1. Stellar Classification/Evolution

2. Learning Objectives 1. Review of HR Diagram 2. Some Stellar Patterns
3. LOW MASS STARS
Creation
Fusion
Evolution

3. Classification (Review of HR Diagram
Measures luminosity and temperature

4. Some common stellar relationships
1. Higher temperatures burn blue
2. Lower temperatures burn red
3. As mass increases, luminosity increases
(And vice versa)
4. On the main sequence, as temperature increases, luminosity increases

5. Mass/Luminosity Relationship
It should make sense that…
If a star is larger, it has more surface area to emit light
Thus….
It has much more Luminosity

6. Beginning of Stellar Evolution
We already have a good idea about this… Remember “Formation of Solar System…”

7. 1. Starts with a Interstellar Gas Cloud (Nebula) that condenses into…
As the cloud condenses, spinning increases, heat increases, pressure increases.
PROTOSTAR FORMED
The Pillars of Creation (Eagle Nebula)

8. 2. Gravity contracts cloud
As gravity works, the protostar spins faster (Conservation of angular momentum)
This spin creates more and more heat…
When 10 million Kelvin is reached, FUSION occurs and a star is born!

9. On the HR Diagram

10. Different mass stars
Depending on the mass and size of the condensing gas cloud, each protostar enters a different part of the HR diagram
O Stars- Most Massive (and hottest)
M Stars- Least Massive (and coldest)

11. Star Formation

12. 3. Fusion / Main Sequence
A star must always battle gravity its entire life If there is no opposite force to stop gravity, the star will collapse in on itself

13. Hydrostatic Equilibrium
When a star creates enough outward pressure to balance gravity
Molecules heat up
move faster
and thus
increase pressure
Eventually, pressure matches Gravity

14. When Fusion energy = Gravity A star is born!
Considered to be on the main sequence

15. 3. Fusion- The Proton-Proton Chain
Low Mass Stars- first fuse HYDROGEN into HELIUM
1 Proton
0 Neutrons
Mass of 1
2 Protons
2 Neutrons
Mass of 4.0

16. Proton-Proton Chain Four Hydrogen make
One Hydrogen-2…which fuses with one more Hydrogen to make
Helium-3, and two Helium-3 make
Regular Helium

17. Let’s Simplify… 6 H He + 2 H + energy
Just know 6 Hydrogen (6 protons) combine to make 1 Helium (with 2 leftover Hydrogens)
6 H He + 2 H + energy
Six hydrogen: mass of 6
One Helium: mass of 4
Two leftover hydrogen: mass of 2

18. Fusion Video

19. 3. Fusion and Hydrostatic Equilibrium
Fusion continuously turns H into He in the core, and He moves inward

20. 4. Core Exhaustion
Eventually, the H runs out in the core, or stops burning
‘Runs out of fuel’
Because of this, the star loses its pressure pushing outward
Gravity condenses the material inside the star

21. 5. Red Giant Phase (1. hydrogen shell burning)
This huge pressure results in much higher temperatures (100,000,000 K)
The hydrogen just outside the core then burns much, much, faster

22. 5. Red Giant Phase
As a result of this HIGH temperature and pressure increase, the star’s fusion pressure is higher than gravity…

23. 5. Red Giant Phase
And becomes much larger

24. What would happen to us?
The sun would strip us of our atmosphere, boil our oceans, and probably engulf the earth.

26. 5. Red Giant Phase (2. Helium Fusion)
Star then starts fusing Helium into Carbon
Helium
Carbon
2 Protons
2 Neutrons
Mass of 4
6 Protons
6 Neutrons
Mass of 12

27. 3 Helium  1 Carbon plus energy
Helium Fusion
3 Helium  1 Carbon plus energy

28. Overview: Main SequenceRed Giant
H burns and the runs out. Contraction from gravity.
Contraction heats H shell- burns quickly, expands
Temp increases to where He can fuse into C
A low mass star’s red giant phase. Near the end.

