The Supernova Event When the Fuel Runs Out in the Core The trap door opens: gravity wins. Imagine a Free-Fall Here On Earth 1. The force of gravity is roughly constant as you fall, because your distance from the center of the Earth changes very little 2. Air resistance slows you down and sets a limiting terminal velocity Terminal Velocity (a parachute makes it even slower) But in a Star When gravity wins, the atoms of the star fall freely towards the centre. The whole star is collapsing inward. As the atoms get closer together, the force of gravity increases the collapse accelerates enormously [Moreover, there is no wind resistance. It is a complete free-fall.] How Long? Felix free-falls from the stratosphere Felix free-falls from the stratosphere Felix free-falls from the stratosphere Simple to Work Out! The time to impact depends on the gravitational force acting on you how far you have to fall [ignoring air resistance] Felix B With no air resistance, he would have fallen 24.2 km in a little more than a minute and hit the ground at twice the speed of sound! A Different Example Lets stop the Earth in its orbit! It will fall into the sun, picking up speed as it goes. How long will it take? Not quite 3 months. But Here: Stellar Collapse Imagine a dense stellar core: one solar mass of material in a ball about the size of the Earth Imagine a dense stellar core: one solar mass of material in a ball about the size of the Earth Remove the sustaining force (gravity wins!) Time taken to fall to the center? About one second - very fast! About one second - very fast! A Crowd Forms [the particles come crashing together] More Violent than Rugby! a whole stars worth of nuclei rushing inward accelerating as they go attracted by a fantastically strong and growing gravitational force at densities which are approaching a trillion times that of water. What happens? What happens? What Happens to a Building? The bricks and mortar are torn asunder! All That Hard Work Undone! Similarly in the Star The vigorous collapse: releases gravitational potential energy causes extreme heating and lots of very energetic radiation (gamma rays). Disruption The gamma rays rip apart many (but not all!) of the nuclei that have been so gradually fused together over millions of years. Back to Particles The disruption of the nuclei leads to a sea of inward-falling protons and neutrons (plus electrons, of course! They have been present ever since the star formed: one per proton!) Can Chandrasekhar Save Us?No! In the crush, these electrons are squashed together with the protons. This converts a lot of the protons to neutrons Result: there is no electron degeneracy to resist further collapse! Neutronization Consequences Much of the infalling material the equivalent of a few times the mass of the sun, perhaps is more or less instantly turned into neutrons. An enormous flux of neutrinos also comes out. Neutron Degeneracy Kicks In! Suddenly the infalling neutrons reach the critical density and come to a halt! A (Neutron) Star is Born! The deep core of the huge star has hatched a tiny neutron star (about the size of a city!). There is a Rebound An Abrupt Reversal Material that falls in just fractions of a second later rebounds off the now-dense core, and is exploded out into space. [This is aided by a momentary over-contraction of the neutron star core, which itself re-expands slightly and gives an extra kick to the infalling material.] [This is aided by a momentary over-contraction of the neutron star core, which itself re-expands slightly and gives an extra kick to the infalling material.] The envelope of the star is blasted into space - a Supernova! Rather Like This Follow the link: Making a Ball Bounce Very High Making a Ball Bounce Very High Neutrino Wind In addition, the rush of fantastic numbers of emitted neutrinos helps to expel the outer shell of material. (Not all of the elusive neutrinos get through the dense outer layers! Those that are captured push the shell of material outwards.) End of Story BUT WAIT! Theres a Serious Problem Earlier I said that the ultra-heavy elements are produced in supernovae But now I seem to be saying that the collapse tears apart any moderately heavy elements that have so far been built up!! the collapse tears apart any moderately heavy elements that have so far been built up!! Which Is It? If the heaviest elements are not made deep in the core, where do they come from? It Depends on Location The neutron star itself forms deep in the core, in the dense innermost parts. Down there, things are indeed torn apart, and the star neutronizes completely. But somewhat farther out, other things happen. Remember the Newly Created Neutrons Neutrons have no trouble getting into a nucleus, since they are uncharged. What happens if we bombard a heavy nucleus (say, Iron) with neutrons? Heavier Elements Can Be Built Up [dont worry about the details] And Even More r (rapid) and s (slow) process In the Turmoil The heaviest elements of all are built up, in very small quantities, by the flood of neutrons created in the collapse. Some of these are subsequently blasted out into space, enriching the interstellar medium. Eventually, thats where we come from. The Alchemists Dream Realized This process produces Gold, plus the other heavy elements well beyond iron (like radium, uranium, etc). There is a Cost It requires an input of energy to make the heaviest elements. The supernova provides it! (Really it comes from gravity.) Finishing the Story: Heres What We See What We See (Later) What It Produces A very bright source of light A very bright source of light Neutrinos in unimaginable numbers Neutrinos in unimaginable numbers An expanding shell of heavy-element-enriched hot gas (at speeds of tens of thousands of km per sec) An expanding shell of heavy-element-enriched hot gas (at speeds of tens of thousands of km per sec) A compact neutron star (often). A compact neutron star (often). Why So Bright? A supernova is bright because (a) It has a huge, hot, expanding surface a large radiating area (a) And the shell contains certain newly- formed radioactive elements that decay, producing light. The Light Curve A supernova becomes as bright as a whole galaxy (billions of stars). Lets Reconsider SN 1987A Note how the decay of radioactive Cobalt plays a role. Computers allow us to model all this behaviour! Amazing Numbers! The visible light output can match that of a whole galaxy of 100 billion stars. But there is 100x as much energy again in the form of rapidly-moving material (kinetic energy) and 100 times as much again in the form of neutrinos. That is, the energy in the form of neutrinos is 10,000 times as much as the light we see!! In Other Words Supernovae matter to us because they produce heavy elements (thats where we come from) but what they do most effectively is flood the universe with fantastic numbers of neutrinos! Supernova 1987A We see a dramatic increase in brightness. We claim that there is also a huge flood of neutrinos. Neutrinos Indeed! The Kamioka detector in Japan detected 11 neutrinos from the 1987 supernova, just as the light arrived! (Sadly, SNO was not yet operational.) [Disappointing numbers? NO! Remember that the source is 150,000 light years away! And that neutrinos are elusive, hard to capture!] Here is One Neutrino Event One Special Application: Very Distant Supernovae They can be used to tell distances to remote galaxies (by how faint they appear) Using them, we can unravel the size, structure and evolution of the universe. This won the Nobel Prize in 2011, for reasons we will come back to later. At the Limits of Detectability! Note the faint supernova just to the right of the fuzzy galaxy