1. Looking at the Interstellar Medium
A Star Is Born
Looking at the Interstellar Medium

2. The Stuff in Space Material exists between the stars and planets
“Building Blocks” for more stars and planets
Interstellar Medium
Composed of Gas and Dust

3. Interstellar Medium Seen in telescopes
Large clouds of gas and dust are stellar “nurseries”
ISM is clumpy!
Obscures objects beyond it

4. Interstellar Gas Gas is mostly individual atoms
Individual neutral atoms and ions
Molecules
Electrons
Hydrogen is main component
Very tenuous, doesn’t block light
Some parts cool, some parts hot

5. Interstellar Dust More complex composition 1% of ISM
Clumps of atoms and molecules
Much larger than gas particles so…
Can block light
Composition is not well known

6. Interstellar Dust Light passes through dust cloud
Extinction-dimming of light
Longer Wavelengths pass through
Short Wavelengths get absorbed
Reddening- scattering of blue light
Red light makes it through
Changes star’s apparent color

7. Dust and Gas
Dust is believed to come from mass loss winds in stars (like solar wind)
Gas of wind can cool and solidify
Dust grains provide coagulation seeds for molecules

8. Evidence of Gas and Dust
Observed Nebulae (clouds)
Dust and Gas form different types of nebulae
4 basic types
Emission (Bright) nebula
Dark nebula
Reflection nebula
Molecular Clouds

9. Emission Nebula Example: Orion Nebula
Spectra has emission lines (hot gas)
Does not shine under it’s own light
Powered by hot stars
H II region (ionized hydrogen)

10. Emission Nebula OB associations
Form H II region hot spot on molecular cloud
Drives new star formation
Reddish hue is from Hydrogen
H II regions are star “nurseries”
Very bright!

11. Emission Nebula
Heart and Soul Nebulae
Astronomy Picture of the Day

12. Emission Nebula
The Eagle Nebula and a close up of a star forming region
Astronomy Picture of the Day

13. Molecular Clouds Composed mostly of H2 (hard to detect)
Use other molecules as tracers
80 known ISM molecules
Many are organic molecules
Associated with H II regions
Majority of ISM is here

14. Molecular Clouds 10’s of ly across (6 trillion miles= 1ly)
Cool clouds (Dark)
Occur in huge complexes
Contain enough gas to make millions of stars
complexes in our Galaxy

15. Molecular Cloud
Barnard 68
APOD

16. Molecular Cloud Horsehead Nebula
Note: there are several types of nebulae in this panorama
APOD

17. Dark Nebula Example: Snake Nebula Contain gas and dust
Dust blocks light
Cool (10’s K)
Larger than our Solar System
Also, dust lanes w/ H II regions

18. Dark Nebula
Snake Nebula
APOD

19. Reflection Nebula Gas and Dust Absorption line spectra (stars)
Doesn’t generate own light
Scatters blue light from starlight passing through
Nebula appears blue (like sky)
Example: Pleiades Nebula

20. Reflection Nebula
Witch Head Nebula
APOD

21. A Panorama in Orion Can you ID the different types of nebula here?
APOD

22. Neutral Hydrogen Presence was suspected H II regions come from it
Not observed until 1951
Need to observed from its own radiation
Low-energy Radio emissions from the gas itself

23. 21-cm Radiation From single electron orbiting nucleus
Not from excitation, from spin of electron
2 possible configurations
Parallel
Anti-parallel
Lower energy one preferred
Spin flipping emits 21-cm photon

24. Coronal Interstellar Gas
Very hot gas
Highly Ionized
Very low density
Exists between clouds
Why?

25. Star Birth
Star’s life is a dance between gravity and radiation pressure
All stars have a similar origin
Cold, dark molecular clouds
Collapse to form stars

26. Cloud Collapse Giant molecular cloud
Something to trigger collapse (shockwave)
Cloud begins to collapse under it’s own weight
Jean’s Instability
Cloud will begin to heat up as it collapses

27. Cloud Collapse Heating causes outward pressure
Balances out gravity pushing inward
Collapse is “lumpy”, fragmentation
Pockets collapse faster (denser)
Centrally dense region is where star will eventually form

28. Cloud Collapse Fragmentation occurs in several ways
Dozens of Massive Stars
Hundred or Thousands of Sun-like Stars
No evidence for single star formation
Single stars must escape after formation
Collapse can occur with or without rotation

29. Cloud Fragmentation Sun sized star comes from
2 solar mass fragment
100 times radius of Solar System
Less than 100K
Fragmentation ceases as density of each fragment increases
Interior becomes opaque
Radiation is trapped

30. Fragmentation to Protostar
10,000 + years passed
Central part 10,000K
Outer part cool
Dense, opaque central region=Protostar
Still contracting, material raining down on it

31. Protostar (Sun predecessor)
1,000,000 years have passed
Not hot enough for P-P chain
Still about size of Mercury’s orbit
More luminous than Sun (bigger)
Surrounded by a dusty shroud
Vaporizes nearby dust

32. Protostar Protostar Dust Free Zone IR photon
Outer Envelope of Gas and Dust

33. Protostar to Star Protostar first appears on HR diagram
Protostar loses shroud
Vaporizes
Falls onto Star
Blown away by wind
Contracts, Luminosity , Temp 
Hayashi track on HR Diagram
Violent Surface Wind (T Tauri Star)

34. T-Tauri Stars Exhibit strong winds Bipolar flows
Clear gas and dust away from young star so it is at last visible
Where the outrushing gas impacts stationary gas in the ISM a bright hot spot appears
Herbig-Haro object

35. T-Tauri Stars Also appear to vary in brightness
Likely associated with star’s magnetic field, much like the active Sun
Have “star spots” like sun spots
Are still collapsing to final size
Larger in size and therefore brighter than they will be as main sequence stars

36. T-Tauri Star
A false color image of the T-Tauri system
Note jet
APOD

37. T Tauri Tantrum

38. HH object
APOD
Hubble Heritage

39. Pre-Main Sequence Star
10,000,000 Years
Now a “True Star”
P-P Chain has begun
Larger and Cooler than Main Sequence Star
Still slowly contracting

40. At last, the Sun! Contraction continues Central Temp=15,000,000K
Outward pressure balances inward gravity
Hydrostatic Equilibrium (HSE)
Contraction Stops, Balance Reached
Main Sequence Star!
All main sequence stars are in HSE

41. Massive Stars Steps occur faster
Collapse of Cloud occurs in similar way
Central part still collapses faster
Fragments are larger
No T Tauri phase
Chain Reaction from OB association
>100 M Gravity can’t hold together

42. Failed Stars Some cloud fragments are too small Don’t get hot enough
H-fusion never occurs
Warm due to collapsing
Brown dwarfs
Cooling objects
<0.08M (Jupiter)

43. Collapse with Rotation
Rotation during collapse leads to orbiting clumps around protostar
These clumps can attract more material via gravity and form planetismals
These could be a future solar system

44. Summary All stars have a “cloudy” beginning Different types of nebula
Stars collapse from clouds