susan cartwrightour evolving universe1 the deaths of stars n what happens to stars when the helium...

Our Evolving Universe 1Susan Cartwright

The Deaths of Stars

What happens to stars when the helium runs out?

do they simply fade into oblivion? NO!

stellar deaths producesome of the mostspectacular phenomenain the universe

and also play a vital rolein enriching with heavyelements the materialof which new stars aremade


Stellar lifetimes

Our binary star data suggest that the lifetimes of stars with mass < 0.9 Msun are longer than 15 billion years (the age of the universe)

But massive stars have lifetimes of only a few million years

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

0.01 0.1 1 10 100

mass (solar masses)

esti

mate

d m

ain

-sequence

lif

eti

me (

years

)

Popper T2

Popper T4/5

Popper T8

Malkov

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

0.01 0.1 1 10 100

mass (solar masses)

esti

mate

d m

ain

-sequence

lif

eti

me (

years

)

Popper T2

Popper T4/5

Popper T8

Malkov

HR Diagram

-10

-5

0

5

10

15

-0.5 0 0.5 1 1.5 2

B - V

Abso

lute V

isual

Magn

itude

HR Diagram

-10

-5

0

5

10

15

-0.5 0 0.5 1 1.5 2

B - V

Abso

lute V

isual

Magn

itude


Stellar statistics

A census of nearby stars: most stars are low mass

red dwarfs a few percent are 1–2 x

the Sun’s mass very massive stars are

very rare (only 3 B and no O class blue stars in the 3800 stars within 75 light years of the Sun)

Stellar deaths are rare (but crucial to our

existence!)

III

IIIIV

VWD

B

A

F

G

K

M

0

200

400

600

800

1000

1200

Stars within 25 pc

III

IIIIV

VWD

B

A

F

G

K

M

0

200

400

600

800

1000

1200

Stars within 25 pc


The deaths of Sun-like stars

Giant stars are very unstable

especially those which are fusing helium

Outer layers of star easily lost

mass loss seen in spectra of these stars

rate increases towards end of helium fusion


Mass loss in Sun-like stars

1.52

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

1.6E+09 1.7E+09 1.7E+09 1.8E+09 1.8E+09 1.9E+09 1.9E+09

time (years)

mass (

sola

r m

asses)

1.52

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

1.6E+09 1.7E+09 1.7E+09 1.8E+09 1.8E+09 1.9E+09 1.9E+09

time (years)

mass (

sola

r m

asses)

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 of 1.7 solar masses


Death of a star

At end of helium fusion star has lost all its outer layers

central, very hot, carbon core surrounded by expelled gas

ultraviolet radiation from hot core causes gas to produce emission lines

planetary nebula nothing to do with planet,

just looks like one!


Planetary nebulae


The legacy of Sun-like stars

Giant stars have convective outer layers

elements formed in core are transported to surface

thus ejected in planetary nebula

Planetary nebulae thus enrich the interstellar gas with many elements

nitrogen, barium, zirconium, etc.

Stellar core is eventually revealed as small, hot white dwarf star


Fusing heavy elements

Massive stars have much hotter cores

successfully fuse elements up to iron But this is a very temporary respite:

for a star of 20 solar masses, hydrogen fusion lasts around 15 million

years helium fusion lasts around one million

years carbon fusion lasts 300 years oxygen fusion lasts for 7 months silicon fusion lasts for two days and

produces an iron core

H + He

He

C

O

Si

Fenot to scale!


The fate of massive stars

Fusing iron requires (does not generate) energy

iron core cannot support itself against gravity

collapses to form neutron star neutron star is about 50% more massive

than Sun, but is only 20 km across basically a gigantic atomic nucleus:

protons and electrons have combined to form neutrons

in extreme cases even this may not be stable, and a black hole is formed instead

outer layers expelled in massive explosion: a supernova

for a few weeks star is nearly as bright as a whole galaxy

SN1994D: HST


The legacy of massive stars

Supernova remnants expanding gas cloud,

formerly star’s outer layers

Pulsars rapidly rotating neutron

stars observed by their “lighthouse

beam” of radio emission (sometimes also optical) emitted from magnetic poles of star

Heavy elements formed in the core of the

star as it implodes


The Crab pulsar

Spin axis and magnetic axis misaligned: see pulseIn Crab see 2 pulses/cycle: angle must be ~90°


Supernova remnants

Cas A in x-rays (Chandra)

Vela

SN1998bu

Remnant of SN386, with central pulsar (Chandra)

Cygnus Loop (HST): green=H, red=S+, blue=O++


Building the elements

Routes to building elements:

Start with Add In Making

carbon protons H-fusing stars nitrogen

carbon helium nuclei(α-process)

He-fusing stars oxygen-16, neon-20, etc.

carbon-13, neon-22

helium nuclei He-fusing stars free neutrons

silicon, sulphur

silicon, sulphur pre-supernova stars iron

iron, neon neutrons, slowly(s-process)

He-fusing stars most stable isotopes up to Bi

iron neutrons, rapidly(r-process)

supernovae neutron-rich isotopes

iron add photon, lose neutron? (p-pr)

supernovae rare neutron-poor isotopes


Adding neutrons—why does the speed matter? Slow addition of neutrons (s-process) cannot make

isotopes that are “shielded” by unstable isotopes (e.g. 116Cd)

Rapid addition of neutrons (r-process) cannot make those shielded by stable nuclei (e.g. 116Sn)

Some isotopes (p-process) cannot be made by either method (e.g. 112Sn)51

Sb

121 57.3%

122 2.7 d

123 42.7%

50

Sn 112 1.01%

113 115 d

114 0.67%

115 0.38%

116 14.6%

117 7.75%

118 24.3%

119 8.6%

120 32.4%

121 27.1 h

122 4.56%

49

In 112

14 m

113 4.3%

114 71.9 s

115 95.7%

116 14.1 s

117 44 m

48

Cd 110 12.5%

111 12.8%

112 24.1%

113 12.2%

114 28.7%

115 53.4 h

116 7.5%

117 2.4 h

118 50 m

62 63 64 65 66 67 68 69 70 71 72


1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

0 50 100 150 200

Mass number

No o

f ato

ms p

er

1000000 s

ilic

on

Summary: the abundance of the chemical elements

The iron peak:made in the laststages of fusion

Heavy elements:made by addingneutrons to iron

not madein stars

CNO

p-process isotopes

susan cartwrightour evolving universe1 the deaths of stars n what happens to stars when the helium...

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