When Stars Outshine Galaxies

gravity is pretty darn weak every time you pick something up you're counteracting the entire gravitational attraction of the earth and the earth is big therefore there's a hint of irony that the weakest of fundamental forces is responsible for one of the most extreme and aggressive environments in the universe [Music] not every massive star will form a black hole at the end of its life however they do almost always produce a supernova whatever you think you know about supernovas it's unlikely you've seen the full picture i sure haven't the more i read about supernovas the more i focused on the core collapse event the more questions i had and the more information i found and well we are nearing the end of a half-century-long effort into researching this phenomenon the end of a massive star's life is composed of cycles as fusion occurs the ashes or products of this fusion settle at the core these ashes are themselves non-fusionable in their current environment and thus continue building up the closest layer to the core burns the hottest and fastest and will eventually run out of fuel this causes the volume around it to collapse raising the temperature and density within the core until it can trigger a new wave of fusion this continues in cycles where heavier and heavier elements collect at the core until they are unable to trigger more fusion this principally occurs when the core is composed of iron normally fusion occurs because the resulting new element is a more stable configuration and lower energy state for its components iron however is the most stable configuration you can create giving iron more energy actually causes it to lose stability as we will learn in just a bit above the iron core the gravitational pressure from the star's mass is counteracted by the thermal pressure of fusion but inside the core this doesn't exist instead what's keeping the core intact is degeneracy pressure of electrons as we learned in the last video degeneracy pressure actually weakens when densities and pressures get too high as the silicon layer above burns during the last day of the star's life its iron ash is deposited onto the core the core's mass continues to increase until reaching the chandrasekhar limit once our degenerate core reaches this limit it collapses in a tenth of a second our roughly 5 000 kilometer diameter core shrinks to a diameter of less than 30 kilometers this brief action moving that much mass further down the gravity well produces as much energy as about 80 suns would release over their lifetime this dramatic collapse is the product of an even more dramatic positive feedback loop the unimaginable increase in density forces electrons to fuse with protons in a process called neutronization producing a neutron and a neutrino this loss of electrons within protons alleviates the electron degeneracy pressure and thus the core collapses faster as the density of the core spikes so does the temperature at about 10 billion kelvin the gamma rays produced shred through the iron nuclei that took millions of years to form breaking them into particle components of protons neutrons and alpha particles iron is the most stable configuration for these particles thus by fragmenting these nuclei we reduce the stability and internal energy of the core in its weakened state the core collapses faster this feedback loop serves to generate the hottest object in the universe as it collapses the momentum of the shrinking core actually causes its density to overshoot the average density inside of a nucleus the strong force acts like a spring if you're too close it pushes you away if you get too far it pulls you back in this case our neutrons are too close and the strong force pushes them back this causes the core to rebound out to a diameter of about 50 kilometers in an event called the bounce this rebounding motion slams into the infalling matter in the star creating a shock wave of unimaginable intensity that begins traveling outwards as intense and extreme as the shock wave is if there's a significant amount of iron falling towards the core the iron will efficiently absorb the energy from the shock wave and photo disintegrate rapidly weakening it after about a tenth of a second the outward traveling shock wave loses enough energy to stall at a distance of roughly 200 kilometers the outward pressure of the shock wave is equal to the inward pressure of the infalling material you can see the stall in this model of supernovae explosions so if our shock wave stalls as it travels out it begs the question how do extremely large stars explode neutrinos during and after collapse neutrinos are produced in immense quantities with greater and greater energy similar to how high energy uv light cannot pass through glass these high-energy neutrinos cannot pass so easily through the incredibly dense matter of the collapsing core this environment is opaque to neutrinos this trapping of neutrinos causes the core to reach temperatures i've seen range from 100 to 500 billion kelvin depending on the source of information but where do these neutrinos come from to understand this we need to take a step back and learn a bit about the nature of neutron stars the neutron configuration of quarks isn't very stable in order for a neutron to stay in neutron it needs to interact with the quarks and the strong force of a proton otherwise neutrons will decay into a proton electron and an antineutrino now obviously in a neutron star there are a ton of neutrons and very few protons not enough to satisfy all of those neutrons needs how can such a thing exist let's pretend one of these neutrons loses stability and decays into a proton electron and anti-neutrino well our old friend gravity is still very much present and very intense plus our electron and proton don't go anywhere the anti-neutrino has so little mass that it escapes quite easily the electrons in this highly dense environment are degenerate meaning they aren't particularly happy to be squeezed up against all the other electrons if a neutron wants to decay it must be able to squeeze yet another electron into the electron degenerate environment if it doesn't have the energy to do this then it simply cannot decay this is what normally keeps a neutron star intact but this is not the environment inside a collapsing core a collapsing core is not very degenerate degeneracy occurs at lower temperatures and are freshly collapsed core is likely the hottest object in the universe the unimaginable heat provides our neutrons with