1. WHERE
STARS
ARE
BORN

2. The Interstellar Medium (ISM)
The space between the stars looks empty . . .
But it’s not!

3. Gas
* Mainly hydrogen + helium
* Avg density  1 – 10 atoms/cm3
* 99% of ISM
Dust
* Tiny grains (‘smoke’)
~ cm
* Silicates, carbon, ice (?)
* Avg density  1 grain/million m3

4. ‘Whirlpool’
Galaxy

5. Looking at a galaxy from the inside.

6. Looking for Gas

7. Emission Nebulae
Rosette Nebula
* Ultraviolet
photons from hot
stars are absorbed
by gas atoms –
cause gas to glow.

8. Great Orion Nebula

9. North America Nebula
Trifid Nebula

10. Hot
Stars
H atoms UV
Earth

11. Hydrogen atom UV
Electron ejected
Electron recaptured
Photon emitted

12. Red Balmer
emission Line

13. Molecules – mainly hydrogen (H2).
Carbon monoxide (CO) in Orion

15. Looking for Dust

16. Reflection Nebulae
Pleiades
Star Cluster
* Glows due to scattered
(reflected) starlight.
* Dust scatters blue light more efficiently than red light.

17. Dark Nebulae
Molecules
abundant
here

18. Emission nebula
‘Horsehead’ Nebula
Reflection
nebula

19. Star Formation

20. * Results from collapse of a molecular cloud.
Cloud collisions
Supernova blast wave
Expanding emission nebula
Galactic density wave

21. p. 271

22. Central temp  10 million K: Hydrogen fusion ignites ‘protostars’
Molecular
Cloud
Shrink & heat
Central temp  10 million K:
Hydrogen fusion ignites
‘protostars’
(on main sequence)

23. Star birth in the
Eagle Nebula

24. Star birth in the
Trifid Nebula
Forming star?

25. * Accretion disk may form around young stars . . .
Bipolar Flow

26. Bipolar
Flow
Disk
p. 273

27. Protostellar Disks

28. ‘Evolutionary tracks’ on the HR diagram:
Time required
for contraction
to main sequence
depends on mass.
pgs

29. Limits to Star Formation

30. Protostar mass < 0.08 M: No hydrogen ignition:
‘Brown Dwarf’

31. M > 100 M
Star disrupted
by the pressure
of photons.
The ‘Pistol’ Star