Our Place in the Cosmos

1. Our Place in the Cosmos Lecture 13 Supernovae, neutron stars and black holes

2. Evolution of Massive Stars • In the last lecture we followed the evolution of low mass stars (below about 3M) from themain sequence to planetary nebulae and white dwarfs • More massive stars spend a much shorter time on the main sequence and end their lives in spectacular and dramatic fashion

3. CNO cycle • In the hotter cores of massive main sequence stars hydrogen fusion can occur by an efficient mechanism known as the carbon-nitrogen-oxygen (CNO) cycle (as well as the less efficient proton-proton chain that occurs in low-mass stars) • This explains the dramatically higher luminosity of high mass stars • Note that carbon is not consumed in the CNO cycle - it acts instead as a catalyst

4. CNO Cycle

5. Energy Production by PP and CNO Chains Proton-Proton CNO dominates in low-mass stars …. dominates in high mass stars

6. Post-Main Sequence Evolution • The helium core of a massive star reaches a temperature of 108 K, at which point helium fusion can begin, before it becomes electron degenerate • There is therefore no explosion of the core - the star makes a smooth transition from core hydrogen-burning to core helium-burning • A massive star does not become a red giant but moves horizontally on the H-R diagram as it grows modestly in size while surface temperature falls • Itnowhas the structure of a horizontal branch star

7. After leaving the main sequence, massive stars move horizontally back and forth on the H-R diagram The dotted region is the instability strip where stars pulsate in size

8. Nucleosynthesis • When a high-mass star exhausts the helium in its core, the core shrinks until it reaches a temperature of 8 x 108 K • At this point carbon can fuse into more massive nuclei [low-mass stars never get hot enough to do this] • When the carbon is exhausted, core-burning of neon, oxygen and silicon successively occurs • This synthesis of heavier nuclei from lighter ones is known as nucleosynthesis

10. Pulsating Variable Stars • At this stage in their evolution, some massive stars pass through the instability strip on the H-R diagram • Changes in the ionization state (what fraction of electrons are removed from atoms) alter the transparency of the star to escaping radiation • When energy is trapped inside the star it expands • A change in ionization statethen allows the trapped radiation to escape and the star shrinks • This cycle continues - the star is a Pulsating Variable Star • Examples include Cepheid and RR Lyrae variables with periods of order days increasing in proportion to luminosity

11. Captions

12. Mass Loss • Even main sequence massive stars are losing mass at up to 10-5M per year due to radiation pressure • The most massive stars (20 M or more) may lose 20% of their mass while on the main sequence and 50% over their entire lifetime • An extreme example is Eta Carinae with a mass around 100 M but losing about 1 M every 1000 years

13. End of Fusion • Once all fusable material in the core of a low-mass star is exhausted the star expires relatively gently as outer layers are ejected to form a planetary nebula leaving behind the degenerate core as a white dwarf • Massive stars end their lives in a spectacular explosion known as a supernova • Nucleosynthesis proceeds as far as iron, the most tightly bound atomic nucleus • Whatever other nucleus one tries to fuse with iron, the product will have less binding energy then iron • Therefore no element heavier than iron is fused within stars

14. 1. Fusing elements lighter than iron releases energy 2. Fusing elements more massive than iron requires energy - iron and more massive elements do not burn

15. Neutrino Cooling • Once carbon starts to burn, fusion proceeds extremely rapidly as neutrinos efficiently carry energy away from core - neutrino cooling • Nuclear reaction rate must increase to balance energy lost by neutrinos • Hydrogen burning lasts for millions of years • Helium burning lasts for ~ 100,000 years • Carbon burning lasts or ~ 1000 years • Oxygen burning lasts for ~ 1 year • Silicon burning lasts for a few days • By now the star is radiating nearly all its energy in the form of neutrinos

