Our Our Place Place in in the the Cosmos Cosmos planetary - PDF document

Evolution of Massive Stars • In the last lecture we followed the evolution of low mass stars (below about 3 M � ) from the main sequence to Our Our Place Place in in the the Cosmos Cosmos planetary nebulae and white dwarfs • More massive stars spend a much Lecture 13 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 10 8 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 h orizontally 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 branch star dominates in low-mass stars …. dominates in high mass stars

After leaving the Nucleosynthesis main sequence, massive stars • When a high-mass star exhausts the helium in move horizontally its core, the core shrinks until it reaches a back and forth temperature of 8 x 10 8 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 to 10 -5 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 massive elements do not • Whatever other nucleus one tries to fuse with iron, 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 • Neutrinos from this Suns supernova had already - • One hundred times more energy, however, is released but unknowingly - been in the form of the kinetic energy of the ejected gas detected by neutrino • One hundred times more energy again is released in telescopes, confirming the form of neutrinos theories of supernova explosions Neutron Stars Spreading it Around • 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 • If the remnant is no more massive than about within stars 3 M � further collapse is halted by neutron • They are also responsible for degeneracy synthesizing elements heavier 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 white dwarfs and 10 15 times denser than Remnant of material from exploding stars! water SN 1987A 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 3 M � 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 gravity increases until the escape velocity Schwarzschild radius r S = 2 GM / c 2 v esc = � [2 G 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 3 M � or if radiation, as in Cygnus X-1 - a ~ 30 M � the neutron star accretes sufficient mass supergiant and ~ 10 M � 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 3 M � ) or black holes (> 3 M � )