1. Stellar Evolution The birth, Life and Death of stars
Assigned Reading: Chapter 12 + Chapter 13

2. The main sequence begins as soon as the star is supported by hydrogen fusion
The main sequence exists because stars balance their weight with energy outflows, produced by nuclear fusion in their core

3. A main-sequence star can hold its structure for a very long time. Why
A main-sequence star can hold its structure for a very long time. Why? Time = c2 M / L = c2 M / M3.5 = 1 / M2.5
Gravitational Contraction
Thermal Pressure

5. Main Sequence Stars
Main Sequence stars are all fusing H to He in their cores.
Life time of a star is determined by its mass.
Nature makes more low-mass stars than high-mass stars. Low-mass stars also live longer. That is why there are a lot more low-mass stars.
KEY QUESTION:
What happens after the main sequence (when hydrogen in the core runs out)?

6. Low-End of Main Sequence
Most common stars, but very hard to see
This one is CHRX 73 A+B, a 0.3 Mo red dwarf plus a 15 MJ brown dwarf

7. High-End of Main Sequence
Very luminous byt very rare
Stars.
Very hard to measure the mass
Also, very hard to find stars with
M>100 Mo.
Large mass ejection
This one is Eta Carinae: two
Stars, one of 60 Mo and the
The other of 70 Mo.

8. When core hydrogen fusion ceases, a star leaves the main sequence and becomes a giant
The thermal pressure in the core can no longer support the weight of the outer layers.
The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion.
This fusion takes place at very high temperatures and the new thermal pressure causes the outer layers to expand into a giant star.
Both the cooling/collapsing inert He core and the H-burning shell contributes to energy output. Star overproduces energy: it expands, surface cools, and becomes a luminous giant

9. Anatomy of a Star that is leaving the Main Sequence
Hydrogen burning core
shell
Hydrogen fuel
ABSOLUTELY NOT
IN SCALE:
In a 5 Mo star,
if core has size of a
quarter, envelope has
size of a baseball
diamond.
Yet, core contains
12% of mass
Helium “ash”

10. Up the red giant branch
Eventually, hydrogen will burn only in the outer parts of the mostly-helium core. The star will swell to enormous size and luminosity, and its temperature will drop, becoming a red giant.
Sun in ~5 Gyr
Sun today

11. How does the Helium core push back?
As matter compresses, it becomes denser (and heats up!)
Eventually, the electrons are forced to be too close together. A quantum mechanical law called the Pauli Exclusion Principle restricts electrons from being in the same state (i.e., keeps them from being too close together).
The resulting outward pressure which keeps the electrons apart is called electron degeneracy pressure – this is what supports the core
Stars with M > 3 Mo never develop degenerate He core
Indistinguishable particles are not allowed to stay in the same quantum state.

12. Helium fusion begins at the center of a giant
While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon (if M > 0.5 Mo)
Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.
He fuses through a number of reactions, generally referred to as the “3-a” reactions
He + He + He = C + energy
… and produces an element “crucial” to our existence:
CARBON
For a star with M<Msun, the carbon core never gets hot enough to ignite nuclear fusion (star needs 600,000,000 K to do so).

13. After helium fusion gets going…
The Sun will expand and cool again, becoming a red (super) giant. Earth, cooked to a cinder during the red giant phase, will be engulfed and vaporized within the Sun. At the end of this stage, the Sun’s core will consist mostly of carbon, with a little oxygen.

14. For low mass stars

15. Planetary Nebula At the center of the nebula there is the dying star.
Destiny of stars with roughly M < 8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD

16. Nuclear burning in massive stars (>4 Mo)

18. The lead-up to disaster in massive stars
Iron cores do not immediately collapse due to electron degeneracy pressure.
If the density continues to rise, eventually the electrons are forced to combine with the protons – resulting in neutrons.
What comes next … is core collapse.

19. Massive Star Explosions: Supernovae
The gravitational collapse of the core releases an enormous amount of energy.
All the shells ignite, and the stars literally explodes
It can fully disintegrates, nothing is left of it (Type Ia)
Or a neutron star or black hole (core cadaver) is left (Type II)
100 times the total amount of energy produced by the Sun in its lifetime is released in a matter of seconds.
For a few days, the star is ~as luminous as a whole galaxy!!!
Then luminosity decays in following months:
E.g. A Type Ia SN dims by a factor of 100 in about 170 days
Chart of light intensity versus time is called “Light Curve” (see fig13-13, page 300).