more energy than they can handle causing them to rapidly decay into electrons and protons the resulting electron and proton are then subsequently fused back together due to the immense pressure forming another neutron each time a neutron decays or is formed an antineutrino or neutrino is released respectively this is called the direct erka process and serves to produce neutrinos in unimaginable abundance as long as the temperature is ludicrously high these neutrinos can then either bounce off and transport energy to a nucleon or electron or be reabsorbed by a nucleon to produce the other nucleon and a lepton we return to our stalled shock wave as the shock wave remains motionless roughly 200 kilometers from the core the end falling material gathers or accretes on the proto-neutron star creating what's called the gain radius in between the gain radius and the shock wave is a small volume called the gain region this is a relatively low density region where neutrinos can diffuse out from the central core the densities and pressure of the shock wave creates additional opacity for neutrinos called the neutrino sphere when we say opacity for neutrinos we mean relatively speaking between four to ten percent of neutrinos are absorbed which if you know neutrinos is a very very high percentage this gain region is thought to be highly abundant with neutrino energy and serves to fuel the shock wave against the in-falling matter pressure now comes the crux of what i've learned has been over 50 years of research why does it explode we'll approach the solution from two different perspectives first we'll look at some models huge thanks to dr vartanian for providing high definition footage of their simulations and taking the time to answer some of my questions afterwards we'll look at a simple english explanation of what's happening first the model the new material in the gain radius is highly turbulent it is after all a fluid the material is being superheated by neutrinos and this turbulence helps turn over the mantle to aid in more efficient neutrino heating and increase the time neutrinos spend in the mantle in large stars the material is not homogeneous and thus there are some regions of slightly lower density than others as the turbulence becomes more extreme it will disrupt the spherical nature of the shock wave based on those inhomogeneities this stimulates the production of a dipole structure this newly formed axis serves as the direction our explosion will propagate the belief is that the dipole structure separates outward explosion on one axis and accretion of material on another this permits the liberated gravitational energy from the accreting matter on the core to continue the creation of neutrinos to further fuel the explosion a spherical explosion would mean no more accretion and therefore no further fueling of our explosion for very large stars it's unlikely this would provide enough of an initial burst to explode outright therefore in these models large stars almost always explode asymmetrically and smaller stars can explode spherically that's a very technical explanation let's see if we can't explain it more conceptually the two biggest obstacles are shockwave faces are photodissociating nuclei absorbing its energy and the physical ram pressure of this matter itself in smaller stars this is not such a big obstacle and we see the shock wave only stalls for an instant in larger stars this poses more of an issue both photo dissociating nuclei and ram pressure decreases as density decreases thus as long as the gain region can sustain the shock wave the amount of matter or ram pressure and density above it decreases while the amount of neutrino heating below it increases eventually the pressure from neutrino heating overcomes the ram pressure overhead and our shock wave restarts in larger stars it takes longer to reach this point this causes more neutrino energy to build up in the gain region known as accretion luminosity so when the shockwave finally does release it is accompanied with greater neutrino release as well therefore larger stars explode with greater intensity after a longer delay as the shock wave dissipates the superheated gain radius is flung away and neutrino opacity drops over roughly the next 10 seconds the direct erka process serves to cool the neutron core from its hundreds of billions of kelvin all the way down to one billion kelvin it's just mind-bogglingly mind-bogglingly fast i can't even say these words at about 1 billion kelvin the neutron core becomes degenerate enough to prevent further cooling through this method all of these neutrinos released have about 1 in 1 000 chance of colliding with the matter in the star around them this may sound very little but even 1 1 000th of the amount of neutrinos released is an insane amount of energy as the shock wave propagates out the pressure and energy facilitates the lighter elements to undergo fusion releasing additional energy but this nuclear fusion is magnitudes less energy than that released from the core and neutrinos that's how much energy was created from the core collapse a literal wave of nuclear fusion contributes only a tiny fraction of the entire energy released from a supernova there is certainly more to discuss but i have to draw the line somewhere someday in the future we will certainly return to related topics but to wrap things up i just want to say that core collapse supernovae are the most energetic events known to science a single supernova can release around 10 to the 46 joules but this is mostly in the form of neutrinos so we barely see one percent of that energy some of this energy is converted into kinetic and thermal energy to jettison the solar mass away as well as absorb to form larger elements but we still have an insane amount of energy to get rid of the supernova assassin 15 was observed to be radiating away 2.2 times 10 to the 38 watts of energy or 10 to the 38 joules every second all of the stars all of the fusion in the milky way galaxy radiates away about 8 to 15 times 10 to the 36 watts depending on the calculation for about a month this singular supernova was producing more electromagnetic energy than our entire galaxy mind-boggling [Music] you

Channel: But Why?

Views: 175,998

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Keywords: supernova, supernovae, core collapse, direct urca process, neutron star, star, solar, evolution, solar explosion, neutrinos, neutrino heating

Id: Yt-SBT7nNfU

Length: 14min 27sec (867 seconds)

Published: Sun Oct 31 2021