Transcript Introduction: The Night Sky

The Deaths of Stars

What happens to stars when the helium runs out?


do they simply fade into oblivion?
NO!
stellar deaths produce
some of the most
spectacular phenomena
in the universe
 and also play a vital role
in enriching with heavy
elements the material
of which new stars are
made

Susan Cartwright
Our Evolving Universe
1
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
Susan Cartwright
Our Evolving Universe
2
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)
Stars within 25 pc
12 00
10 00
80 0
60 0
40 0
 Stellar deaths are rare

M
20 0
K
G
F
0
A
I
II
III
B
IV
V
WD
(but crucial to our existence!)
Susan Cartwright
Our Evolving Universe
3
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
Susan Cartwright
Our Evolving Universe
4
Mass loss in Sun-like stars
Star of 1.7
solar masses
Susan Cartwright
Our Evolving Universe
5
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

Susan Cartwright
nothing to do with planet,
just looks like one!
Our Evolving Universe
6
Planetary nebulae
Susan Cartwright
Our Evolving Universe
7
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

nitrogen, barium, zirconium, etc.



Planetary nebulae thus enrich the
interstellar gas with many
elements
Stellar core is eventually
revealed as small, hot white
dwarf star
Susan Cartwright
Our Evolving Universe
8
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
not to scale!
Susan Cartwright
Our Evolving Universe
H + He
He
C
O
Si
Fe
9
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
SN1994D:
HST
outer layers expelled in massive
explosion: a supernova

Susan Cartwright
for a few weeks star is nearly as bright as
a whole galaxy
Our Evolving Universe
10
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
Susan Cartwright
Our Evolving Universe
11
The Crab pulsar
Spin axis and
magnetic axis
misaligned: see pulse
In Crab see 2
pulses/cycle: angle
must be ~90°
Susan Cartwright
Our Evolving Universe
12
Supernova remnants
Cygnus Loop (HST):
green=H, red=S+,
blue=O++
Cas A in x-rays
(Chandra)
Vela
Remnant of SN386, with
central pulsar (Chandra)
SN1998bu
Susan Cartwright
Our Evolving Universe
13
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
Susan Cartwright
Our Evolving Universe
14
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
121
Sb
50
Sn
49
112
1.01% 115 d
112
In
48
Cd
Susan Cartwright
113
110
14 m
111
114
115
116
117
118
119
0.67% 0.38% 14.6% 7.75% 24.3% 8.6%
113
114
115
116
117
4.3%
112
71.9 s 95.7% 14.1 s
113
114
115
44 m
116
12.5% 12.8% 24.1% 12.2% 28.7% 53.4 h 7.5%
62
63
64
65
66
67
68
Our Evolving Universe
122
57.3% 2.7 d
120
121
123
42.7%
122
32.4% 27.1 h 4.56%
117
118
2.4 h
69
50 m
70
71
72
15
Summary: the abundance of
the chemical elements
CNO
The iron peak:
made in the last
stages of fusion
Heavy elements:
made by adding
neutrons to iron
not made
in stars
p-process isotopes
Susan Cartwright
Our Evolving Universe
16