What's the lethal radius of a supernova?

[halfeye]

I've read 1000 lightyears, but that's ridiculous, I think that might be missing "cubic", which would take it to about 6 light years.

[factotum]

In the Wikipedia article on near-earth supernovae it says that a supernova within 1000 ly would have a noticeable effect on the biosphere, but it doesn't go on to clarify what that would be--the article mostly talks about ozone layer damage from a supernova within 30ly or so.

[Kato]

I've read 1000 lightyears, but that's ridiculous, I think that might be missing "cubic", which would take it to about 6 light years.

I'm not sure.. I mean, I've read the neutrino emission could kill you up close. But I neither have a precise idea of the energy emitted or how durable a human is, not considering how it probably varies between super novae of different types.

My best rough estimate.. Kepler's super nova, 20k light years away, could be seen with the naked idea. I'm not saying it's likely, but I'd say it's possible at 1k light years to still be pretty damaging.

[Lord Torath]

I'd recommend Phil Plait's book, Death from the Skies. I've borrowed it from the library, and he talks about all the various ways the universe can kill us, including gamma ray bursts and rogue black holes. I'm pretty sure there was a chapter on supernovas, but I don't recall how close we'd need to be for it to kill us all. I do recall him assuring readers that there are currently no supernova candidates close enough to cause a problem for Earth.

His Crash Course Astronomy (https://thecrashcourse.com/courses/astronomy) video on High Mass Stars (https://www.youtube.com/watch?v=PWx9DurgPn8) might also contain the answer.

Good luck!

Edit: I thought Randall Munroe's "What If" on Lethal Neutrinos (https://what-if.xkcd.com/73/) might have an answer, but it only tells how close you need to be for the neutrinos to kill you (assuming you magically survived everything else about a supernova that could kill you).

[hamishspence]

I'm not sure.. I mean, I've read the neutrino emission could kill you up close. But I neither have a precise idea of the energy emitted or how durable a human is, not considering how it probably varies between super novae of different types.

My best rough estimate.. Kepler's super nova, 20k light years away, could be seen with the naked idea. I'm not saying it's likely, but I'd say it's possible at 1k light years to still be pretty damaging.

Kepler's appears to be rather weak. At 1/20 the original distance (1000 light years) it would be 400x as bright. 6 and a bit magnitudes. So it would go from -2.5 to a little bit over -8.5 (but a long way short of -9.5. Maybe magnitude -9 at most)

But the 1006 supernova:


https://en.wikipedia.org/wiki/SN_1006

was already magnitude -7.5 at 7200 light years away. At 1/7.2 x the original distance it would be nearly 52x as bright (4 and a bit magnitudes), So it would go from -7.5 to a little over magnitude -11.5 (maybe magnitude -12) at 1000 light years. Still not as bright as the full moon, but close. And on the order of 100x as bright as Kepler's Supernova would be at the same distance.

[halfeye]

Hm,


It is estimated that a Type II supernova closer than eight parsecs (26 light-years) would destroy more than half of the Earth's ozone layer.[5

https://en.wikipedia.org/wiki/Near-Earth_supernova

That's less damage than I was thinking of, at a greater range, which is probably expected.

If there were jets I'd want to discount them in the first analysis, because they are so unlikely to impact any particular planet.

I'd expect different types of supernovae to have different lethal radii, that seems reasonable to me.

I do remember that xkcd "what if" that mentioned a supernova at one AU being brighter than an Hbomb on your eyeball, but what we don't visualise very well is just how huge a light-year is.


1 light-year = 9460730472580800 metres (exactly)
≈ 9.461 petametres
≈ 9.461 trillion kilometres
≈ 5.878625 trillion miles
≈ 63241.077 astronomical units
≈ 0.306601 parsecs

https://en.wikipedia.org/wiki/Light-year

[wumpus]

Edit: I thought Randall Munroe's "What If" on Lethal Neutrinos (https://what-if.xkcd.com/73/) might have an answer, but it only tells how close you need to be for the neutrinos to kill you (assuming you magically survived everything else about a supernova that could kill you).

I think the takeaway from "what if 73" is that no matter how far away from a supernova you think you can be and still survive, you need to be *much* further away than that.

