Our Evolving Universe 1Susan Cartwright

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?

HR Diagram of nearby stars

-5

0

5

10

15

20

-0.5 0 0.5 1 1.5 2 2.5

B - V

absolu

te v

isual m

agnit

ude

Our Evolving Universe 2Susan Cartwright

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

Nuclear stability

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

Mass p

er

nucle

on

helium 4

hydrogen 1

iron 56

Nuclear stability

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

Mass p

er

nucle

on

helium 4

hydrogen 1

iron 56

E=mc2

Our Evolving Universe 3Susan Cartwright

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,,,

ttt eeemmmpppeeerrraaatttuuurrreee,,,

dddeeennnsssiiittt yyy

iiinnncccrrreeeaaasssiiinnnggg

Our Evolving Universe 4Susan Cartwright

Hydrogen fusion reactions

Reaction rateincreases astemperatureincreases

more massive starshave higher fusionrates

Reaction can be direct or use carbon-12as catalyst

this tends to increase abundanceof nitrogen and oxygen

pp chain

CNO cycle

Our Evolving Universe 5Susan Cartwright

Getting the heat out

Energy generated in stellar core has to be transported to surface

Two options: radiation

absorption and re-emissionof photons

convection hot gas rising towards surface,

cool gas falling

Giant stars have convective outer layers transports out the heavy elements

produced by fusion

Our Evolving Universe 6Susan Cartwright

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

log

L

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

Our Evolving Universe 7Susan Cartwright

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

6

7

8

9

10

11

12

13

14

15

16

-0.5 0 0.5 1 1.5 2

B-V

V

core hydro-gen fusion

starting to fuse out-side core

establishedfusion in

shell

Our Evolving Universe 8Susan Cartwright

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)

Our Evolving Universe 9Susan Cartwright

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

log

L

Star of 1.7 solar masses

core heliumstarts to

fuse

helium fusion in core:star is smaller and

hotter, but less bright

carbon core with heliumfusion outside: star

becomes a giant again

1.86 Gyr

1.76 Gyr

1.82 Gyr

Our Evolving Universe 10Susan Cartwright

M3

12

13

14

15

16

17

18

19

20

21

22-0.3 0 0.3 0.6 0.9 1.2

B-V

V

M3

12

13

14

15

16

17

18

19

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

Nuclear stability

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

Mass p

er

nucle

on

helium 4

hydrogen 1

iron 56

Nuclear stability

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

Mass p

er

nucle

on

helium 4

hydrogen 1

iron 56

helium corefusing stars:

the hori-zontal branchof a globular

cluster

Our Evolving Universe 11Susan Cartwright

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 exactly the

right place otherwise carbon would be a rare element and we would not exist!

Fred Hoyle, 1953

Our Evolving Universe 12Susan Cartwright

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

Our Evolving Universe 13Susan Cartwright

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!