Slide 1

Death of StarsDeath of Stars

“All hope abandon, ye who enter here” Dante

Slide 2

The Dumbbell nebula (M76): a dying sun-like star

Slide 3

Life of stars:Gravity is everything

• Stars are born due to gravitational collapse of gas clouds• Star’s life is a battle between thermal pressure generated by

nuclear reactions and gravity

• Eventually, a star loses this battle, and gravity overwhelms

Slide 4

What happens when all hydrogen is converted into helium in the core??

Mass defines the fate of the star

Slide 5

Evolution on the Main Sequence

Luminosity L ~ M3.5

A star’s life time T ~ energy reservoir / luminosity

T ~ M/L ~ 1/M2.5

Energy reservoir ~ M

Massive stars have short

lives!

Slide 6

Evolution on the Main Sequence

Zero-Age Main

Sequence (ZAMS)

Main-Sequence stars live by

fusing H to He.

Finite supply of H => finite life time.

MS evolution

Slide 7

1. Accumulation of helium ash in the core

Helium burning requires higher temperaturesThe star loses the ability to generate nuclear energy

Slide 8

2. Core collapses and gets hotter, while the envelope expands and cools down

Note the hydrogen fusion in a shell surrounding the core!It is now hot enough there.

Slide 9

Evolution off the Main Sequence: Expansion into a Red Giant

Hydrogen in the core completely converted into He:

H burning continues in a shell around the core.

He Core + H-burning shell produce more energy than needed for pressure support

Expansion and cooling of the outer layers of the

star Red Giant

“Hydrogen burning” (i.e. fusion of H into He) ceases in the core.

Slide 10

Red Giant Evolution

4 H → He

He

H-burning shell keeps dumping He

onto the core.

He-core gets denser and hotter until the

next stage of nuclear burning can begin in

the core:

He fusion through the

“Triple-Alpha Process”

4He + 4He 8Be + 8Be + 4He 12C +

Slide 11

If M > 0.4 Msun, the temperature reaches 100 million K Nuclear fusion of helium and heavier elements starts

Carbon and then oxygen are produced

Slide 12

Red Dwarfs: no red giant phase

Stars with less than ~ 0.4

solar masses are completely

convective.

Hydrogen and helium remain well mixed throughout the entire star.

No phase of shell “burning” with expansion to giant.

Star not hot enough to ignite He burning.

Mass

Slide 13

Sunlike Stars

Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.

Expansion to red giant during H burning shell phase

Ignition of He burning in the He core

Formation of a degenerate C,O core

Mass

Slide 14

In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.

Slide 15

Outer layers expand due to radiation pressure from a hot core

The star becomes a Red Giant

• Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times

Slide 16

Future of the Sun

(SLIDESHOW MODE ONLY)

Slide 17

Supergiant

Slide 18

A star leaves the main sequence and becomes a red giant

Slide 19

Simulated evolution of the main sequence

Slide 20

While outer layers are expanded, inner helium core contracts, and its temperature rises

Slide 21

Helium Fusion

When pressure and temperature in the He core

become high enough,

He nuclei can fuse to build heavier elements:

Slide 22

• For stars with M > 4 Msun, helium fusion proceeds gradually as the core heats up.

• For stars with 0.4 Msun < M < 4 Msun, electrons in helium core becomes degenerate and their pressure does not increase with temperature. Helium fusion results in runaway explosion – helium flash.

• In any case, carbon ash accumulates in the core. Core contracts and becomes degenerate. Then helium burns in the shell. Subsequent evolution depends on the core mass. Eventually all reactions stop and the star dies.

Slide 23

Inactive He

C, O

Red Giant Evolution (5 solar-mass star)

Slide 24

Fusion Into Heavier Elements

Fusion into heavier elements than C, O:

requires very high temperatures; occurs only

in very massive stars (more than 8 solar masses).

Slide 25

The Life “Clock” of a Massive Star (> 8 Msun)

Let’s compress a massive star’s life into one day…

12 12

3

45

67

8

9

1011

12 12

3

45

67

8

9

1011

Life on the Main Sequence

+ Expansion to Red Giant: 22 h, 24 min.

H burning

H He

H He

He C, O

He burning:

(Red Giant Phase) 1 h, 35 min, 53 s

Slide 26

The Life “Clock” of a Massive Star (2)

H HeHe C, O

C Ne, Na, Mg, O

Ne O, Mg

H He He C, O

C Ne, Na, Mg, O12 1

23

4567

8

910

11

C burning:

6.99 s

Ne burning:

6 ms 23:59:59.996

Slide 27

The Life “Clock” of a Massive Star (3)

H HeHe C, O

C Ne, Na, Mg, ONe O, Mg

O burning:

3.97 ms 23:59:59.99997

O Si, S, P

H HeHe C, O

C Ne, Na, Mg, ONe O, Mg

Si burning:

0.03 ms

The final 0.03 msec!!