20. Supernova 1987a before/after

21. Supernova Remnant Cassiopeia A

22. Stellar Evolution in a Nutshell
Mass controls the evolution of a star!
M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mcore > 3MSun

25. End Products of Stars
M < 0.08 Msun  Brown dwarf (fusion never starts)
0.08 Msun < M < 8 Msun  White dwarf
Helium White Dwarf: 0.08 Msun < M < 0.4 Msun
Carbon White Dwarf: 0.4 Msun < M < 4 Msun
Oxygen-Neon White Dwarf: 4 Msun < M < 8 Msun
M > 8 Msun  Supernova II+ neutron star or a black hole
If Supernova Ia (explosion of the degenerate core), the whole star disintegrates, nothing is left of it

26. All of the Heavy Elements are Made During Supernovae

27. The Key Point in the Production of Elements in the Universe
Hydrogen and Helium are initially created in the Big Bang
Stars process Hydrogen and Helium into heavier elements (elements lighter than iron) during the nnuclear-burning phase of their lives.
Elements heavier than iron are generated only in the deaths of high mass stars (supernovae).
We were all once fuel for a stellar furnace. Parts of us were formed in a supernova.

28. The evolution of stars Points to remember:
How is the helium core of a star supported? (electron degenerate pressure)
What causes the expansion of a star to become a red giant? (shell burning, energy over-production)
What is a supernova? (the consequence of an unstoppable gravitational collapse)
When does a massive star explode as a supernova? (once it cannot support itself anymore)

29. Where does the energy come from in a star like the Sun? Why?
Nuclear fusion.
What elements can such a star produce?
Carbon and Oxygen.
Why cannot the star produce heavier element?
not enough mass to reach the temperature.
Why more massive stars have higher central temperatures?
high pressure to balance the gravity.
What is the heaviest element that can be fused into in a star? Why?
Iron, which is the most bound nucleus.

30. Observing Stellar Evolution
How can we see stellar evolution in action?
Stellar Clusters, a group of coeval stars, I.e. all born at the same time, but with different masses (hence different life time)
How can one estimate the age of a stellar cluster?
By looking at the HR diagram of the cluster, namely at the evolutionary phase of stars of the same age but with different mass

31. Our First Measurement of Age Star Clusters
Open cluster: 103 stars, up to 30 pc
in size, found in disk of galaxy.
All have mostly young stars
Globular cluster: up to 106 stars and 150 pc in size, in disk and halo of galaxy. All have old stars

32. Why are clusters useful to astronomers?
All stars in a cluster are at about same distance from Earth.
All stars in a cluster are of about the same age.
Clusters therefore are natural laboratory in which mass, rather than age, of stars is only significant variable.

33. The Hertzsprung-Russell Diagram
More mass, more fuel, very fast burning.
Shorter
Lifetime of Star
Less mass, less fuel, slow, steady burning.
Longer
How do we know the age of a star?

34. The H-R Diagram of a Cluster
We can date a cluster by observing its population of stars.
All these stars in the cluster have burned themselves out!
Turn-Off point:
Age indicator
The oldest clusters known have been measured to be ~14 billion years old.

35. Variable Stars Chepeids RR-Lyrae Variability due to Instability
Variability is PERIODIC
Instability caused by presence of ionized He
More luminous variable stars have large Period
Variability is EXTREMELY USEFUL, because it is an absolute distance indicator

36. Cepheid Variable Stars
Cepheid variable stars have variable brightness that is very regular.
The period of the variation can be from days to weeks
Pulsation due to instability: He ionization layer acts a energy sponge
it seems to be a reliable indication of the star’s luminosity!

37. Cepheids: the Period-Luminosity Relation
Henrietta Leavitt
Henrietta Leavitt ( ).
Luminosity=4D2B

38. Standard Candles
If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness.
An object that has a known luminosity is called a standard candle.

