Hi Everyone!
How close would a star (other than the very special case of our Sun), have to be to the Earth, to cause harm by going nova or supernova?
This guy: http://beyondeinstein.nasa.gov/press/2003/2003articles/030108_gro_ozone_toronto.pdf says 26 lightyears.
I think nova and supernova are different cases.
If a Supernova is a hundred times more powerful, for instance, it would be damaging at ten times the distance.
Tris
“Sic transit gloria mundi. And Tuesday’s usually worse.” ~ Robert A. Heinlein ~
Canopus is currently the largest closest star, tho farther away than once assumed (310 LY being the current estimate). Betelgeuse (430) and Antares (600) are also relatively close and may be better candidates for a supernova in the near future. Given proper motion and enough time they may move closer or farther away of course and new candidates might come into the picture.
Well, if our Sun had any chance of going supernova I’d be very very very worried… 
There’s a new theory that says some gamma ray bursts are a byproduct of supernovae, and there’s a theory that the Ordovician-Silurian extinction event was caused by a gamma ray burst. But the thing with gamma ray bursts, AIUI (IANAgamma ray astronomer), is that one of the poles of the star that goes supernova has to be pointing in your direction for the gamma ray burst to affect you. If that happens, and the star that created the gamma ray burst is within 500 light-years, it’s a problem.
So a supernova needs to be within 26 light-years of Earth to cause an extinction-level event, unless one of the star’s poles is pointed at us, in which case a supernova up to 500 light-years away could do it.
I recommend Charles Sheffield’s books Aftermath and Starfire for a fictional treatment of what happens to Earth after the supernovadom of Alpha Centauri, only 4.3 light years away. It’s a fun read, and Sheffield was a physicist, so it’s a fairly detailed story. (I’m not saying it’s completely or even mostly accurate, but there’s a lot of interesting stuff in there.)
Except for the problem that there’s pretty much no way in hell either of the primary stars in the Alpha Centuari system will ever become supernovas. α Cen A is a G2 V, only slightly more massive than Sol, and α Cen B is a K1 V, slightly less massive. (α Prox is a small M-type dwarf that orbits the main system and won’t ever nova much less supernova.) The only way you could get a supernova is if the metals-heavy α Cen B falls off the main sequence and the orbit degrades such that A and B crash into one another. This is about as likely as the Bush Administration admitting that it was a mistake to invade Iraq.
The closest possible supernova candidate is Altair, an A7 IV star, but it’ll be around a billion years before it starts to wander off main sequence and will probably just puff out into a red giant. The most realistic candidate is IK Pegasi A, but I think by the time it falls off the main sequence it will have flown on past and be even further than its current ~150 ly distance. I’m not sure where the 26 ly estimate for lethal distance comes from, but there are a lot of variables (including unknowns on how fast the Earth can replenish the ozone layer) so I wouldn’t place a lot of credence in that number, nor would I be particularly in panic about a supernova extinction event. I’d be much more concerned about being hit by one of the errant rocks flying in unmapped orbits in the Solar System, which is a threat we could actually do something about.
Stranger
Why, you lousy book-spoiling bastard!
That’s one of the points – they don’t understand how any of the Alpha Centauri stars could have gone supernova, and they’re worried that their understanding of astrophysics is majorly flawed. Eventually (particularly in the second book) there are very strange effects in addition to the gamma flux and other “expected” supernova effects, and they figure out that the supernova was not at all a natural event.
(To all: Don’t read that spoiler box if you’re at all interested in reading the two books mentioned.)
Altair can’t go supernova- it’s only 1.7 solar masses. A Main Sequence star needs to be somewhere around 7 to 9 times as massive as the Sun to go supernova.
There is another type of supernova, originating from a white dwarf that manages to accumulate a mass of more than the Chandrasekhar limit of 1.44 solar masses. That usually happens when the white dwarf is stealing material from a binary companion (in theory, it could happen from another star colliding with a white dwarf, but that’s extremely improbable).
There is a white dwarf very near us that will probably eventually become a Type Ia supernova- Sirius B, which is 8.6 light years away. Sirius B is massive for a white dwarf- 1 solar mass, while a typical white dwarf is more like 0.5-0.6. But it’s not stealing material from Sirius A now, and probably won’t until Sirius A becomes a red giant, in about a billion years.
Might be interesting if it did happen, though- I wonder how much difference it would make that much of the Northern Hemisphere would be shielded from the supernova by the fact that Alpha Centauri is too far south to ever be above the horizon there…
Also, within 20 light years, a gamma ray burst has a %90 chance of causing acute Hulkism.
Not per current theory, though I recall reading an article in some respected journal several years back regarding a theory that smaller mass stars with high metallicity might be create a new type of supernova without additional mass. I can’t find a cite to it, and clearly the hypothesis hasn’t been accepted into the mainstream, which is why I indicated that it was unlikely and will probably just expand into a red giant.
Is it likely to supernova even then? It was my understanding that the two members are far enough apart that accretion would be sufficiently gradual to cause B to flare or nova but not supernova as A moves off main sequence and becomes a giant.
Stranger
I read a magazine article once that said it would, but that was a long time ago and not from the most authoritative source.
So if Alpha Centauri did go supernova and send a gamma ray burst in our direction, those of us living in the Northern Hemisphere would be facing a horde of Incredible Hulks from further south… :eek:
I’ll be under the bed if you need me.
I’ll just say that in the book, the gamma ray burst from the supernova causes a gigantic … nay, catastrophic! … problem for everybody, worldwide.
Actually, a gamma ray burst is over in a matter of seconds, so only half the Earth would feel the immediate effects (the area to be spared would include a fair chunk of the southern hemisphere, too). The long-term global effects would be mostly from photoproduction of opaque nitrogen oxides in the atmosphere. These gasses would relatively quickly mix through a hemisphere, so what you’d get would be a somewhat darkened sky everywhere in the northern hemisphere, and a more severly darkened sky everywhere in the southern hemisphere.
All right, I’ll post the gamma-ray burst results from the book here, but don’t read the spoiler if you care about the books.
In Aftermath, the gamma ray burst ionizes the upper atmosphere and causes a huge EMP that destroys all the electronic equipment in the world except a few military machines and university computers and whatnot that were specifically shielded.
I don’t have the book handy to go into further details, so I have no idea how plausible that is.
Betelgeuse is less than 500 light years away and it appears to be heading towards a supernova in the next few centuries. Closer than you’d ideally want a supernova to be but probably outside the danger zone. But it’s likely to be real noticable when it happens.
And anybody who wants a good story involving Betelgeuse should read Robert J. Sawyer’s Calculating God!
What’s that? Too many book recommendations in a GQ thread, you say? OK, OK…