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This question already has an answer here:

If our Sun was to suddenly undergo a supernova, what would the signs be? Would there be any signs we could notice before our death?

Assume that the Sun was to gain an increase in handwavium and thus go supernova despite its inadequate mass.

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marked as duplicate by a CVn, kingledion, Murphy, Frostfyre, James Jan 18 '17 at 14:53

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First, if the Sun went supernova scientists would be terribly, terribly confused.

In order for a star to go supernova, it has to have a mass greater than at least 8 solar masses. Although there is some debate about the exact threshold, the Sun is not nearly massive enough, not even close. So if it went supernova it would be really weird. In fact, it would be “totally defying the laws of physics” weird.

But let’s pretend for a moment that the Sun could go supernova…

In essence, a supernova is a violent stellar explosion. Perhaps “violent” is an understatement. These explosions are roughly the equivalent of a few octillion nuclear warheads, and a few octillion nuclear warheads going off in your neighborhood is extremely detrimental to any life in the area. An explosion of this magnitude releases incredible amounts of energy—as much as the sun creates over the course of its entire life.

Which means radiation, lots and lots of radiation.

This is not so good for our ozone. Scientists theorize that the Earth’s ozone layer would be damaged if a star less than 50 light-years away went supernova. And as a reminder, the sun is about 8.3 light-minutes from Earth. Big frown face for us, because 8.3 light-minutes is a lot closer than 50 light-years.

Dr. Mark Reid, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics, has said:

Were a supernova to go off within about 30 light-years of us, that would lead to major effects on the Earth, possibly mass extinctions. X-rays and more energetic gamma-rays from the supernova could destroy the ozone layer that protects us from solar ultraviolet rays. It also could ionize nitrogen and oxygen in the atmosphere, leading to the formation of large amounts of smog-like nitrous oxide in the atmosphere. Of course, just as a reminder, the Sun is not nearly as massive as the stars that actually do go supernova, so assuming that it did (for some strange reason), it would still be very bad.

If our ozone was impacted enough to cause even a 10 percent increase in ground-level ultraviolet (UV) radiation, it could kill much of the Ocean’s phytoplankton, effectively bringing an end to all marine life. If we lose 2/3 of our ozone, the UV radiation on mid-latitude cities like Washington, D.C. would be strong enough to give you a nasty sunburn in just five minutes.

If the Sun went supernova it would have a much more dramatic effect. We would have no ozone. With no ozone, skin-cancer cases would skyrocket. All living things would suffer from severe radiation burns, unless they were underground or in protective suits. Plants would fry, animals would fry…we would all die.

However, if the Sun went supernova the loss of ozone would be the least of our concerns.

There would be no escape. On the side of Earth that faced the Sun, the explosion would boil away the surface of the Earth at hundreds of meters per second. People on the night side wouldn’t do much better. Scattered light would heat Earth to lethal temperatures. Scientists estimate that the planet would be roughly 15 times hotter than the surface of the Sun currently is. Far above the boiling point of any known material, and much hotter than any human can withstand (obviously).

At best, the Earth would take a few days to vaporize.

Even were we to flee to Pluto (which would take roughly a 10 years with today’s technology) we still wouldn’t survive. The tiny dwarf planet would also be heated to temps hotter than the surface of the Sun. Poor Pluto, first it’s not a planet and now this.