Don't forget the "heavy metal* bombardment". Just about everything heavier than about carbon comes from supernovas, so expect it to be pelting out the complete periodic table. I'd also expect that the proportions we see on Earth are after billions of years of decay of said production: expect nuclei being emitted in a random hodge-podge of protons and neutrons and decaying (in your face) as they keep decaying until they land on something more or less stable.

* metal (to an astronomer) means "anything not hydrogen or helium". But in this case probably means "things past iron that typically release energy when they fall apart".

[BritishBill]

I dunno what it is, but it probably sucks being in one lol.

[veti]

I do remember that xkcd "what if" that mentioned a supernova at one AU being brighter than an Hbomb on your eyeball, but what we don't visualise very well is just how huge a light-year is.

You missed the "nine orders of magnitude" in that difference, there. Which suggests that at one light year, a supernova would still be about as bright as an H bomb a few inches from your face. Which still sounds pretty bad to me.

[hamishspence]

You missed the "nine orders of magnitude" in that difference, there. Which suggests that at one light year, a supernova would still be about as bright as an H bomb a few inches from your face. Which still sounds pretty bad to me.Actually, using Kepler's Supernova as a basis, it would go from -2.5 to -24 or so, in the process of changing it from 20000 light years away to 1 light year away (400 million times as bright, and 15 magnitudes is 1 million times as bright).

For comparison, the Sun is -26.7.

So, it still wouldn't be as bright as the Sun.

[factotum]

So, it still wouldn't be as bright as the Sun.

It would be an academic difference, though--apparent magnitude of -24 would still make it by far the brightest object in the sky after the Sun (the full moon only gets to magnitude -12.6). It might even be bright enough to cause visual damage if you looked at it directly, although that would be the least of your problems when the shockwave from the explosion arrived...

[Lvl 2 Expert]

It might even be bright enough to cause visual damage if you looked at it directly, although that would be the least of your problems when the shockwave from the explosion arrived...

A shockwave carried pretty much only by the stuff itself propels. I'm sure it's a huge boom, but that would probably limit the radius quite a bit.

Isn't it the gamma ray bursts that are the real killers? (For those supernova's that even eject them.) They're directional, so it can't really be captured in a radius, but they travel at light speed, they're barely affected by traveling through empty space and they are especially harsh on complex organic molecules.

[hamishspence]

A shockwave carried pretty much only by the stuff itself propels. I'm sure it's a huge boom, but that would probably limit the radius quite a bit.

Isn't it the gamma ray bursts that are the real killers? (For those supernova's that even eject them.) They're directional, so it can't really be captured in a radius, but they travel at light speed, they're barely affected by traveling through empty space and they are especially harsh on complex organic molecules.

I'd be interested in knowing the gamma ray exposure at 1 light year. If the exposure at 30-odd light years is "potentially enough to strip the planet's ozone layer" then it's going to be a lot, and at 1 light year it'll be much worse.

Still, "just outside our solar system" might be the minimum needed for enough of the gamma rays to pass through the atmosphere, that everyone on the exposed hemisphere dies of radiation sickness:

https://www.quora.com/Could-anything-on-Earth-protect-a-human-from-a-supernova-hypernova-or-gamma-ray-burst

3% of the speed of light is typical for the shockwave - so the wave itself would take some 30 years to reach 1 light year away from the supernova progenitor.


It would be an academic difference, though.

True, but "almost as bright as the Sun" is a long way from "as bright as an H-bomb a few inches away".

[halfeye]

Actually, using Kepler's Supernova as a basis, it would go from -2.5 to -24 or so, in the process of changing it from 20000 light years away to 1 light year away (400 million times as bright, and 15 magnitudes is 1 million times as bright).

An order of magnitude is a power of ten (and is usually only used where more exact details are irrelevant), so six orders of magnitude is one million times.


You missed the "nine orders of magnitude" in that difference, there. Which suggests that at one light year, a supernova would still be about as bright as an H bomb a few inches from your face. Which still sounds pretty bad to me.

You really have to do the maths, or at least have a very good understanding of it, "suggests" without at least an approximation is inadequate. I'm not saying that one lightyear would be safe. I wouldn't be at all surprised if six lightyears was still unsafe, but I am very sure that 10^3 lightyears is overdoing the safety margin by a huge distance.

