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. The Various Appearances of the ISM

4. Three kinds of nebulae 1) Emission Nebulae (HII Regions)
1) Emission Nebulae (HII Regions)
Hot star illuminates a gas cloud;
excites and/or ionizes the gas (electrons kicked into higher energy states);
electrons recombining, falling back to ground state produce emission lines.
The Fox Fur Nebula
NGC 2246
The Trifid Nebula

5. 2) Reflection Nebulae Star illuminates gas and dust cloud;
Star illuminates gas and dust cloud;
star light is reflected by the dust;
reflection nebula appears blue because blue light is scattered by larger angles than red light;
Same phenomenon makes the day sky appear blue (if it’s not cloudy).

6. Emission and Reflection Nebulae

7. 3) Dark Nebulae
Dense clouds of gas and dust absorb the light from the stars behind;
appear dark in front of the brighter background;
Barnard 86
Horsehead Nebula

8. Interstellar Reddening
Blue light is strongly scattered and absorbed by interstellar clouds
Red light can more easily penetrate the cloud, but is still absorbed to some extent
Infrared radiation is hardly absorbed at all
Barnard 68
Interstellar clouds make background stars appear redder
Infrared
Visible

9. Interstellar Absorption Lines
The interstellar medium produces absorption lines in the spectra of stars.
These can be distinguished from stellar absorption lines through:
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 components
b) Small line width (too low temperature; too low density)
c) Multiple components (several clouds of ISM with different radial velocities)

10. Structure of the ISM HI clouds: Hot intercloud medium:
The ISM occurs in two main types of clouds:
HI clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.

11. The Various Components of the Interstellar Medium
Infrared observations reveal the presence of cool, dusty gas.
X-ray observations reveal the presence of hot gas.

12. Shocks Triggering Star Formation
Henize 206 (infrared)

13. The Contraction of a Protostar

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

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

16. 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

17. Protostellar Disks and Jets – Herbig-Haro Objects (II)
Herbig-Haro Object HH34

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

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

20. 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

21. 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.

22. 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).

23. 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

24. Hydrostatic Equilibrium (II)
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.

25. 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

26. 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

27. 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
Rotation also offers some support against pressure perpendicular to the axis of rotation.
A star’s mass (and chemical composition) completely determines its properties.
That’s why stars initially all line up along the main sequence.

28. 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.

29. 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.

30. The Lifetimes of Stars on the Main Sequence

31. The Orion Nebula: An Active Star-Forming Region

32. The Trapezium
The 4 trapezium stars: Brightest, very young (less than 2 million years old) stars 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

33. 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
Visual image of the Orion Nebula