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

2. The Interstellar Medium (ISM)
The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky.
We are interested in the interstellar medium because:
a) Dense interstellar clouds are the birth place of stars.
b) Dark clouds alter and absorb the light from stars behind them.

3. Compression of the ISM by Winds from Hot Stars

4. The Contraction of a Protostar

5. From Protostars to Stars
Star emerges from the enshrouding dust cocoon
Ignition of H → He fusion processes

6. Factors in Nuclear Fusion
Hydrogen atoms are ionized (bare nuclei)
Nuclei repel each other (Coulomb barrier)
High enough temperature means a small percentage will have a high enough energy to get close enough for strong interaction to occur (Maxwell distribution of velocities)
Sufficiently high pressure ensures that enough reactions occur to supply energy needs of star

7. Evidence of Star Formation
Nebula around S Monocerotis:
Contains many massive, very young stars,
including T Tauri Stars: strongly variable; bright in the infrared.

8. T Tauri Stars Very young stars, still in the forming stage
Very young stars, still in the forming stage
Typically 100,000 – 10 million years old

9. Protostellar Disks and Jets – Herbig Haro Objects
Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects

10. Contracting to form protostars
Globules
Bok Globules:
~ 10 – 1000 solar masses;
Contracting to form protostars

11. Globules
Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars.

12. Winds from Hot Stars
Very young, hot stars produce massive stellar winds, blowing parts of it away into interstellar space.
Eta Carinae

13. The Orion Nebula An Active Star-Forming Region

14. The Trapezium
Only one of the trapezium stars is hot enough to ionize hydrogen in the Orion nebula.
Infrared image: ~ 50 very young, cool, low-mass stars
X-ray image: ~ very young, hot stars
The Orion Nebula

15. Spectral types of the trapezium stars
Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared
The Becklin-Neugebauer Object (BN): Hot star, just reaching the main sequence
Spectral types of the trapezium stars B3 B1 B1 O6
Visual image of the Orion Nebula
Protostars with protoplanetary disks

16. The Source of Stellar Energy
Recall from our discussion of the sun:
Stars produce energy by nuclear fusion of hydrogen into helium
In the sun, this happens primarily through the proton-proton (PP) chain.

17. Energy Source 1H + 1H  2H + e+ + ν 2H + 1H  3He + γ
2H moving fast
e+ annihilates an electron producing Gamma rays
Neutrino escapes from sun
2H + 1H  3He + γ
3He + 3He  4He + 1H + 1H

18. The CNO Cycle The CNO Cycle
In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over.
The CNO Cycle

19. Fusion into Heavier Elements
Fusion into heavier elements than C, O:
requires very high temperatures; occurs only in very massive stars (more than 8 solar masses)

20. Hydrostatic Equilibrium
Imagine a star’s interior composed of individual shells.
Within each shell, two forces have to be in equilibrium with each other:
Gravity, i.e. the weight from all layers above
Outward pressure from the interior

21. Hydrostatic Equilibrium
Outward pressure force must exactly balance the weight of all layers above everywhere in the star.
This condition uniquely determines the interior structure of the star.
This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram.

22. Regulation of Energy Production
If the energy production were to be insufficient then temp of core would decrease.
Pressure would decrease which would cause star to contract causing temp to increase again because of energy release from gravity.
If energy production were to be too much then the steps would occur in reverse.

23. Energy Transport Radiative energy transport Convection
Energy generated in the star’s center must be transported to the surface.
Inner layers of the sun:
Radiative energy transport
Outer layers of the sun (including photosphere):
Convection

24. Stellar Structure Sun Flow of energy Energy transport via convection
Energy transport via convection
Sun
Energy transport via radiation
Flow of energy
Energy generation via nuclear fusion
Basically the same structure for all stars with approx. 1 solar mass or less.
Temperature, density and pressure decreasing

25. 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.

26. 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.

27. 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.

28. The Lifetimes of Stars on the Main Sequence