1. The Formation and Structure of Stars
Chapter 9
The Formation and Structure of Stars

2. The structure and evolution of a star is determined by the laws of:
Stellar Models
The structure and evolution of a star is determined by the laws of:
Hydrostatic equilibrium
Energy transport
Conservation of mass
Conservation of energy
A star’s mass (and chemical composition) completely determines its properties.
That’s why stars initially all line up along the main sequence.

3. Interactions of Stars and their Environment
Supernova explosions of the most massive stars inflate and blow away remaining gas of star forming regions.
Young, massive stars excite the remaining gas of their star forming regions, forming HII regions.

4. The Life of Main Sequence Stars
Stars gradually exhaust their hydrogen fuel.
In this process of aging, they are gradually becoming brighter, evolving off the zero-age main sequence.

5. Chapter 10
The Deaths of Stars

6. Evidence that Stars Die
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.

7. Evolution off the Main Sequence: Expansion into a Red Giant
Hydrogen in the core completely converted into He:
→ “Hydrogen burning” (i.e. fusion of H into He) ceases in the core.
H burning continues in a shell around the core.
He Core + H-burning shell produce more energy than needed for pressure support.
Expansion and cooling of the outer layers of the star → Red Giant

8. Expansion onto the Giant Branch
Expansion and surface cooling during the phase of an inactive He core and a H- burning shell
The Sun will expand beyond Earth’s orbit!

9. Degenerate Matter degenerate matter:
Matter in the He core has no energy source left.
→ Not enough thermal pressure to resist and balance gravity
→ Matter assumes a new state, called
degenerate matter:
Electron energy
Pressure in degenerate core is due to the fact that electrons can not be packed arbitrarily close together and have small energies.

10. Red Giant Evolution H-burning shell keeps dumping He onto the core.
H-burning shell keeps dumping He onto the core.
He-core gets denser and hotter until the next stage of nuclear burning can begin in the core:
He fusion through the
“Triple-Alpha Process”:
4He + 4He → 8Be + g
8Be + 4He → 12C + g
The onset of this process is termed the
Helium Flash.

11. Evidence for Stellar Evolution: Star Clusters
Stars in a star cluster all have approximately the same age!
More massive stars evolve more quickly than less massive ones.
If you put all the stars of a star cluster on a HR diagram, the most massive stars (upper left) will be missing!

12. HR Diagram of a Star Cluster
High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence

13. Estimating the Age of a Cluster
The lower on the MS the turn-off point, the older the cluster.

14. Red Dwarfs Recall: Mass
Recall:
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

15. Sunlike Stars
Sunlike 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

16. 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

17. 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

18. 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

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

20. 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

21. Low luminosity; high temperature => White dwarfs are found in the lower center/left of the Herzsprung-Russell diagram

22. The Chandrasekhar Limit
The more massive a white dwarf, the smaller it is.
→ Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity.
WDs with more than ~ 1.4 solar masses can not exist!

23. 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.

24. Recycled Stellar Evolution
Recycled Stellar Evolution
Mass transfer in a binary system can significantly alter the stars’ masses and affect their stellar evolution.

25. White Dwarfs in Binary Systems
X-ray emission
Binary consisting of WD + MS or Red Giant star => WD accretes matter from the companion
Angular momentum conservation => accreted matter forms a disk, called accretion disk
T ~ 106 K
Matter in the accretion disk heats up to ~ 1 million K => X-ray emission => “X-ray binary”

26. 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

27. Recurrent Novae
In many cases, the mass transfer cycle resumes after a nova explosion.
T Pyxidis
→ Cycle of repeating explosions every few years – decades

28. The Fate of our Sun and the End of Earth
The 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

30. 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

31. Numerical Simulations of Supernova Explosions
The details of supernova explosions are highly complex and not quite understood yet.

32. Remnant of a supernova observed in a.d. 1054
Supernova Remnants
X-rays
The Crab Nebula:
Remnant of a supernova observed in a.d. 1054
The Veil Nebula
Optical
Cassiopeia A
The Cygnus Loop

33. Synchrotron Emission and Cosmic-Ray Acceleration
The shocks of supernova remnants accelerate protons and electrons to extremely high, relativistic energies.
→“Cosmic Rays”
In magnetic fields, these relativistic electrons emit
Synchrotron Radiation

34. The Famous Supernova of 1987: SN 1987A
Before
At maximum
Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987

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

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

37. Type I and II Supernovae
Core collapse of a massive star: Type II Supernova
If an accreting White Dwarf exceeds the Chandrasekhar mass limit, it collapses, triggering a Type I Supernova.
Type I: No hydrogen lines in the spectrum
Type II: Hydrogen lines in the spectrum