Are there observable changes in a star about to become supernova, minutes or hours before the explosion?

  • I am writing a science fiction novel, where a ship is stranded in a single star system (a red supergiant). One of the plot points is the star becoming supernova in several hours, so the characters have to fix their ship before that happens.



    I have basic knowledge of how it works: Iron generated from nuclear fusion gets accumulated in the core, until it reaches a point when iron fusion starts. As iron fusion is an endothermic reaction, the core is no longer able to generate enough energy to hold against its own gravity and external layers pressure, so it collapses, and explode.



    I have read that once the iron fusion starts inside the core, the collapse occurs within minutes, that the collapse itself lasts a few seconds (even less than a second), and that the shockwave takes several hours to reach the surface. Is all that correct?



    The thing is that I need the characters to bee able to predict the explosion in a short term. A few hours or even minutes. It would be great if they could be aware of the core collapse and start a countdown.



    So, are there any external cue of these events, like changes in luminosity or color? Does the star spectrum change when iron fusion starts, or when the core collapses? I know that the core collapse generates a huge amount of neutrinos. Is this amount so intense that it can be easily detectable? (that is, without a huge detector in an underground facility). Can the amount of iron in the core be estimated from tha star spectrum and size, so the aproximate time of the collapse could be predicted?


    The earliest warning you could get of an impending core-collapse supernovae would come from neutrinos because they react very-weakly with matter, however this is also why such large detectors are needed to measure their presence so its a catch 22 situation. EM clues are there but they are on much shorter timescales.

    I think you could get by with a smaller tub of cleaning fluid if you were in orbit around the star that went supernova!

    This science of this question is probably fine for this site. However, the associated story issues and most potential follow-on questions may be more appropriate on [worldbuilding.se].

    @Makyen Actually, on Worldbuilding there is a *highly* related question at How can we extinguish a supernova? The title is a little misleading -- it's more about *preventing* the star from going supernova late in the process, than about extinguishing or reducing it once it's started -- but I do think that question will be useful for the OP.

    The Currents of Space. Isaac Asimov.

    @Dean A geiger counter will suffice for your neutrino detector in this case. On the other hand, if you're too close it's simply going to tell you that you're dead: https://what-if.xkcd.com/73/

    Related, but I never understood how something collapsing results in an explosion. If it's collapsing doesn't that mean it's getting smaller? How can getting smaller make it explode? Why didn't the explosion happen earlier?

    @Mehrdad You should ask that as a separate question. As far as I can see, there are a number of mechanisms. One is that, as the core collapses, it's_moving_ inwards, so it has kinetic energy. When the collapse stops, that energy has to go somewhere. Another is essentially that of a plutonium bomb: as the density increases, fusion reactions that weren't previously feasible become feasible. That's my 300-character summary of Wikipedia's material on core collapse. I probably mangled it horribly because I'm _far_ from an expert.

    @RobJeffries, David Richerby: Thank you!! Taking a look at those links right now!

    @LorenPechtel As far as i'm aware Geiger counters can only detect Alpha, Beta & Gamma radiation and cant even distinguish between the three. I stand by my statement that neutrinos are such weakly interacting particles you need a lot of mass just to detect them.

    @Dean You need a certain number of nuclei to detect a certain *fraction* of neutrinos. It doesn;t matter if that fraction is tiny if you have enough neutrinos. And you do have enough neutrinos in this case. A GM tube detects anything that ionises the inert gas in it. Neutrinos can do this via inverse beta decay. The cross-section is tiny, but as I say, if you have enough neutrinos that doesn't matter.

    @RobJeffries Yes that is true, but what would the background count rate be that close to a red supergiant? Would you even be able to tell if neutrinos were contributing to your signal?

    @Dean Alpha and Beta particles get stopped by the hull of your spaceship. What source of gamma rays were you thinking of?

    @RobJeffries fair enough, I was assuming the GM tube would be an external sensor, but I guess it makes sense to shield it from outside noise if its just looking for inverse beta decay that occurs in the inert gas inside it. I'm actually rather impressed such a feat is possible, though you wont see me riding a spaceship near a red supergiant!

  • Peter Erwin

    Peter Erwin Correct answer

    5 years ago

    I think your best bet would be detecting neutrinos generated by nuclear burning inside the star (as we do for the Sun). Once the star hits the carbon-burning stage, it's actually putting out more energy in neutrinos than in photons. During the silicon-burning phase, which lasts for a few days and is what creates the degenerate iron core (that collapses once it is massive enough), the neutrino flux increases to about 1047 erg/s a few seconds before core collapse. (The peak flux during core collapse is about 1052 to 1053 erg/s). This paper by Asakura et al. estimates that the Japanese KamLAND detector could detect the pre-supernova neutrino flux for stars at distances of several hundred parsecs, and provide advance warning of a core-collapse supernova several hours or even days in advance. Since your characters are in the same system as the star, they'd hardly need a large underground detector to pick up the neutrinos.



