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. Three kinds of nebulae 1) Emission Nebulae (HII Regions)
1) Emission Nebulae (HII Regions)
A 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
The Trifid Nebula
NGC 2246

4. 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)

5. Emission and Reflection Nebulae

6. 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

7. Hubble Telescope Pictures
Slide show on internet titled 20 years of Hubble pictures.
Many resources like this on the Hubble site
Start for to find many different resources which can be downloaded.

8. Cone Nebula - Monoceros

9. Cats Eye Nebula

11. Matter ejection from star

12. Orion Nebula

13. Butterfly Nebula

14. Large Gas and Dust cloud

15. Trifid Nebula

16. Pillars of Creation nebula

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

18. 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)

19. 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

20. Shocks Triggering Star Formation
The gas in the ISM needs to be compressed in order to collapse and form stars: Shocks traveling through interstellar space can do this.

21. Shocks Triggering Star Formation

22. Compression of the ISM by Winds from Hot Stars

23. The Contraction of a Protostar

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

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

26. 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

27. 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

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

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

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