the lives of stars - university of sheffieldstellar evolution note step is in log (age): each frame...

1

Susan Cartwright Our Evolving Universe 1

The Lives of Stars

� From studying nearby stars and stellar clusters� most stars are on the main

sequence

� stars become red giants after leaving the main sequence

� How does this relate to the internal structure of the stars and their nuclear fusion reactions?

-5

0

5

10

15

20

-0.5 0 0.5 1 1.5 2 2.5

B - V


Fusion reactions

� Generate energy up to iron

� But, need to get two positively charged nuclei close enough to fuse together� need fast movement

� high temperature (and high density)

� Converting hydrogen-1 to helium-4 is the easiest and most efficient fusion reaction� 0.7% of initial mass converted

to energy 0.998

0.999

1

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 560.998

0.999

1

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 56

E=mcE=mc22

2


Stellar structure and fusion

� To keep star stable need pressure to increase downwards� temperature increases

� density increases

� fusion most likely in central core of star

� Stars are mainly hydrogen� expect main sequence

stars to fuse hydrogen to helium in core

HHH ———>>> HHHeee

iiinnn cccooorrreee

ppprrreeessssssuuurrreee,,,

ttteeemmmpppeeerrraaatttuuurrreee,,,

dddeeennnsssiiitttyyy

iiinnncccrrreeeaaasssiiinnnggg


Hydrogen fusion reactions

� Reaction rateincreases astemperatureincreases� more massive stars

have higher fusionrates

� Reaction can be direct or use carbon-12as catalyst� this tends to increase abundance

of nitrogen and oxygen

pp chainpp chain

CNO cycleCNO cycle

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Getting the heat out

� Energy generated in stellar core has to be transported to surface

� Two options:� radiation

� absorption and re-emission

of photons

� convection� hot gas rising towards surface,

cool gas falling

� Giant stars have convective outer layers� transports out the heavy elements

produced by fusion


What happens when the core

hydrogen runs out?

0

0.5

1

1.5

2

2.5

3

3.5

4

3.53.553.63.653.73.753.83.853.93.95

log T

Star of 1.7 solar masses

star now has helium core (not hot enough to fuse)

on main sequence10x Sun

core shrinks under gravityuntil hydrogen outside starts to fuse

star expands and cools,becoming luminousred giant (1000x Sun)

1.61 Gyr

1.65 Gyr

1.69 Gyr

1.76 Gyr

4


Stages of hydrogen fusion

� Main sequence stars fuse hydrogen to helium in core

� Red giants (and subgiants) fuse hydrogen to helium in shell outside helium core

� Stars have nearly constant luminosity on main sequence, but red giants get brighter as they age

� Red giant stage lasts only 10% as long as main sequence

M67

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9

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12

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16

-0.5 0 0.5 1 1.5 2

B-V

V

core hydro-gen fusion

starting to starting to fuse outfuse out--side coreside core

establishedestablishedfusion infusion inshellshell


Helium fusion

� Neither beryllium-8 nor boron-8 is stable� need to combine three

helium nuclei to get stable carbon-12

� beryllium-8 serves as intermediate stage

� need high temperature and density (else 8Be decays before it gets converted to 12C)

5


Stages of helium fusion

0

0.5

1

1.5

2

2.5

3

3.5

4

3.53.553.63.653.73.753.83.853.93.95

log T

Star of 1.7 solar masses

core heliumcore heliumstarts to starts to

fusefuse

helium fusion in core:helium fusion in core:star is smaller andstar is smaller and

hotter, but less brighthotter, but less bright

carbon core with heliumcarbon core with heliumfusion outside: starfusion outside: star

becomes a giant againbecomes a giant again

1.86 Gyr

1.76 Gyr

1.82 Gyr1.82 Gyr


M3

12

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20

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22

-0.3 0 0.3 0.6 0.9 1.2

B-V

V

M3

12

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20

21

22

-0.3 0 0.3 0.6 0.9 1.2

B-V

V

Helium fusion

on the HR diagram

� Helium fusion is much less efficient than hydrogen fusion (0.07% instead of 0.7%)� helium fusion stage lasts for a

much shorter time

0.998

0.999

1

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 560.998

0.999

1

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 50 100 150 200

Number of nucleons

helium 4

hydrogen 1

iron 56

helium corefusing stars:the hori-

zontal branchof a globularcluster

6


Side effects of helium fusion

� Adding more helium nuclei to carbon can produce the alpha-process elements� oxygen-16, neon-20, etc.

� Adding helium to carbon-13 or neon-22 produces free neutrons� which can easily combine

with nuclei (no charge) to produce different elements

� Why does helium fusion make mostly carbon?� because carbon nuclei have

an energy level at exactlythe right place

� otherwise carbon would be a rare element

� and we would not exist!

Fred Hoyle, 1953


Stellar evolution

� Note step is in log (age): each frame is 60% older than the one before� massive stars evolve very

quickly

� post-main-sequence life of star is always comparatively short

� massive stars change colour a great deal, but don’t change brightness much

� less massive stars become much brighter as red giants

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After helium fusion

� Fusion of heavier elements gets more difficult� higher mass means lower

speed at given temperature

� higher charge means more electrostatic repulsion

� Stars like the Sun never get beyond helium fusion

� More massive stars (>8 MS) can fuse elements up to iron

� What happens to Sun-like stars when the helium is used up?

� What happens to massive stars when they reach iron? � fusion beyond iron requires

energy

� How are the heavy elements formed in stellar cores dispersed into space?

…next lecture!

the lives of stars - university of sheffieldstellar evolution note step is in log (age): each frame...

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