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    $\begingroup$ The scientists would be confused, but fortunately their confusion will last for a very very short time. $\endgroup$ – AlexP Jan 11 '17 at 21:25
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    $\begingroup$ Regarding Dr. Reid's quote, it is kind of silly to make a statement about supernovas within 30 ly. There are no stars within 10 parsec (32.6 ly) large enough to have a core collapse supernova (approx 8 sun masses). The list of white dwarfs that could have a Ia supernova is also small, with only Sirius B being close to the Chandrasakar limit of 1.44 sun masses. As far as I can tell, the closest star that will supernova is Bellatrix at 240ly. $\endgroup$ – kingledion Jan 12 '17 at 2:23
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    $\begingroup$ What would REALLY confuse scientists is if the sun went nova... and we survived it! $\endgroup$ – SRM Jan 12 '17 at 6:37
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    $\begingroup$ Informative but the actual question seems to be: "Would there be any signs we could notice before?" $\endgroup$ – Denis de Bernardy Jan 12 '17 at 6:38
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    $\begingroup$ Poor answer, because it understates the effects by a few billion billion billion times. "We would have no ozone...skin cancer...plants would fry..." -> "earth vaporised in moments, quite probably by being dissociated into fundamental particles and photons" if sun went sn, would be a closer description :) $\endgroup$ – Stilez Jan 12 '17 at 21:47
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@SandyBeach is absolutely correct: The sun should not go supernova, and if it did so, we'd all be dead by the time we could tell.

However, assuming there's a handwaved mechanism to keep some sensors alive, there would be noticeable changes.


It's important to know what causes supernovas first and foremost

Supernovas occur under specific conditions, so it's important to know what they are, and what causes them, to explain what we'd see right before one.

Fusion occurs in layers

When any star fuses hydrogen into helium, it will run into a problem called helium poisoning (not to be confused with the problem that kills people occasionally).
Helium gets in the way of hydrogen nuclei colliding, and fusion efficiency decreases. This problem is eventually solved when the heavier helium sinks to the core - hydrogen is more buoyant - and hydrogen fusion continues in the outer shell. Efficiency is boosted: helium is next to helium, hydrogen is next to hydrogen, and the star can keep going if it's massive enough.

Eventually, helium itself may begin to fuse into carbon - and then carbon will fuse - and so on, adding new layers to the star that our sun does not have. This ends when iron is created, which is not sustainable.

All things must end

The star will waste energy trying to combine iron nuclei, it will not maintain stellar equilibrium, gravity will overpower energy output, and the star will collapse. Imagine hitting a rubber ball with a hammer: the hammer will fly right back off. That's what happens here: the ball is the core of the star, and the hammer represents all that material.


#1: Orbits would change

You suggest adding enough mass via handwavium to cause the a supernova. Let's pretend that mass is dark matter, so it doesn't block out all the sun's light, and that it's on the outside of the star, so fusion can continue to occur uninterrupted. Even before changes in the sun itself, the orbits of all planets would noticeably in potentially hazardous ways.

#2: The sun's spectra would change

If you handwaved in enough mass, gravity would provide the energy to continue fusion of helium into other layers. Let's pretend it's a lot of mass, so these changes don't take millions of years. New elements would be visible in the spectra of the sun, and scientists would fairly quickly see these substances:

  • Carbon
  • Neon
  • Oxygen
  • Silicon
  • Magnesium
  • Nickel
  • Iron

The iron would be a dead giveaway that a supernova was imminent.

3: The sun would leave the main sequence

enter image description here The sun is about in the middle of that diagram: it's average in brightness, temperature, mass, and volume. By increasing the energy output of the sun - "wringing it out" by applying more pressure - it would change unpredictably. On one hand, it would get smaller due to pressure, and hotter, moving toward the rare "blue dwarf" category - yet on the other hand, stars close to supernova tend to expand greatly as layers are added. Regardless, we would see a color change, and a size change, in our star.

