1. Where do gold earrings come from?
Stellar Evolution
Where do gold earrings come from?

4. Interstellar Clouds Where new stars form
Orion Nebula – copyright Robert Gendler
Interstellar Clouds
Where new stars form

5. Concept Test These interstellar clouds appear red because they are:
hot and things that are hot glow red.
ionized hydrogen which appears red because the brightest emission line is red.
cold and things that are cold appear red.
full of red stars.
Composed of dust between the observer and the region which blocks the blue light, but lets the red light through.

6. Cold Dark Clouds
If dust clouds block light, then inside thick dust clouds there should be no light at all.
Without light, there is little energy.
With little energy, gas inside is very, very cold.

7. Gravity vs. Pressure
Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure).
Gravity comes from the mass of the cloud or star.
Pressure comes from the motion of the atoms or molecules.
Think of hot air balloons.
The hotter the air, the bigger the balloon.

8. Star Formation Remember: Cold interstellar clouds:
No heat = no velocity = no outward pressure.
Gravity wins.
Gas begins to contract.
HOTTER
COOLER

9. How to Make a Star Star “turns on” when density and pressure1 2 3
Star “turns on” when density and pressure
at the core enough to fuse H into He..

10. The Main Sequence
A star is a delicate balance between the force of gravity pulling in, and pressure pushing out.
Stars on the main sequence fuse hydrogen in their core to produce thermal pressure.
Longest phase of a star’s life.

11. The Main Sequence Stars characterized by what holds them up.
90% held up by heat of Hydrogen fusion
4H  He + Energy

12. What then?
When the hydrogen in the core is almost consumed the balance between gravity thermal pressure pushing out and gravity pushing in is disturbed.
The structure and appearance of the star changes dramatically.
What happens then, depends on the star’s mass.
Two cases:
Low-mass (< 8 x mass of Sun)
High-mass (> 8 x mass of Sun)

13. Helium Ash Heavier elements, sink to the “bottom.”
After 10 billion years, core is “choked” with helium “ash”.
H  He continues in shell around non-burning core.

14. The Red Giant Branch Without fusion pressure in core:
Helium core collapses (no counter to gravity)
Density in core increases.
3He  C + Energy in core
4H  He + Energy in shell
Extra energy results in extra pressure. Star expands.
The star gets bigger while its outside gets cooler.

15. The Onion Sun Red Giant Stars Layers of: Non-fusing H Fusing H
Fusing He
Non-fusing C “ash”

16. …And the Solar System? In a few million years: “a last perfect day”:
Sun becomes slightly brighter
Ocean’s begin to evaporate
“Hot House” Earth
In a few billion years:
Sun swells up
Engulfs the inner Solar System
Certain death for terrestrial planets
Possible “spring” on the Jovian ocean-moons!

17. Death Core is contracting and heating.
Surface is cooling and expanding.
Will it finally become hot enough in core for Carbon to fuse?
For the Sun: No.
Gravity keeps contracting the core: 1000 kg/cm3!
What stops it?
Electron degeneracy pressure!

18. Electron Degeneracy
Pressure from motion of atoms

19. Electron Degeneracy
Pressure from electron shells

20. Where are we now? Core dead – nothing happening.
Shells – burning H and He, but soon stop too.
Outside atmosphere of star still cooling and expanding.
…and expanding
Force of radiation from burning shells blows the atmosphere away.

21. NGC3242 – HST – Bruce Balick

22. M57 – Ring Nebula

23. M27 – Dumbbell Nebula – copyright VLT, ESO

24. Cat’s Eye

25. Eskimo Nebula

26. Hourglass Nebula

27. A million times denser than lead Slowly cool off
White Dwarf
Mass of Sun
Radius of Earth
Hot as Sun’s core
A million times denser than lead
Slowly cool off
NGC2440 – HST – Bruce Balick

29. High-Mass Stars Think back to the first carbon core.
How they get from main sequence to the carbon core stage is a little different.
Now however, there is enough mass that it becomes hot enough to fuse carbon?
Hot enough to eventually fuse lots of elements.

30. The Iron Core 4H  He + Energy 3He  C + Energy C + He  O + Energy
The ash of one reaction, becomes the fuel of the next.
Fusion takes place in the core as long as the end result also yields energy.
This energy causes pressure which counters gravity.
But Iron doesn’t fuse.

31. Core-Collapse Iron core – no outward pressure. Gravity wins!
Star collapses rapidly!
Electron degeneracy can’t stop it.
Atomic structure can’t stop it.
Electrons and protons crushed together to produce neutrons.
Neutrons pushed together by force of gravity.

32. Supernova

33. Supernova

34. Supernova

35. Supernova

36. Supernova
The result of the catastrophic collapse is the rebound and explosion of the core.
From start of collapse to now: 1 second!
Matter thrown back into the interstellar medium.
Matter rushing outwards, fuses with matter rushing inwards.
Every element after Fe is made in the instant of a supernova!

37. M1 – Crab Nebula – copyright VLT

38. Veil Nebula – Lua Gregory (English ’05)

39. NGC 4526 – 6 Million parsecs away

40. Discussion Question For Wednesday:
What is our cosmic connection? How does it change (or does it) your opinion of the world around you?
Consider the statement, “The Universe makes me feel so small.” In what way is this statement true? In what way is this statement false. Defend or refute the statement.