1. The Deaths of Stars

2. Less massive stars will die in a less dramatic event, called a nova
The End of a Star’s Life
When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die.
High-mass stars will die first, in a gigantic explosion, called a supernova.
Less massive stars will die in a less dramatic event, called a nova

3. Red Dwarfs
Stars with less than ~ 0.4 solar masses are completely convective.
Mass
Hydrogen and helium remain well mixed throughout the entire star.
No phase of shell “burning” with expansion to giant.
Star not hot enough to ignite He burning.

4. Sun-like Stars
Sun-like stars (~ 0.4 – 4 solar masses) develop a helium core.
Mass
Expansion to red giant during H burning shell phase
Ignition of He burning in the He core
Formation of a degenerate C,O core

5. The more massive the star, the stronger its stellar wind.
Mass Loss From Stars
Stars like our sun are constantly losing mass in a stellar wind (solar wind).
The more massive the star, the stronger its stellar wind.
Far-infrared
WR 124

6. The Final Breaths of Sun-Like Stars: Planetary Nebulae
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ light years
Expanding at ~10 – 20 km/s (Doppler shifts)
Less than 10,000 years old
Have nothing to do with planets!
The Helix Nebula

7. The Formation of Planetary Nebulae
Two-stage process:
Slow wind from a red giant blows away cool, outer layers of the star
The Ring Nebula in Lyra
Fast wind from hot, inner layers of the star overtakes the slow wind and excites it => Planetary Nebula

8. The Dumbbell Nebula in Hydrogen and Oxygen Line Emission

9. Planetary Nebulae Often asymmetric, possibly due to Stellar rotation
Magnetic fields
Dust disks around the stars
The Butterfly Nebula

10. Planetary Nebulas

11. Planetary Nebulas

12. The Remnants of Sun-Like Stars: White Dwarfs
Sunlike stars build up a Carbon-Oxygen (C,O) core, which does not ignite Carbon fusion.
He-burning shell keeps dumping C and O onto the core. C,O core collapses and the matter becomes degenerate.
 Formation of a White Dwarf

13. White Dwarfs White Dwarfs: Mass ~ Msun Temp. ~ 25,000 K
Degenerate stellar remnant (C,O core)
Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!!
Chunk of WD material the size of a beach ball would outweigh an ocean liner!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun

14. White Dwarfs
Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.

15. Mass Transfer in Binary Stars
In a binary system, each star controls a finite region of space, bounded by the Roche Lobes (or Roche surfaces).
Lagrange points = points of stability, where matter can remain without being pulled towards one of the stars.
Matter can flow over from one star to another through the Inner Lagrange Point L1.

16. Nova Explosions
Hydrogen accreted through the accretion disk accumulates on the surface of the WD
Very hot, dense layer of non-fusing hydrogen on the WD surface
Nova Cygni 1975
Explosive onset of H fusion
Nova explosion

17. The Fate of Our Sun and the End of Earth
Sun will expand to a Red giant in ~ 5 billion years
Expands to ~ Earth’s radius
Earth will then be incinerated!
Sun may form a planetary nebula (but uncertain)
Sun’s C,O core will become a white dwarf

18. The Deaths of Massive Stars: Supernovae
Final stages of fusion in high-mass stars (> 8 Msun), leading to the formation of an iron core, happen extremely rapidly: Si burning lasts only for ~ 1 day.
Iron core ultimately collapses, triggering an explosion that destroys the star:
A Supernova

19. Observations of Supernovae
Supernovae can easily be seen in distant galaxies.

20. Supernova Remnants

21. Supernova Remnants

22. Supernova Remnants

23. The Famous Supernova of 1987: SN 1987A
Before
At maximum
Supernova in the Large Magellanic Cloud in Feb. 1987

24. The Remnant of SN 1987A
Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays.

25. Local Supernovae and Life on Earth
Nearby supernovae (< 50 light years) could kill many life forms on Earth through gamma radiation and high-energy particles.
At this time, no star capable of producing a supernova is < 50 ly away
Most massive star known (~ 100 solar masses) is ~ 4,900 ly from Earth. Vy Canis Majoris