    This plot shows an example of neutrino luminosity (for anti-electron neutrinos) versus time for a pre-supernova star (from Asakura et al. 2016, based on Odrzywolek & Heger 2010 and Nakazato et al. 2013); core collapse begins at t = 0s.



    Figure 1 from Asakura et al. 2016



    By measuring the spectrum of energies for different types of neutrinos and their time evolution, you could probably get a very good idea of how far along the star was, particularly as we can probably assume your characters have much better models for stellar evolution than we currently do. (They'd also want to get accurate measurements of the star's mass, rotation rate, maybe internal structure via astroseismology, etc., in order to fine-tune the stellar-evolution model; these are all things they could do pretty easily.)



    The core collapse itself would be signalled by the enormous increase in neutrino flux.



    This "What If" article by Randall Munroe estimates that the neutrino flux from a core-collapse supernova would be lethal to a human being at a distance of around 2 AU. Which, as he points out, could actually be inside of a supergiant star, so your characters would probably be a bit further away than that. But it does show that the neutrino flux would be easily detectable, and that your characters might well get radiation poisoning from it if they were closer than 10 AU. (Of course, you'd want to detect it more directly than just waiting around till you started to feel sick, since that might take longer than the shock wave takes to reach the surface of the star.) This is just to bring home the fact that they wouldn't have any problem detecting the neutrinos....


    Great answer! @Alfonso It might help your realism to address the fact that neutrinos are notoriously hard to detect. Some simple line implying that neutrino detection capabilities have increased ten-fold for your future-tech would help sell the realism that you can actually detect neutrinos on a small space ship without something like current neutrino detectors (which are massive).

    Thank you very much, Peter. That's exactly the info I needed.

    @zephyr The volume of the neutrino detector can be scaled down by the relative flux of neutrinos you expect. You don't need a large detector to observe the neutrino pulse if you are in orbit around the star that blows up.

    @RobJeffries Sure, but they'd need to detect these neutrinos before the major burst happens. What's more, who knows how far out the space ship is.

    @zephyr From Table 6 of the paper I linked to, let's say KamLAND can detect pre-SN flux 10 hours before core collapse for a star 150 pc away (after a 48 hour integration). The neutrino flux for a spaceship 100 AU from the star is about 100 billion times higher. So, as Rob Jeffries pointed out, you could use a detector 100 billion times smaller than KamLAND. To cut the integration time to a more practical 1 second, you need a detector 500,000 times smaller than KamLAND: about 2 kg of liquid scintillator. (Assuming no better technology.)

    Even without actual neutron detector, that neutrino flux might deposit enough energy to the star that it would be detectable from stars surface layers, or show as specific kind of noise in other detectors (at least cameras, radars and radios) the ship must have.

    The ship is orbiting a planet a few AU from the star surface. From your comments I see it is possible to detect the neutrino burst with wathever sensors has the ship.

    @RobJeffries The problem isn't really the volume you need to detect the neutrino flux. It's the fact that you carry around a neutrino detector in the first place. Although as noted by hyde the flux might well be large enough to induce noise in other detectors.

    @Taemyr The immense size of neutrino detectors is because of the sheer amount of stuff you have to have in order for neutrinos to interact with. Neutrino detectors could be much smaller (yes, the volume of detection medium would indeed scale with the neutrino flux). Yes of course the associated electronics etc. wouldn't scale with volume, but neither can we fly a spaceship to a nearby supergiant... The flux might be large enough to kill you. Would that count as a detection..?.

    @RobJeffries What I mean is that you wouldn't normally carry a small deticated neutrino detector. - Because there are very few situationations where this is useful. If you knew the star you where visiting was about to go nova you might get one built - the science value could be considerable, and of course the early warning would be useful. But for a run of the mill starship having a device whose sole purpose is that it goes ding if you are close to a star that is about to go nova is a waste of space, mass and money. I rate getting killed as noise induced in other detectors.

    @Taemyr Neutrinos are produced by nuclear reactions, so a neutrino detector might be useful for detecting nearby nuclear reactors (hidden bases or ships?) in addition to its astronomical uses. Alternately, the characters might decide they need to *build* a detector on their ship, given the situation they're in.

    What a great answer.

    @AlfonsodeTerán here is a amazingly small neutrino detector! I think you could consider asking a question in Physics SE based on the data in this answer (luminosity 10^45 erg/sec a few hours beforehand) about expected count rate per kg of detector versus distance. The more specific information you add to the question (and link here) the better.

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Content dated before 7/24/2021 11:53 AM