[hamishspence]

An order of magnitude is a power of ten (and is usually only used where more exact details are irrelevant), so six orders of magnitude is one million times.

i'm talking about the astronomical magnitude system of brightness.

In that, a magnitude -2 object is 1/100 the brightness of a magnitude -7 object, which is 1/100 the brightness of a magnitude -12 object, which is 1/100 the brightness of a magtitude -17 object.

"15 magnitudes of difference" corresponds to "1 million times as bright" in that system.

[halfeye]

i'm talking about the astronomical magnitude system of brightness.

In that, a magnitude -2 object is 1/100 the brightness of a magnitude -7 object, which is 1/100 the brightness of a magnitude -12 object, which is 1/100 the brightness of a magtitude -17 object.

"15 magnitudes of difference" corresponds to "1 million times as bright" in that system.

Well the "what if" article referred to "orders of magnitude", not "magnitudes of difference", so if that's where you picked magnitudes up from, you were mistaken. Otherwise, I'm very dubious that we'd be safe from a supernova at one light year.

I still think that a light-year being about 64k AU is pretty significant for the rate of diminution of the effects of a supernova, but supernovae are still vastly energetic in light and particle emission. So I think I'd want to be tens of light-years away from one if I had any choice in the matter at all.

[hamishspence]

Their "A supernova at 1 AU is nine orders of magnitude (1 billion times) brighter than a nuclear bomb mm away from eyeball" is something I'd want checked.

1 AU is 1.5 x 10^-5 x 1 light year.

1 light year is 63241 AU.

An object would be 63241x63241 (4 billion, roughly) times as bright at 1 AU as it is at one light year.

Kepler's Supernova at 1 light year would be a bit less than the Sun's brightness. Kepler's Supernova at 1 AU would be on the order of 1 billion times the Sun's brightness.

For comparison, the average type 1a supernova is 5 billion times the Sun's brightness.

https://en.wikipedia.org/wiki/Type_Ia_supernova


The typical visual absolute magnitude of Type Ia supernovae is Mv = −19.3 (about 5 billion times brighter than the Sun), with little variation.

How bright is an atomic bomb, really, at "eyeball distance"? Only as bright as the Sun? Not a chance.

This thread:

https://10minuteastronomy.wordpress.com/2013/11/28/a-hydrogen-bomb-detonated-against-your-eyeball/

discusses the "1 billion times brighter than bomb against eyeball" claim, and concludes there's an error somewhere.

[Lvl 2 Expert]

I'd be interested in knowing the gamma ray exposure at 1 light year. If the exposure at 30-odd light years is "potentially enough to strip the planet's ozone layer" then it's going to be a lot, and at 1 light year it'll be much worse.

Still, "just outside our solar system" might be the minimum needed for enough of the gamma rays to pass through the atmosphere, that everyone on the exposed hemisphere dies of radiation sickness:

https://www.quora.com/Could-anything-on-Earth-protect-a-human-from-a-supernova-hypernova-or-gamma-ray-burst

I find that article fairly optimistic though, even though apparently the author did his doctorate work on the subject. If there is enough radiation to kill everyone on the exposed side of the planet directly I would expect lots of fallout, toxic skies and some flavor of nuclear winter preventing the other half from being "just fine". (Possibly an oxygen imbalance affecting sea life etc.) It feels a bit like saying duck and cover is a great way to defend against nuclear weapons with no drawbacks whatsoever.

[hamishspence]

If there is enough radiation to kill everyone on the exposed side of the planet directly I would expect lots of fallout, toxic skies and some flavor of nuclear winter preventing the other half from being "just fine".



This is "pure gamma radiation" though - no alpha, no beta, no massively irradiated soil being launched into the air in a huge plume.

Thus, it's not quite like the fallout created by an atomic explosion.

It's suggested here:

https://hps.org/publicinformation/ate/q10108.html

that a lot of the gamma rays would be absorbed by the supernova's own gas and dust debris - and that the ultraviolet radiation would be a bigger problem.

[Lvl 2 Expert]

It's suggested here:

https://hps.org/publicinformation/ate/q10108.html

that a lot of the gamma rays would be absorbed by the supernova's own gas and dust debris - and that the ultraviolet radiation would be a bigger problem.

The sunburnpocalypse. Nice.