Ast 4 Lecture 19 Notes 1 Evolution of Low Mass Star Hydrogen Shell Burning Stage • After 10 billion years a Sun-like stars begins to run out of fuel • Inner core composed of helium • Core begins to shrink and heat up • Hydrogen shell around the core generates energy at a faster rate Red Giant • Faster rate of energy production in the hydrogen shell means higher lumi- nosity • Core is still shrinking • Outer layers increase in radius • The star becomes a red giant Helium Fusion • Temperature in the core reaches 108 K • Helium can now fuse into carbon • Triple-alpha process 4He + 4He → 8Be + energy 8Be + 4He → 12C + energy Helium Flash • Inside the helium burning core of a red giant pressure is almost independent from temperature • Electron degeneracy pressure • The core is not in equilibrium • The rate at which helium fuses increases rapidly during helium flash Horizontal Branch and Asymptotic-Giant Branch • After helium flash the core is back into equilibrium • During the stable burning of helium the star is at the horizontal branch of the H-R diagram • Asymptotic-giant branch star: carbon core and a helium-burning shell 2 Death of a Low-Mass Star Death of a low-mass Star • Core is not hot enough to fuse carbon • Helium shell is unstable • Outers layers of the star are ejected into space Planetary Nebula - Abell 39 Planetary Nebula - M57 2 Image Credit: AURA/STSci/NASA Planetary Nebula - NGC 5882 Image Credit: H. Bond(STSci)/NASA Planetary Nebula - NGC 7027 3 3 End of a High-Mass Star End of a High-Mass Star • High-mass star; M > 8M • High-mass stars can continue fusing elements beyond helium (carbon,oxygen,neon,magnesium,silicon) • Iron is the most stable element - energy cannot be produced from fusing iron • Once there is an iron core the star is no longer in equilibrium End of a High-Mass Star • Gravity causes the star to shrink • Temperature in the core is around 10 billion K • High-energy photons disintegrate the core into electrons, protons, neutrons • Photodisintegration absorbs energy thus lowering the temperature; gravity cause the collapse to accelerate 4 End of a High-Mass Star • High density causes the electrons and protons to form neutrons 1H + e− → neutron + ν • Neutron degeneracy pressure stops the collapse at the core and the star rebounds • The high-speed expansion creates a shock-wave that ejects the layers of the star into space in an event called a supernova (Type II) Kepler Supernova SN 1604- Visible Light Image Image Credit: NASA/ESA/R. Sankrit and W. Blair (Johns Hopkins University) Kepler Supernova - Infrared Image (Dust) 5 Image Credit: NASA/ESA/R. Sankrit and W. Blair (Johns Hopkins University) Kepler Supernova - X-ray Image Credit: NASA/ESA/R. Sankrit and W. Blair (Johns Hopkins University) Leftover Ball of Neutrons • The core remnant of a Type II supernova is called a neutron star 6 • Typical diameter: 20 km • Strong magnetic fields and extremely fast rotation 7.