29. Let’s review Protostar Condenses enough to start Fusion
Is in hydrostatic equilibrium
Main Sequence Star
Fuses 6 Hydrogen into 1 Helium in core
Core exhausts
Material contracts/heats up
Burns H quickly, expands
3. Red Giant Phase
Fuses He into C

30. 6. Planetary Nebula Stage
As the sun continuously gets hotter and fuses He into C, the outer layers are ‘boiled off.’
This decreases mass, which decreases pressure, which decreases temperature
Fusion shuts down and all that is left is the ‘core’ which is incredibly bright and still incredibly hot

31. 6. Planetary Nebula
The outer layers ejected into space are ‘illuminated’ by the white hot core

32. 6. Planetary Nebula

35. 7. White Dwarf Stage
White dwarf- the remaining core of the star, made mostly of carbon and helium (the result of fusion)
VERY DENSE
A teaspoon of a White Dwarf would weigh 5 tons on earth!

36. White Dwarves
Sirius, the white dwarf

37. More Overview

38. Low Mass Star on HR

39. Binary Star Systems
Tatooine (Fictional home planet of Luke and Anakin Skywalker in Star Wars)

40. Binary Star Systems
Binary Stars: Two stars that orbit each other by a fixed point
More common than you think, 80%

41. What the orbit looks like…

42. Can a Planet have two suns?

43. Some pictures… Sirius (Really Sirius A and B)
Cygnus X-1 (It is thought one of the stars became a black hole)

44. So, what happens if one star ‘dies?’
Lets assume the LEFT star goes to the red giant phase (hydrogen shell burning)
AND
The RIGHT star is a white dwarf…

45. Continued
Red giant expands past ‘Roche Lobe,’ and enters white dwarfs gravitational pull

46. Binary Stars
White dwarf gains more mass increases pressureincreases heat
Fusion re-awakens in the white dwarf!

47. Nova
This extra mass is then expelled out into space (because fusion started again)
This happens many times a year! (20x a year)
Nova that happened in The white dwarf is in the middle.

48. Or a Type I Supernova

49. Type I Supernova
If a white dwarf absorbs more than 1.4 solar masses, it will explode!

50. Type I Supernova

51. Binary Stars and other Astronomy Topics

52. High Mass Stars… Take the same first steps of a low mass star…
Interstellar Gas CloudProtostarMain SequenceRed Giant….
But then things get a bit more complicated

53.
Star sizes and scale

54. Low vs High Mass Stars

55. Low vs. High Mass Stars Low Mass Stars- 200 Billion Years
Average Mass Star – 10 Billion Years
High Mass Star – 10 Million Years
Larger Stars die quicker (more energy, higher molecule speeds)

56. Hydrogen shell burning
5,000,000 yrs
2. Red supergiant
Hydrogen shell burning
1. Core burns Hydrogen
106 yr

57. 3. Blue Supergiant (Helium fusing into Carbon in CORE)
4. Red supergiant
Helium Shell burning

58. C Burning Core
5. More collapse, Carbon now fuses into oxygen

59. This process continues…
Fusion (shell), collapse, fusion (core), collapse
Each collapse creates higher temperatures (which allows for fusion of heavier elements)

60. These heavy element fusion periods become shorter and shorter
As you can see…
These heavy element fusion periods become shorter and shorter
Stops at Iron (Fe)

61. Why does it stop at Iron?
Fusing Fe requires more energy than it releases, so it does not happen

62. After fusion stops…
Like low mass stars, once fusion stops a great collapse occurs…
But unlike low mass stars, there is an incredible amount of mass to collapse in on itself

63. The collapse…
Is so great, all protons and electrons are smashed together, and everything turns into neutrons

64. Supernova Some large mass stars just go neutron star
More massive ones go supernova

65. Supernova
A result from a mass collapse and giant explosion of star

66. Supernovas A remnant of a supernova A before and after of a supernova
Beginning of a supershell
A before and after of a supernova

67. So…

68. On HR diagram