Just to add to the supernova database in the thread: there’s also Eta Carinae, a hypergiant star of 100+ solar masses about 7500 LY away. Eta Car actually had a failed supernova event in 1730. If it finally blows, which could be any time from 1 minute from now to a zillion years, it will likely be visible in the daytime on Earth. If one of its poles were pointed toward us, pretty bad stuff would likely happen. Luckily, the poles are not pointed our way, so all we expect is a nice visible bang-bang.
But we’re more excited about Betelgeuse, because Eta Carinae is in the far southern sky. A supernova there wouldn’t be visible to most observers in the northern hemisphere, no matter how bright it is. Betelgeuse is near the celestial equator, so if and when it goes supernova, it will be visible throughout most of the world.
:eek:
I wonder what colour it would be…
I asked Mr. Neville this very same question when he got his name put in as one of the authors on a paper about gamma-ray bursts (although he doesn’t work on GRBs). He didn’t know. I would still like very much to find out (though preferably not by seeing an actual dangerous gamma-ray burst go off).
Where’s your motivation? I would certainly take a trip to SA to see a hypernova!
A GRB would have to be very close indeed to blow the atmosphere away, and short of that, it’s not going to wipe out life on Earth (or probably even human life). Although GRBs are very energetic, they’re also very short-duration: One that lasts ten seconds would be considered very long duration indeed. So the Earth isn’t going to rotate appreciably under the burst, and so only half of the planet would be flash-sterilized (a few thousand kilometers of rock makes one heck of a radiation shield).
However, what it could do is photodissociate the oxygen and nitrogen in the atmosphere, turning it (for a while, at least) into a soup of opaque brownish NO[sub]x[/sub] compounds. Exactly how much impact this would have would depend on the latitude of the burst: Air doesn’t easily cross the equator, so a burst at high latitude in one hemisphere wouldn’t have much effect on the other, but a burst near the equator would effect both. This opacity of the atmosphere would result in a “nuclear winter” effect over the affected hemisphere(s). Oh, and the whole planet would smell like Los Angeles, so maybe we’d be glad to go extinct.
And while neutrinos do have mass, they’re miniscule at best (most experimental upper bounds are in the vicinity of one or two eV, compared to half a million eV for the next lightest known particle, the electron). And there is no known lower bound for the mass of the lightest neutrino. So while they don’t quite travel at the speed of light, they come awfully darned close. They might even get closer to the speed of light than light does itself: The interstellar medium is not quite a perfect vacuum, and therefore has a very slight index of refraction, and so it’s possible that neutrinos might travel through the ISM faster than light does.
Regardless, though, the neutrinos from a supernova explosion are nothing to worry about. Even the neutrino flux at the surface of the star wouldn’t be a problem, with how weakly-interacting neutrinos are (though a lot of other things would, of course, be a problem there). The same is also true, but even more so, of the gravitational waves produced in a supernova (which travel at exactly c).
What about using Cephid-type variable stars as a means of more accurate measurement? A recent NASA PotD did a piece on RS Pup and explained its usefulness in improving the accuracy of measuring stellar distances:
From here, plus a page on Cephid-type variable stars.
I’d want to, too. But if it happened to a star that was visible from the northern hemisphere, it would be like having a bright comet around- everybody gets excited about it. That’s a lot of fun.
Plus, if the supernova were a star that is only visible from the southern hemisphere, it wouldn’t quite be the same- it wouldn’t be a change in the stars I’ve known all my life.
The problem with that is that a lot of the stars we want to know distances to are not Cepheid variables.
The Mega Disasters show about gamma ray bursts depicted this.
It might smell even worse than that, if whoever they consulted for Mega Disasters is right. On the Mega Disasters show about a gamma ray burst, they said it might cause red tides, which would add a lovely rotting fish element to the smog smell.
Anne (or whoever cares to respond):
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I think Mindfield’s point is that stars in association with Cepheids could be reliably distanced by that means – just as is done with galaxies containing them. I suppose the problem is that open clusters containing Cepheids are still relatively rare.
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Would someone kindly explain in simple terms what provokes the different kinds of supernova? I understand about the iron crisis at the cores of irregular variables; I do not understand the mechanism for a white dwarf going supernova. And what’s the other type – if they’re Type Ia and II respectively, what’s Type Ib?
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SN 1987a was, so I’m told, odd because it happened to a blue giant rather than a red giant – and IIRC the former is the term for O and B main sequence stars, which supposedly aren’t ready to go supernova yet. Anyone want to sort my confusion here, and explain what we actually learned from 1987a?
White dwarfs go supernova because you can’t have two electrons with the same spin in the same state.
After a low-mass red giant throws off its outer layers, there’s a core of material left over. That core isn’t going to be able to generate energy by fusion, so it’s going to collapse.
In a normal star, the size of the star is determined by a balance between gravity, which wants to pull all the material into the center of the star, and thermal pressure, which wants to make the star expand. The heat for the thermal pressure is, of course, generated by fusion. Normal stars stay in equilibrium because the rate of fusion goes up rapidly when gravity squeezes the star inward, raising the temperature and making the star expand. This keeps the thermal pressure and gravity in equilibrium.
The upshot of this is that, if you don’t have fusion going on, a star is going to collapse inward until something else stops it. To form a white dwarf, the leftover core of a star collapses inward until it is so dense that all its available electron states are occupied.
Matter in a state like this is said to be supported by electron degeneracy pressure. In addition to being very dense (a white dwarf about the same mass as the Sun is about the size of Earth), it’s also rather weird- if you add mass to a white dwarf, its radius will actually decrease (the radius is inversely proportional to the cube root of the mass). There’s a limiting mass at which point the radius would have to shrink to zero, called the Chandrasekhar limit (probably spelled wrong- this and Schwarzschild are some of the hardest names in astronomy to spell…). The limiting mass is about 1.4 solar masses, assuming the white dwarf is not rotating (the limiting mass is higher if it is).
The current model for Type Ia supernovae says that white dwarfs never actually reach this limiting mass. Instead, they get massive enough and hot enough that they manage to restart fusion, this time of the carbon that was formed as a result of helium fusion during the red giant phase. But there’s a difference here- a red giant or Main Sequence star is supported by thermal pressure, which is (obviously) dependent on temperature. If fusion tries to go faster in one of those stars, the thermal pressure pushes the material further apart, which slows down the fusion. A white dwarf is supported by electron degeneracy pressure, which is independent of temperature. When the fusion speeds up, the white dwarf doesn’t get bigger. You can get runaway fusion here, which goes until the white dwarf is so hot it blows itself up.
Supernovae are classed as Type I or II based on whether they have significant hydrogen lines in their spectra. There are also Type Ib and Ic supernovae, which differ in some other details of their spectra. We think they come from massive stars, and differ from Type II supernovae in that the massive stars have thrown off most of their hydrogen before the supernova. This happens either because the progenitor stars of Type Ib/c supernovae have stronger stellar winds than those of Type II, or because they have a nearby companion that has sucked away a lot of their hydrogen.
The fact that SN 1987a came from a blue giant and not a red giant was (and remains) puzzling and goes against our understanding of what causes supernovae. The current theory is that the progenitor of this supernova was an unstable blue star created by the merger of a binary pair of stars about 20,000 years before the supernova.
One thing we learned is that this was the first supernova from which we detected neutrinos. This gives some support to supernova models where neutrinos play a major role.
We learned that the neutrinos are given off in a supernova before the light is, since the neutrinos were detected before we saw the supernova.
We actually detected both neutrinos and antineutrinos from SN 1987a. They both arrived at the same time, which tells us that antimatter and matter interact with gravity in the same way.