O Si, S, PSi Fe, Co, Ni

Slide 28

Inside Stars

(SLIDESHOW MODE ONLY)

Slide 29

The Fate of Our Sun and the End of Earth• Sun will expand to a

Red giant in ~ 5 billion years

• Expands to ~ Earth’s radius

• Earth will then be incinerated!

• Sun may form a planetary nebula (but uncertain)

• Sun’s C,O core will become a white dwarf

Slide 30

The Final Breaths of Sun-Like Stars: Planetary Nebulae

The Helix Nebula

Remnants of stars with ~ 1 – a few Msun

Radii: R ~ 0.2 - 3 light years

Expanding at ~10 – 20 km/s ( Doppler shifts)

Less than 10,000 years old

Have nothing to do with planets!

Slide 31 p. 192

Slide 32

The Dumbbell Nebula in Hydrogen and Oxygen Line Emission

Slide 33

Slide 34

Slide 35 p. 193

Slide 36 p. 193

Slide 37 p. 193

Slide 38 p. 193

Slide 39

What is left??

A stellar remnant: white dwarf, composed mainly of carbon and oxygen

Slide 40

Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A.

Wobbling motion of Sirius A

1850: a strange star was discovered

Slide 41

Slide 42

Sirius B is very hot: surface temperature 25,000 KYet, it is 10,000 times fainter than Sirius A

It should be very small: R ~ 2 Rearth

Its mass M ~ 1 Msun

It should be extremely dense!

M/R3 ~ 106 g/cm3

Slide 43

White DwarfsDegenerate stellar remnant (C,O core)

Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!

White Dwarfs:

Mass ~ Msun

Temp. ~ 25,000 K

Luminosity ~ 0.01 Lsun

Chunk of WD material the size of a beach ball would outweigh an ocean liner!

Slide 44

All atoms are smashed and the object is supported by pressure of degenerate electrons

Slide 45

Degenerate gas

-The core is compressed until the inter-particle distance = de Broglie wavelength-One particle occupies finite volume in space and in momentum space- we can decrease the particle size by increasing its velocity (energy)- Pauli exclusion principle permits only one particle per each state

Slide 46

secJ102

; 34

hh

kmvp

Perhaps one of the key questions when Bohr offered his quantized orbits as an explanation to the UV catastrophe and spectral lines is, why does an electron follow quantized orbits? The response to this question arrived from the Ph.D. thesis of Louis de Broglie in 1923. de Broglie argued that since light can display wave and particle properties, then perhaps matter can also be a particle and a wave too.

Energy and momentum of a particle are related to wavelength:

Wave-particle duality

Wave packetm

k

m

pE

22

222

mv

Slide 47

Your de Broglie wavelength:

msmkg

sJ

mv31

2

34

10/1010

10

de Broglie wavelength for the electron in an atom:

msmkg

sJ

mv10

529

34

10/1010

10

Note the velocity dependence!

Slide 48

Degenerate matter

N particles in volume V

Density of particles = N/V

Share of volume per particle: V = V/N = 1/nDistance between particles = d

V ~ d3 ; therefore d ~ (V)1/3 = n-1/3

d

Particle density

3

3deBroglie )(

1~

mv

V

Nn

When particles are so close that d ~ deBroiglie, gas becomes degenerate

Slide 49

Pauli exclusion principle:Only one electron per quantum state

With increasing density, particles occupy states with higher velocity and kinetic energy

Pressure increases; however the temperature does not increase!

Slide 50

Equation of State:

Tk

P

Molecular weight =1 for pure hydrogen, 4 for helium, etc.

Classical gas

Slide 51

Equation of State:

3/5KP

Degenerate gas

Low density,small kinetic energy

High density, large kinetic energy

Slide 52

Strange properties of degenerate matter

• It strongly resists compression: P ~ 5/3

• Pressure does not depend on temperature

• Gas becomes more ideal with increasing density

Slide 53

Evolution of sun-like stars on H-R diagram

Slide 54

White Dwarfs (2)

Low luminosity; high temperature => White dwarfs are found in the lower left corner of the

Hertzsprung-Russell diagram.

Slide 55

White dwarfs in a globular cluster

Slide 56 p. 152

Slide 57

Slide 58

As it cools, carbon crystallizes into diamond lattice.Imagine single diamond of mass 1030 kg!Don’t rush, you would weigh 15,000 tons there!

Slide 59

Summary of Post Main-Sequence Evolution of Stars

M > 8 Msun

M < 4 Msun

Evolution of 4 - 8 Msun stars is still uncertain.

Fusion stops at formation of C,O core.

Mass loss in stellar winds may reduce them all to < 4 Msun stars.

Red dwarfs: He burning never ignites

M < 0.4 Msun

Supernova

Fusion proceeds; formation of Fe core.