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

2. The Big Picture Stars exist because of gravity.
Gravity causes interstellar material to collapse to form stars.
Gravity determines how much energy stars generate.
Gravity dictates how stars evolve and die.
Mass determines gravity; Mass is the #1 property of a star.

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

4. Various Appearances of the ISM

5. Three Kinds of Nebulae
Emission Nebula
Reflection Nebula
Dark Nebula

6. 1) Emission Nebulae (HII Regions)
A very hot star illuminates a cloud of hydrogen gas;
Its ultraviolet light ionizes hydrogen;
Electrons recombine with protons, cascade down to the ground state, and produce emission lines, dominated by red Hα photons.
The Keyhole Nebula
NGC 2246
The Fox Fur Nebula

7. 2) Reflection Nebulae
Star illuminates a nearby cloud of gas and dust;
Blue light is much more likely to be scattered by dust than red light;
Reflection nebula appears blue.
*The same physics for the blue sky and the red sunset!

8. Emission and Reflection Nebulae

9. 3) Dark Nebulae
Dense clouds of gas and dust absorb the light from the stars behind;
Appear dark in front of the brighter background, which is often an emission nebula.
Barnard 86
Horsehead Nebula

10. Interstellar Reddening
The physics of reflection nebula revisited!
Blue light is strongly scattered and absorbed by interstellar (dust) clouds.
Red light can more easily penetrate the cloud, but is still absorbed to an extent (“interstellar extinction”).
(Infrared radiation is hardly absorbed.)
Interstellar clouds make background stars appear redder.
Barnard 68
Infrared
Visible

11. Interstellar Absorption Lines
The interstellar medium produces absorption lines in the spectra of stars.
Distinguished from stellar absorption lines via:
a) Absorption from wrong ionization states
Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple lines of same transition
b) Narrow (sharp) lines (temperature & density too low)
c) Multiple components (several clouds of ISM with different radial velocities)

12. Four Components of the ISM
EXTRA
Component
Temperature (K)
Density (atoms/cm3)
Gas
Percent of total mass
Molecular clouds
20 – 50
103 – 105
Molecules (H2 and others)
~ 25%
HI clouds
50 – 150
1 – 1000
Neutral hydrogen Other atoms ionized
Intercloud medium
103 – 104
0.01
Partially ionized
~ 50%
Coronal gas
105 – 106
10–4 – 10–3
Highly ionized, from hot stars and supernovae
~ 5%
Note: Emission nebulae (HII regions) occur only near very hot stars, so they comprise very small fraction of the ISM.

13. Various Views of the Interstellar Medium
Infrared observations reveal the presence of cool, dusty gas.
X-ray observations reveal the presence of hot gas.

14. Shocks triggering star formation
Henize 206 (infrared)

15. The Contraction of a Protostar

16. Ignition of 41H → 4He fusion processes
From Protostars to Stars
Star emerges from the enshrouding dust cocoon (T Tauri stage)
Ignition of 41H → 4He fusion processes

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

18. 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 or bipolar outflows) seen as Herbig-Haro objects

19. Example:
Herbig-Haro Object HH34

20. Globules Contracting to form protostars Bok globules:
~ 10 – solar masses
Contracting to form protostars

21. EGGs
Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

22. Energy Generation (§7.2) nuclear fusion
Energy generation in the sun (and all other stars):
nuclear fusion
Binding energy due to the strong force
= fusing together 2 or more lighter nuclei to produce heavier ones.
Nuclear fusion can generate energy up to the production of iron.
For elements heavier than iron, energy is produced by nuclear fission.

23. Energy generation in the Sun: The Proton-Proton Chain (§7.2)
Need large proton speed (high temperature) to overcome Coulomb barrier (electrical repulsion between protons).
Basic reaction: 41H → 4He + energy
4 protons have x10-27 kg (= 0.7 %) more mass than 4He.
T ≥ 107 K = 10 million K
Energy gain = mc2
= 0.43x10-11 joules per reaction
Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second!.

24. The Solar Neutrino Problem
The solar interior cannot be observed directly because it is highly opaque to radiation.
But neutrinos can penetrate huge amounts of material without being absorbed.
Early solar neutrino experiments detected a much lower flux of neutrinos than expected (→ the “solar neutrino problem”).
Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem.
Davis solar neutrino experiment

25. The Source of Stellar Energy
Recall from our discussion of the Sun:
Stars produce energy by nuclear fusion of hydrogen into helium.
Basic reaction: 41H → 4He + energy
In the Sun, this happens primarily through the proton-proton (PP) chain

27. Fusion into Heavier Elements than C & O:
Requires very high temperatures (why?).
Occurs only in very massive stars (more than 8 solar masses) —why?.

28. Stellar Models—the theory of stars
Divide a star’s interior into concentric shells — “Onion skin layer model”
Within each shell and between neighboring shells, require that the laws of physics are obeyed:
Four laws of stellar structure:
• Conservation of Mass
• Conservation of Energy
• Hydrostatic Equilibrium
• Energy Transport

29. Hydrostatic Equilibrium
In each layer:
Inward force of gravity (weight of all layers above)
Outward force of thermal pressure =
This condition uniquely determines the interior structure of the star.
Stable stars on a narrow strip (main sequence) in the H-R diagram.

30. Energy Transport Radiation Convection
Energy generated in the star’s center must be transported to the surface.
Inner layers of the Sun:
Radiation
Outer layers of the Sun:
Convection
Energy carried by photons
Energy carried by convective motion of large masses

31. Stellar Structure Sun Flow of energy Energy transport via convection
Energy transport via radiation
Flow of energy
Energy generation via nuclear fusion
Star’s total mass determines which part of the star has convection or radiation (cf. Ch. 10)
Temperature, density and pressure decreasing

32. Calculating the stellar structure:
Take the four equations representing the four laws of stellar structure and solve them simultaneously!
Conservation of mass
Conservation of energy
Star’s mass (and chemical composition) completely determines the properties of the star.
Energy transport
Hydrostatic equilibrium

33. 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.

34. The Life of Main-Sequence Stars
As stars gradually exhaust their hydrogen fuel,
they become brighter, and evolve off the zero-age main sequence.

35. The Lifetimes of Stars on the Main Sequence

36. The Orion Nebula: a region of active star formation

37. The Trapezium
The 4 trapezium stars: Brightest, young stars (< 2 million years old) in the central region of the Orion nebula
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

38. 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
Protostars with protoplanetary disks

39. Stellar Structure— Cause and Effect
Mass (M)
Available Fuel →
~ M
Gravity (weight) →
Hydrostatic equilibrium
Pressure
Temperature
Pressure-Temperature Thermostat → →
Density
Fusion Rates → →
Radius
Luminosity →
Mass-Luminosity Relation
L ~ M 3.5
Time of Stability
Main Sequence Lifetime (“Life Expectancy”)
Lifetime = M/L ~ M –2.5

40. “Red” in Astronomy red emission nebulae red supergiants/giants/dwarfs
red shift (in the Doppler effect)
Interstellar reddening
blue reflection nebulae
red sunset
blue sky