1 Evolution of Low Mass Star

Ast 4 Lecture 19 Notes

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