1. The Deaths of Stars
Chapter 14

2. Types of Stars
Although gravity always wins in the end, a star’s fate is determined by its mass.
3 Categories according to mass:
Low-mass red dwarfs
Medium-mass sunlike stars
Massive upper-main-sequence stars
**One important difference between 1. and 2. is the amount of interior convective mixing.

3. The Steady Reds!
1. Low mass stars (<0.4 Msun) are completely convective inside. This means that a) H is completely consumed and b) they never form a He core with a H shell around it.
Therefore, these stars never become giants!
They merely use up all their fuel and cool down like cinders of a campfire. Boring!
Lifetimes on the order of trillions of years; much older than the life of our universe (~14 billion years) so none of them have died yet.

4. Red and Brown Dwarfs
The “big” star is a red dwarf, while the smaller companion is believed to be a brown dwarf (not a true star) about 12 Jupiters in mass. HST image.

5. The Ghost of X-Sun Future!
2. Stars between ~0.4 and ~4 solar masses are able to “burn” H and He, but stop at C (carbon). Not hot enough!
The cores are not well mixed, so shells around the core ignite and expand the star into a giant. The outer layers are only weakly held, so can be significant mass loss; enough to change how it evolves. Ex. thermal pulses, periodic eruptions.
Since C cannot fuse, then the core must contract.

6. Beautiful Results!
This mass loss often results in a planetary nebula , composed of ionized gases expelled by the dying star. The star itself will collapse into a white dwarf.
A white dwarf contracts (because of the lack of fuel) into degenerate matter, which resists further collapse. Its interior will lock together into a crystal lattice of C and O; no longer a star but a compact object (1st type).
Eventually cools down into a black dwarf.
Red Giant Horizontal Branch Star Asymptotic Red Giant Planetary Nebula White Dwarf

7. Evolutionary Path of a White Dwarf

8. First Discovered White Dwarf
Main star is called Sirius A.
B was discovered due to wobble in A’s motion.

10. The Ring Nebula (M57)
Note: white dwarf in center and filaments extending outward.

11. Saturn Nebula, NGC 7009
Note central white dwarf, barrel shape, and red jets. Green gas came first, then the inner region. The fast winds caught up with the slower initial winds.

12. How Binary Stars Evolve
Remember, more than half of all stars are part of a binary system. If they are far apart, one will not greatly affect the other.
However, if they are close enough, then mass can be transferred from one to the other (and back).
First, we need to realize that each star controls the surrounding region. These teardrop-shaped regions are called Roche lobes. Their sizes depend on the stars’ mass.

13. Mass Exodus!
Mass can move from one star to another either by a stellar “wind” or if it becomes bloated enough that the other’s gravity pulls the outer layers through the L1 Lagrangian point (center of gravity).

14. Roche Lobe and Mass Transfer

15. Hot Pancakes in Space!
Due to conservation of angular momentum, matter just doesn’t “fall” onto the other star. Just like water down a drain, it must swirl around the star.
As a result, a hot, spinning accretion disk is formed around the receiving star.
The disk heats up to 1 million K due to friction and tidal forces. This build up can create a massive explosion, called a nova.

16. Accretion Disk to Nova

17. A “New” Star is Born!
A nova often appears as a “new” star in the sky (hence the name, meaning new).
It is NOT new, but the result of an explosion involving a white dwarf in a binary system.
As H matter (about 100 Earths) falls on the star’s surface, it becomes so hot and dense that it becomes degenerate and ignites nuclear fusion. The PP chain first, then the CNO cycle. The process happens too fast for any control, so an explosion takes place.
This process tends to be periodic over years.

18. The Tarantula Nebula, in the Large Magellanic Cloud.
Nova in the LMC
The Tarantula Nebula, in the Large Magellanic Cloud.
Note the “new” star and the Tarantula Nebula below it.

19. Nova Cygnus 1992
Note the “new” star above and the debris left behind (right).

20. The Big Boys!
3. Massive stars, like the less massive ones, “burn” their H and He and become giants or supergiants.
Though all stars lose mass, if there is more than 4 solar masses left, then heavier elements are fused… until iron!
These reactions become more and more rapid! Why? A) less energy released per fusion reaction and B) there are fewer atoms to react.

21. The Dead Head End
The nucleus of iron (Fe) is the most tightly bound of all—fusion stops working beyond Fe.
When the Fe core exceeds ~1.3 Msun, then it begins to collapse, in only a few thousandths of a second!
The collapse triggers a supernova, an explosion that destroys the star and may even outshine its host galaxy!
Two possible outcomes: neutron star or black hole (2nd and 3rd type of compact object).

22. Anatomy of a Supernova
When the core collapses, the outer layers follow. However, the matter rebounds off the condensed core and forms a shock wave going out.
The shock wave has enough energy to destroy the star 100 X, but the outer layers smother it.
99% of the energy is released in the form of neutrinos, which ordinarily zip through matter without hindrance. Since the center is so dense, some of the neutrinos are absorbed, heating those layers and making the core collapse faster.

23. The Key
It turns out that when the shock wave runs into the collapsing matter, it creates turbulent convection that boosts the stalled shock wave.
It takes a few hours for the explosion to burst out. We see the brightening of the star as the outer layers are blasted away with 1 trillion Earths of TNT!!
There are 2 main types of supernovae (SN).

24. SNL
Type I: reaches maximum luminosity (4B suns), fades quickly, then slowly. Lacks H, results from a white dwarf collapse in binary system that exceeds the Chandraekhar limit (1.4 Msun). No compact object is left behind! Destroyed.
Type II: less luminous than type I (600 M suns), declines in irregular way, results from collapse of iron core in massive star; contains H lines from the star.

25. Comparing the Light Curves
II should be less bright?!

26. The Crab Nebula, M1, SN 1054
Type II

27. The Heart of the Crab
The nebulous remains of a supernova explosion is called a supernova remnant. Caused by collision between fast SN “wind” and the ISM.
The bluish center of the remnant is caused by synchrotron radiation, caused by electrons spiraling around magnetic field lines. Since the charged electron is accelerating, it must give off photons.
A continuous spectrum is produced by various electrons moving at different speeds.

28. Highlights of SN 1987a First visible with naked eye since 1604
In southern hemisphere
Unusual type II light curve
Detection of neutrinos indicates that the core became a neutron star and confirmed SN theories

29. SN 1987a
Type II