16. Supernova • Once silicon is all fused to iron in the star’s core no more fusion can occur • The iron core then collapses beyond the electron-degenerate stage to densities of 10 tonnes per cubic cm and temperatures of 10 billion K • A process called photodisintegration then kicks in, which breaks up the iron nuclei and squeezes electrons into nuclei to produce neutron-rich isotopes • Within about 1 second the core is collapsing at a rate of of about one quarter of the speed of light

17. Supernova • At very high densities, the strong nuclear force becomes repulsive, causing the collapsing core to “bounce”, sending a shockwave through the rest of the star • Over the next couple of seconds, about one-fifth of the mass of the core is converted into neutrinos, some of which are trapped by the huge densities of material within the core, adding to the shockwave • Within about one minute the shockwave has pushed pass the helium shell and within a few hours reaches the surface, heating it to 500,000 K • The star has exploded in a supernova explosion

18. Captions

19. Supernovae • This type of supernova is known as a Type II supernova • Type I supernovae occur due to mass accretion in binary stars • Either type shine with the luminosity of 100 billion Suns • One hundred times more energy, however, is released in the form of the kinetic energy of the ejected gas • One hundred times more energy again is released in the form of neutrinos

20. Supernova 1987A • Supernova 1987A in the Large Magellanic Cloud - a companion dwarf galaxy - was visible to the naked eye in the Southern Hemisphere • Neutrinos from this supernova had already - but unknowingly - been detected by neutrino telescopes, confirming theories of supernova explosions

21. Spreading it Around • Supernovae are responsible for enriching interstellar space with the heavy elements synthesized within stars • They are also responsible for synthesizing elements heavier than iron by the process of neutron capture • Supernovae are thus essential for life • We are literally made up of the material from exploding stars! Remnant of SN 1987A

22. Neutron Stars • The remaining core of a supernova has collapsed to the density of atomic nuclei • If the remnant is no more massive than about 3 M further collapse is halted by neutron degeneracy • The resulting star is known as a neutron star • Typical radii are only 10 km, but with a mass more than 1.4 M • Neutron starsare a billion times denser than white dwarfs and 1015 times denser than water

23. X-Ray Binaries • If the neutron star is part of a binary system material may be transferred from the giant companion • Tiny size but large mass of neutron star leads to large gravitational acceleration of infalling material onto an accretion disk • Disk is heated to high temperatures so that it emits in X-rays - the most energetic form of electromagnetic radiation • A relativistic jet may also form

24. X-Ray Binary

25. Pulsars • Conservation of angular momentum means that many neutron stars are rotating at 10-100 times per second • Any magnetic field is concentrated by the collapsing star to values trillions of times greater than Earth’s magnetic field • Charged particles are accelerated along the field lines towards the magnetic poles • Electromagnetic radiation is beamed out away from the poles like light beams from a lighthouse • We can detect this radiation with radio telescopes - the star appears to “pulse” twice each revolution, hence the name pulsar • The first pulsar was discovered in 1967

27. Black Holes • Neutron stars are supported by neutron degeneracy • Above about 3M the force of gravity can no longer be resisted • As the neutron star collapses its surface gravity increases until the escape velocity vesc = [2G M/r]exceeds the speed of light • Not even light can escape and we have a black hole • A black hole will form if the stellar core left after a supernova explosion exceeds 3M or if the neutron star accretes sufficient mass from a companion to put it over the limit

28. Evidence for Black Holes • If no radiation can escape from a black hole, how can we tell that they are there? • Black holes are located via the effect of their gravity • The size of a black hole is given by its Schwarzschild radius rS = 2GM/c2 • If the Sun were a black hole it would have a radius of only 3km • Closely-orbiting objects or particles will be rapidly accelerated giving rise to X-ray radiation, as in Cygnus X-1 - a ~30M supergiant and ~10M black hole binary

29. Summary • Massive stars “live fast, die young, and leave a beautiful corpse” (supernova remnants) • They are responsible for synthesising all elements heavier than carbon, many of which are essential for human life • Supernovae spread these heavy elements throughout space - they will be incorporated into future generations of stars and humans! • The remnant cores are either neutron stars (below about 3M) or black holes (> 3M)