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    $\begingroup$ "Fusion occurs in layers" - so what you're saying is, stars are like ogres? $\endgroup$ – immibis Jan 12 '17 at 3:25
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    $\begingroup$ @immibis It took so much self restraint for me to not write that... but yes, stars are like ogres $\endgroup$ – Zxyrra Jan 12 '17 at 3:27
  • $\begingroup$ How long would the dark matter need to be there to cause the supernova, I wonder? I'm curious if we'll get a chance to see that color change, given that it takes upwards of thousands of years for light to make it from the core to the surface. image.gsfc.nasa.gov/poetry/ask/a11354.html $\endgroup$ – Jander Jan 12 '17 at 4:26
  • $\begingroup$ @Jander I'm imagining a little while - fusing iron is no quick task, even if supplemented by heat and pressure - and an explosion without that would probably just be a large planetary nebula or a small nova. That may give enough time to see a color change, I'm not sure. $\endgroup$ – Zxyrra Jan 12 '17 at 4:31
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    $\begingroup$ @Jander We're talking about an explosion here - the light wouldn't bounce around the matter inside of the Sun, it would just go and push the outer envelopes on their merry way. Since dark matter doesn't interact electromagnetically, it wouldn't be impeded on its journey to the core (though as a converse, it wouldn't want to stay there either - it'd oscillate back and forth for a long time) - enough of it could cause a reaction violent enough to start a runaway fusion reaction that would blow (most of) the Sun apart. For a more realistic scenario like Ia supernova, it takes about 1000 years. $\endgroup$ – Luaan Jan 12 '17 at 15:38
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Obligatory XKCD link: https://what-if.xkcd.com/73/

Which of the following would be brighter, in terms of the amount of energy delivered to your retina:

  1. A supernova, seen from as far away as the Sun is from the Earth, or

  2. The detonation of a hydrogen bomb pressed against your eyeball?

(Hint: It's not the hydrogen bomb)

In this What-If, Randall Monroe calculates that the neutrino radiation alone (!) would be able to kill you up to about 2.3 AU (the Earth is approximately at 1 AU) for a standard supernova.

So it does not matter if you are on the day or night side of the Earth, since neutrinos don't bother too much with a bit more or less material to pass through.

Arguably, since our sun can't go supernova (with our current understanding of physics), as already stated by the previous answers, this supernova might be slightly less powerful and so the neutrino radiation might be barely survivable. Which is fine, because soon after the neutrinos the electromagnetic radiation arrives (after the neutrinos, because the neutrinos escape easier through the remnants of the sun than photons).

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    $\begingroup$ Hey, I wanted to post the mandatory xkcd link! ;-) By the way, Charles Stross describes the event neatly in Iron Sunrise. You feel a funny event, you see little flashes, and then you start to feel sick, but only briefly (because then you die). His scenario is not entirely implausible: The neutrino flux is indeed traveling unimpeded at full light speed all the way from the implosion/explosion in the sun's core. Other effects are probably slower. E.g. gamma radiation must travel through/push aside the outer layers of the sun, which may take a second or two (instead of the usual 170k years). $\endgroup$ – Peter A. Schneider Jan 12 '17 at 14:19
  • $\begingroup$ @PeterASchneider Well, the neutrinos must travel at slightly less than lightspeed, because they have (a very small) mass. But the effect of the sun slowing the photons down is probably much larger. $\endgroup$ – Graipher Jan 12 '17 at 15:15
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The answer above say it nicely.

Science would be confused (as in, terribly and physics-defyingly confused). Yes. Completely. If they had time to be.

To see why, we need a basic point about quantum physics. Without going into great detail, some particles (known as fermions, and including electrons) simply cannot be packed "too close" because of quantum effects. When forced together under pressure or by some force, this effect manifests as a force arising between them (known as "degeneracy force/pressure") , that resists the force that would otherwise pack them closer. This is the reason behind everyday matter taking up space, it's why chemistry and chemical reactions happen (in basic terms the Periodic Table is largely related to electron "shells" as they are loosely called which is why alkali metals share similar properties, as do halogens, and so on).

In a star, the fusion reactions take place in the core (or for a large star, in layers deep in the star). Although massive and generating a very large gravitional force inward, the star doesn't collapse for 2 reasons - 1) heat, 2) the "exclusion effect" described above, both counteract gravity.

Now consider the same star, when its core runs out of fusable material. If it's below a certain size, the forces between particles (from degeneracy pressure) can counteract the inward gravity indefinitely, alone. So below a certain size, the star simply won't collapse.... and a supernova is a stellar collapse phenomenon. No collapse, no sn. Turns out the size needed for exclusion forces alone, not to be able too counteract gravity, is about 8x the suns mass. So the sun simply can't go supernova, now or ever, left to itself.

