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Our Place in the Cosmos

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Our Place in the Cosmos Evolution of Massive Stars • In the last lecture we followed the evolution of low mass stars (below about 3M!) from the main sequence to Our Place in the Cosmos planetary nebulae and white dwarfs Lecture 13 • More massive stars spend a much shorter time on the main sequence and Supernovae, neutron stars and end their lives in spectacular and black holes dramatic fashion CNO cycle 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 Energy Production by PP and Post-Main Sequence CNO Chains 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 Proton-Proton CNO • It now has the structure of a horizontal dominates in low-mass stars …. dominates in high mass stars branch star After leaving the main sequence, Nucleosynthesis massive stars move horizontally • When a high-mass star exhausts the helium in its core, the core shrinks until it reaches a back and forth temperature of 8 x 108 K on the H-R • At this point carbon can fuse into more diagram massive nuclei [low-mass stars never get hot enough to do this] The dotted • When the carbon is exhausted, core-burning region is the of neon, oxygen and silicon successively occurs instability strip • This synthesis of heavier nuclei from lighter where stars ones is known as nucleosynthesis pulsate in size 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 state then 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 Mass Loss • Even main sequence massive stars are losing mass at up -5 to 10 M! per year due to radiation pressure • The most massive stars (20 Captions 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 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 1. Fusing elements lighter dwarf than iron releases energy • Massive stars end their lives in a spectacular explosion known as a supernova 2. Fusing elements more • Nucleosynthesis proceeds as far as iron, the most massive than iron requires tightly bound atomic nucleus energy - iron and more • Whatever other nucleus one tries to fuse with iron, massive elements do not the product will have less binding energy then iron burn • Therefore no element heavier than iron is fused within stars Neutrino Cooling Supernova • Once carbon starts to burn, fusion proceeds • Once silicon is all fused to iron in the star’s core no extremely rapidly as neutrinos efficiently carry more fusion can occur energy away from core - neutrino cooling • The iron core then collapses beyond the electron- • Nuclear reaction rate must increase to balance degenerate stage to densities of 10 tonnes per cubic energy lost by neutrinos cm and temperatures of 10 billion K • Hydrogen burning lasts for millions of years • A process called photodisintegration then kicks in, • Helium burning lasts for ~ 100,000 years which breaks up the iron nuclei and squeezes • Carbon burning lasts or ~ 1000 years electrons into nuclei to produce neutron-rich isotopes • Oxygen burning lasts for ~ 1 year • Within about 1 second the core is collapsing at a rate • Silicon burning lasts for a few days of of about one quarter of the speed of light • By now the star is radiating nearly all its energy in the form of neutrinos Captions 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 Supernovae Supernova 1987A • This type of supernova is known as a Type II • Supernova 1987A in the Large Magellanic supernova Cloud - a companion dwarf galaxy - was • Type I supernovae occur due to mass accretion in visible to the naked eye in the Southern binary stars Hemisphere • Either type shine with the luminosity of 100 billion Suns • Neutrinos from this • One hundred times more energy, however, is released supernova had already - in the form of the kinetic energy of the ejected gas but unknowingly - been • One hundred times more energy again is released in detected by neutrino the form of neutrinos telescopes, confirming theories of supernova explosions Spreading it Around Neutron Stars • Supernovae are responsible for • The remaining core of a supernova has enriching interstellar space with collapsed to the density of atomic nuclei the heavy elements synthesized within stars • If the remnant is no more massive than about 3 M further collapse is halted by neutron • They are also responsible for ! synthesizing elements heavier degeneracy than iron by the process of • The resulting star is known as a neutron star neutron capture • Typical radii are only 10 km, but with a mass • Supernovae are thus essential more than 1.4 M for life ! • Neutron stars are a billion times denser than • We are literally made up of the 15 material from exploding stars! Remnant of white dwarfs and 10 times denser than SN 1987A water X-Ray Binaries X-Ray Binary • 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 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 Black Holes Evidence for Black Holes • Neutron stars are supported by neutron • If no radiation can escape from a black hole, degeneracy how can we tell that they are there? • Above about 3M! the force of gravity can no • Black holes are located via the effect of their longer be resisted gravity • As the neutron star collapses its surface • The size of a black hole is given by its 2 gravity increases until the escape velocity Schwarzschild radius rS = 2GM/c vesc = ![2G M/r] exceeds the speed of light • If the Sun were a black hole it would have a • Not even light can escape and we have a radius of only 3km black hole • Closely-orbiting objects or particles will be • A black hole will form if the stellar core left rapidly accelerated giving rise to X-ray after a supernova explosion exceeds 3M! or if radiation, as in Cygnus X-1 - a ~30M! the neutron star accretes sufficient mass supergiant and ~10M! black hole binary from a companion to put it over the limit 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!).
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