Susan CartwrightOur Evolving Universe1 The Lives of Stars n From studying nearby stars and stellar clusters l most stars are on the main sequence l stars become red giants after leaving the main sequence n How does this relate to the internal structure of the stars and their nuclear fusion reactions?

Susan CartwrightOur Evolving Universe2 Fusion reactions n Generate energy up to iron n But, need to get two positively charged nuclei close enough to fuse together l need fast movement l high temperature (and high density) n Converting hydrogen-1 to helium-4 is the easiest and most efficient fusion reaction l 0.7% of initial mass converted to energy E=mc 2

Susan CartwrightOur Evolving Universe3 Stellar structure and fusion n To keep star stable need pressure to increase downwards l temperature increases l density increases l fusion most likely in central core of star n Stars are mainly hydrogen l expect main sequence stars to fuse hydrogen to helium in core

Susan CartwrightOur Evolving Universe4 Hydrogen fusion reactions n Reaction rate increases as temperature increases l more massive stars have higher fusion rates n Reaction can be direct or use carbon-12 as catalyst l this tends to increase abundance of nitrogen and oxygen pp chain CNO cycle

Susan CartwrightOur Evolving Universe5 Getting the heat out n Energy generated in stellar core has to be transported to surface n Two options: l radiation  absorption and re-emission of photons l convection  hot gas rising towards surface, cool gas falling n Giant stars have convective outer layers l transports out the heavy elements produced by fusion

Susan CartwrightOur Evolving Universe6 What happens when the core hydrogen runs out? star now has helium core (not hot enough to fuse) on main sequence 10x Sun core shrinks under gravity until hydrogen outside starts to fuse star expands and cools, becoming luminous red giant (1000x Sun) 1.61 Gyr 1.65 Gyr 1.69 Gyr 1.76 Gyr

Susan CartwrightOur Evolving Universe7 Stages of hydrogen fusion n Main sequence stars fuse hydrogen to helium in core n Red giants (and subgiants) fuse hydrogen to helium in shell outside helium core n Stars have nearly constant luminosity on main sequence, but red giants get brighter as they age n Red giant stage lasts only 10% as long as main sequence core hydro- gen fusion starting to fuse out- side core established fusion in shell

Susan CartwrightOur Evolving Universe8 Helium fusion n Neither beryllium-8 nor boron-8 is stable l need to combine three helium nuclei to get stable carbon-12 l beryllium-8 serves as intermediate stage l need high temperature and density (else 8 Be decays before it gets converted to 12 C)

Susan CartwrightOur Evolving Universe9 Stages of helium fusion core helium starts to fuse helium fusion in core: star is smaller and hotter, but less bright carbon core with helium fusion outside: star becomes a giant again 1.86 Gyr 1.76 Gyr 1.82 Gyr

Susan CartwrightOur Evolving Universe10 Helium fusion on the HR diagram n Helium fusion is much less efficient than hydrogen fusion (0.07% instead of 0.7%) l helium fusion stage lasts for a much shorter time helium core fusing stars: the hori- zontal branch of a globular cluster

Susan CartwrightOur Evolving Universe11 Side effects of helium fusion n Adding more helium nuclei to carbon can produce the alpha-process elements l oxygen-16, neon-20, etc. n Adding helium to carbon-13 or neon-22 produces free neutrons l which can easily combine with nuclei (no charge) to produce different elements n Why does helium fusion make mostly carbon? l because carbon nuclei have an energy level at exactly the right place l otherwise carbon would be a rare element l and we would not exist! Fred Hoyle, 1953

Susan CartwrightOur Evolving Universe12 Stellar evolution n Note step is in log (age): each frame is 60% older than the one before l massive stars evolve very quickly l post-main-sequence life of star is always comparatively short l massive stars change colour a great deal, but don’t change brightness much l less massive stars become much brighter as red giants

Susan CartwrightOur Evolving Universe13 After helium fusion n Fusion of heavier elements gets more difficult l higher mass means lower speed at given temperature l higher charge means more electrostatic repulsion n Stars like the Sun never get beyond helium fusion n More massive stars (>8 M S ) can fuse elements up to iron n What happens to Sun-like stars when the helium is used up? n What happens to massive stars when they reach iron? l fusion beyond iron requires energy n How are the heavy elements formed in stellar cores dispersed into space? …next lecture!