The most common way for a star under 8 solar masses to become a supernova is if it isn't left to itself - it gets extra mass from a companion star. One day, enough mass reached, collapse! - called a type 1 (or 1a) supernova. The sun doesn't have a companion star though.

However, if the star is larger than 8 solar masses, its different. There are 3 or 4 possible/known versions of "different", 2 of them give a supernova.

Around 8-9 solar masses isn't quite enough to directly trigger a supernova, but it is enough to trigger a process that gradually removes free electrons from the core, faster than they can be replaced. This reduces the exclusion force instead, so one day - bang! Collapse and supernova.

About 10+ solar masses it's more direct. The star burns all its fuel (in a well-known sequence that sees it fusing its previous fusion products in layers, at even higher temperatures), but it finally runs out of fusable fuel, and it already has a massive enough core that exclusion forces can't counter its own inward gravity without the additional outward forces of active fusion processes. So it collapses. But you only get a supernova from this process up to a certain point - if the star was very, very massive it doesn't get a chance to sn, it either collapses directly to a black hole (or weakly supernova's then collapses back into one), or it blows itself apart via pair production fluctuations. (That's cases 3+4).

Whatever the route, the supernova process broadly follows a similar path. The core - that's the innermost part of the star, not all of it - suddenly finds it can't sustain itself against the inward force of gravity, even with the exclusion principle at work. It suddenly kind of detaches from the rest of the star and collapses inward - and by "suddenly" think in terms of milliseconds and 3/4 of the speed of light. Massive heating, massive outburst of energy (largely in the form of neutrinos), the neutrinos unlike photons can escape from the core easily and add a further runaway loss of energy and support to the collapsing core, it becomes more likely that electrons combine with protons forming neutrons removing even more degeneracy pressure, in the extreme temperatures and pressures elements more massive than iron and silicon are created, 10% or so of its entire mass is converted to energy - and then the inward free-falling core reaches a density around that of neutrons at its heart, and this, finally, stops the collapse. Instead (and we don't fully know the mechanics of it), the infalling core rebounds, and blasts the blissfully unaware outer parts of the star into space in a huge detonation, leaving behind the newly formed neutron core as a neutron star or other similar object, and a nebula of expanding gas.

But that, alas, is one thing our own star will (almost certainly) never do.

What would we see, if it could and did? Well... the best description I ever heard was something like this: "what would deliver the brighter flash to your retina (assuming visibility) - a 50 megaton thermonuclear weapon detonating on your eyeball, or a star going supernova at the same distance as our sun?"

(the comment below says this is from xkcd, and so it is)

One guess :) And by something like a billion times.

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    $\begingroup$ "So the sun simply can't go supernova, now or ever, left to itself." So what you're saying is, another star would have to sneak up on Sol when we weren't looking and merge in additional mass! "Where did the extra mass come from?" "A sneaky star from Beta Quadrant!" "Damn. I knew building that wormhole was a mistake. Well, folks, it's been nice working with you..," $\endgroup$ – SRM Jan 12 '17 at 6:33
  • $\begingroup$ So far as we currently know, and fundamental changes to quantum fields, the setup of the local cosmos, or other basics aside, yes ;-) $\endgroup$ – Stilez Jan 12 '17 at 13:22
  • $\begingroup$ Your last paragraph is from what-if.xkcd.com/73 - please attribute it properly. $\endgroup$ – Mołot Jan 12 '17 at 13:34
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    $\begingroup$ Note that even for a Ia supernova, you need to be a white dwarf. The runaway reaction is due to the fact that the star cannot regulate the rate of the fusion process - if you just added more hydrogen to the Sun, it would slowly increase in brightness, but wouldn't explode. A white dwarf is supported by degenerancy pressure, which is independent of temperature, so it doesn't expand and cool down like a main sequence star would - which means that you get a runaway reaction (a positive feedback loop - more fusion->more heat->even more fusion->...) that results in the supernova. $\endgroup$ – Luaan Jan 12 '17 at 15:42
  • $\begingroup$ In your 6th paragraph, for a 1a supernova to occur in a while dwarf, the mass limit is not the approximately 8x solar masses, it is the Chandrasekhar mass of 1.44 solar masses (not the Chandrasekhar limit, which is 1.39 solar masses). $\endgroup$ – kingledion Jan 13 '17 at 4:57
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Some say the world will end in fire, others say in ice...

