black hole or a pulsar at the center and then this high velocity expanding gas. That type of supernova called a Type 2 Supernova comes from very massive stars that are very luminous before they went supernova. They can be seen at great, great distances. What makes a Type 1A supernova really unusual in that regard is that a white dwarf is very dim. And even white dwarfs in close binaries where mass transfers are going on, they’re really very faint. You don’t see them out to very long distances, and they are a more common type of star. And therefore, since they are more common and fainter, they really pose some reason to be concerned because you don’t see them as easily as more massive stars that are much more luminous. So, I think the main thing that one might be concerned about is the input of high radiation into our atmosphere. But the supernova – if you go on the basis of Type 2 Supernovae, then the current estimates are that if the Type 2 Supernova is within roughly 30 light-years, 30 or 40 light years, or closer, then you’d really have massive input of high radiation. But that’s for Type 2 supernovae.
My collaborator, Dr. Patrick Godon at Villanova, just did a quick back of the envelope calculation and determined that if you had, within a thousand parsecs, a Type 1A supernova go off, and if it was a low tilt such as at the accretion disk and material didn’t block the breakout of the blast, you would have a burst of gamma rays that would essentially be as bright as the sun and the estimate by Peter **** is that 10 to the 48, or 10 to the 50 ergs per second of hard x-rays and gamma rays would be emitted. And we don’t know exactly how far away – we know that it’s closer than a thousand parsecs, but how much closer. Our best estimates right now are – the models allow it to be even as close as 500 parsecs. But that may be too far for it to do real damage to Earth, but we’re still working on this and we’re submitting a paper to the Astrophysical Journal Letters on this topic.
Recorded on January 20, 2010
Interviewed by Austin Allen
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