39. What is burning in stars?
Gasoline
Nuclear fission
Nuclear fusion
Natural gas

40. Survey Questions How is the helium core of a star supported?
What causes the expansion of a star to become a red giant?
What is a supernova?
When does a massive star explode as a supernova?.

41. The death of stars and stellar remnants
He white dwarfs M<0.4 Mo
C White dwarfs (planetary nebulae) <M<4 Mo
Carbon-Neon-Silicon White Dwarfs <M<8 Mo
Two types of Supernovae M>8 Mo
Type Ia, the exploding stars disintegrates
Type II (core collapse), the star leaves remnants:
Neutron stars (basically, a neutron white dwarf, I.e. degenerate gas of neutrons)
Black holes

42. Stellar evolution can be deeply altered in a binary system: mass transfer
An originally massive star can loose mass
and become less massive (longer life)
A nearly dead star can be rejuvenated by
accretion of fresh fuel

43. A Type of Stellar Remnant: Planetary Nebulae
At the center of the nebula there is the dying star.
This is a white dwarf, where small and hot: it photo-ionizes the nebula
The nebula formed out of the mass loss during the red super-giant phase.
Destiny of stars with roughly M < 8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD

44. NOVAE: an example of binary stars
Novae are nuclear explosions on the surface of white dwarf and neutron stars
Brightness changes by a factor of 4000!

45. Two basic types of supernovae
Type Ia – from the thermonuclear detonation of a white dwarf with M ~ 1.4 Msun after accreting matter from its companion.
(1.4 Msun is called Chandrasekhar limit)
Type II – from core collapse of a massive star  neutron star or black hole.

46. Type Ia: White Dwarf Supernova
If a White Dwarf accretes enough matter from a companion star, it will eventually nova.
If, after the nova, it does not shed all the mass it gained, it will continue to accrete mass until it novas again.
If this process continues (accretion, nova, accretion, nova, etc.) such that the WD continues to gain mass, once it has a mass of 1.4Msun, the core will collapse, carbon fusion will occur simultaneously throughout the core, and the WD will supernova.

47. How might it be possible for a White Dwarf to flare back to life?

48. Remnant from a Type Ia supernova: A lot of irons!

49. Another distance indicator: White Dwarf (Type Ia) Supernovae in distant galaxies.
L=4D2 B

50. A 25 Mo star burns:
H in 7 million years
O in 6 month
Si in 1 day
Then… Booom
Core collapse in
~ sec

51. Remnant from a Type II supernova Crab Nebula
The supernova explosion that created the Crab was seen on about July 4, 1054 AD.

52. Another Type of Stellar Remnant: Neutron stars
A neutron star --- a giant nucleus --- is formed from the collapse of a massive star.
Supported by neutron degeneracy pressure.
Only about 10 km in radius.
A teaspoon full would contain 108 tons!
Very hot and with very strong magnetic field

53. Jocelyn Bell
Neutron stars
discovered as
pulsar

54. Pulsar, as a light house A fast rotating, magnetized neutron star.
Emits both strong radiation (radio) and jets pf high-energy particles.
Jets not very well understood; their existence is due to the rotation and to the presence of magnetic fields

55. The Limit of Neutron Degeneracy
The upper limit on the mass of stars supported by neutron degeneracy pressure is about 3.0 MSun (predicted
by Lev Landau)
If the remaining core contains more mass, neutron degeneracy pressure is insufficient to stop the collapse.
In fact, nothing can stop the collapse, and the star becomes a black hole.

56. Black holes
When the ball of neutrons collapses, it keep its mass, but shrinks to smaller and smaller sizes.
No amount of pressure can stop the collapse, because in those extreme situations, pressure itself contributes more to gravity than it does of opposing it.
It forms a singularity – a region in space with the mass of the parent material, but with virtually null volume and hence potentially infinite gravitational field.
In a singularity gravity is so strong, i.e. the space is so tightly
curved, that nothing can escape, even light!
The most interesting aspects of a black hole are not what it’s made of, but what effect is has on the space and time around it.

57. Review Questions What are type-Ia supernovae?
What do a type-II supernova leave behind?
Why does a neutron star spin fast?
What is a pulsar?