If the sun decided to bake us with a solar flare, our satellites would see it coming and we would probably see some increase in its brightness as the super heated plasma approached.

If the sun decided to turn down the heat and freeze us out, this would probably also involve a change in general illumination.

The sun is way out of our weight class. It does not even have to nova or even raise a proverbial sweat to wipe us out. And even if we saw the death blow coming, there is nothing we could do to save our all-too-fragile lives.

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Our sun would 'probably need some help' to go supernova. The chance for spontaneous supernovae is a non-zero number, though, and technically it's... well, no, not possible (unless the Q Continuum are having domestic problems).

If (big if) somehow a supernova were to be seen - its brightness (according to this link) would be nine orders of magnitude greater than a hydrogen bomb detonated on your eyeball.

Quite bright.

Apparently, a supernova would have to be half a light year away before its brightness was equivalent to an H-Bomb pressed against the eye.

Things which might/might not precede or suggest a supernova about to happen with our sun could include a sudden mega-dose of ionizing radiation or at the very least a high-pitched squealing noise.

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The sun will gradually get hotter as it ages. At some point in the next billion years it will become so hot that Earth's cloud cover reaches 100% and then the Earth will undergo runaway global warming to become a cooler version of Venus. This is the end for life here.

As for the sun, it will eventually undergo an ordinary nova event and become a dwarf star. Earth will probably survive minus its atmosphere. Venus probably won't.

Astronomers are watching Betelgeuse with interest. It will go supernova some time in the next million years. The star is a huge giant and is highly unstable even on a human timescale. It is teetering on the brink. It's 600 light years distant so the supernova will not hurt us, but it will be very spectacular in Earth's sky when it blows up. At 60 light years it would be a clear and present danger to us.

Interesting fact. The sun cannot go supernova but if it could and did, the flux of neutrinos would kill us before any light could escape from the exploded core. We'd just feel sick and then keel over without ever knowing why.

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    $\begingroup$ Interesting fact 2: interesting fact is wrong. Neutrino dose from the sun would be (roughly) 50 sieverts, or 10 times a lethal dose. This would certainly be fatal, but hardly instantaneously, so the more conventional radiaion (x-rays and gamma radiation) would have plenty of time to incinerate us first. $\endgroup$ – WhatRoughBeast Jan 11 '17 at 22:16
  • $\begingroup$ I stand corrected about keeling over (although it takes several hours for the photons to get clear of the exploding star) $\endgroup$ – nigel222 Jan 11 '17 at 22:28
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    $\begingroup$ I think @nigel222 is correct about feeling sick and then keeling over. Wikipedia's chart on Whole-body absorbed dose shows >30 Gy doses causing nausea and vomiting in minutes, and diarrhoea, headache, and fever in under an hour, and that the effect on the CNS is "Seizures, Tremor, Ataxia, Lethargy" (compared to mere "Rapid incapacitation" for 8–30 Gy). $\endgroup$ – BenRW Jan 12 '17 at 12:56
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    $\begingroup$ @BenRW To be fair, we're not entirely sure how exactly neutrino radiation would affect the human body, since it's not something we can test easily. It might be that some effect will make it much stronger or weaker than expected - we haven't really had a chance to observe such a massive neutrino flux in a nice, controlled environment. But to the best of our guesses, ouch. $\endgroup$ – Luaan Jan 12 '17 at 15:47
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    $\begingroup$ Our sun won't go nova. Novas only occur in white dwarfs, and only when the white dwarf is sucking material off its partner in a binary star system. Periodically it gets enough material to start fusion again. That sudden brightening is a nova. It is pretty much completely unrelated functionally to a supernova, they share a name because they both appeared as "new stars" in the sky. $\endgroup$ – TheBlackCat Jan 12 '17 at 18:02

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