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> The science of the stars and the cosmos
THE COLLECTION
1 > The atom
2 > Radioactivity
3 > Radiation and man
4 > Energy
5 > Nuclear energy: fusion and fission
6 > How a nuclear reactor works
7 > The nuclear fuel cycle
8 > Microelectronics
9 > The laser: a concentrate of light
10 > Medical imaging
11 > Nuclear astrophysics
12 > Hydrogen
FROM RESEARCH
TO INDUSTRY
11
> Nuclear
astrophysics
THE PRINCIPLE OF NUCLEOSYNTHESIS
THE STARS
THE SUN
SUPERNOVAE
COSMIC RADIOACTIVITY SEEN
BY THE INTEGRAL SATELLITE
© Commissariat à l’Énergie Atomique
aux Energies Alternatives, 2005
Atomique,et2003
Communication
Division
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ISSN 1637-5408.
> CONTENTS
> INTRODUCTION
The aim of nuclear
astrophysics is to
explain the origin,
evolution and
abundance of
elements in the
Universe.
© ESA/Soho - Hubble/AURA/STScI/Nasa - PhotoDisc
THE PRINCIPLE
OF NUCLEOSYNTHESIS
4
What is a nuclear
fusion reaction?
5
Periodic table
and Abundance table 6 and 7
Where does energy
come from?
8
The different types
of nucleosynthesis
8
A
© PhotoDisc
Globular cluster.
THE STARS
What is a star?
Why do the stars shine?
The birth of stars
The life of stars
The death of stars
9
10
10
12
13
13
THE SUN
What is the Sun made of?
A model of the Sun
Life expectancy
of the Sun and stars
15
16
16
19
SUPERNOVAE
20
What is a supernova?
21
The different types
of supernovae
21
Thermonuclear and gravitational
supernovae
21
© D. Malin/Anglo-Australian Observatory
Nuclear
astrophysics
2
Cloud lit from the inside.
Supernovae, the source
of heavy elements
Supernovae, the source
of cosmic radiation
COSMIC RADIOACTIVITY
SEEN BY THE INTEGRAL
SATELLITE
23
24
25
introduction
stronomy deals with the position and observation of the objects in our Universe, from
planets to galaxies. It is the oldest of the
sciences. Astrophysics is the study of the
physical properties of these objects. It dates
from the start of the 20th century.
Nuclear astrophysics is the marriage of nuclear
physics, a laboratory science concerned with
the infinitely small, and astrophysics, the
science of what is far away and infinitely large.
Its aim is to explain the origin, evolution and
abundance of the elements in the Universe. It
was born in 1938 with the work of Hans Bethe,
an American physicist who won the Nobel Prize
for physics in 1967, on the nuclear reactions
that can occur at the center of stars. It explains
where the incredible energy of the stars and the
Sun comes from and enables us to understand
how they are born, live and die.
The matter all around us and from which we
are made, is made up of ninety-two chemical
elements that can be found in every corner of
the Universe. Nuclear astrophysics explains the
origin of these chemical elements by nucleosynthesis, which is the synthesis of atomic
nuclei in different astrophysical environments
such as stars.
Nuclear astrophysics provides answers to
fundamental questions:
• Our Sun and the stars in general shine because
nuclear reactions are taking place within them.
• The stars follow a sequence of nuclear reaction
cycles. Nucleosynthesis in the stars enables us
to explain the origin and abundance of elements
essential to life, such as carbon, oxygen, nitrogen
and iron.
• Star explosions, in the form of supernovae,
disperse the nuclei formed by nucleosynthesis
into space and explain the formation of the heaviest chemical elements such as gold, platinum
and lead.
Nuclear astrophysics is still a growing area of
science.
Designed and produced by Spécifique - Cover photo by © PhotoDisc - Illustrations by YUVANOE - Printed by Imprimerie de Montligeon - 04/2005
The science of the stars and the cosmos
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
3
4
> THE PRINCIPLE OF NUCLEOSYNTHESIS
THE NUCLEAR MICROCOSM
AND ASTRONOMICAL MACROCOSM
ARE CLOSELY LINKED.
The principle of
nucleosynthesis
Nucleosynthesis is the formation of atomic
nuclei in different astrophysical environments.
It is intimately linked with nuclear physics.
WHAT IS A NUCLEAR
FUSION REACTION?
The nuclear fusion reaction produces one
heavy nucleus from two lighter atomic nuclei.
It is accompanied by a huge release of energy.
Fusion is difficult to achieve because it
involves two different, opposing forces:
– the strong nuclear interaction that binds the
neutrons and protons into the nucleus. This
very intense force only works over very small
distances no bigger than the radius of the
nucleus,
– the electromagnetic interaction to which all
charged particles are subject. This works over
a long distance. It prevents positively charged
atomic nuclei from coming very close to one
another, by creating a kind of barrier of repulsion.
To succeed in crossing this barrier and getting
close enough to fuse, the nuclei must be in a
THE ATOM
Matter is made up of atoms, which consist of a nucleus with
electrons orbiting around it.
The atomic nucleus is an assembly of protons and neutrons
concentrated in a very small space. The neutrons are
electrically neutral and the protons have a positive electrical
charge (e+).
Representation of the electron cloud
in an oxygen atom
Atomic nucleus made up
of 8 protons+ 8 neutrons
The electrons have a negative electrical charge (e-). For the
atom to be electrically neutral, the number of protons must be
exactly equal to the number of electrons in orbit.
8 electrons
The protons and neutrons have almost the same mass.
Their mass is a thousand times greater than the mass of an
electron.
A chemical element is defined by the number of protons it has
(e.g. oxygen has eight protons).
Atoms of a chemical element with different numbers of
neutrons are isotopes of that element.
© Digital Vision
For example, in the hydrogen family, hydrogen itself has only
one proton, while deuterium has one proton and one neutron,
and tritium has one proton and two neutrons. Deuterium and
tritium are both isotopes of hydrogen (see also The atom
booklet).
The science of the stars and the cosmos
11 > Nuclear astrophysics
The science of the stars and the cosmos
Electron cloud
11 > Nuclear astrophysics
5
> THE PRINCIPLE OF NUCLEOSYNTHESIS
> THE PRINCIPLE OF NUCLEOSYNTHESIS
Mendeleev’s periodic table of elements
Period
1
1
2
H
1.00794
3
4
2
Li
Be
Na
Mg
5
Al
22.9898 24.3050
21
19
20
Ca
K
4
B
C
7
8
N
9
O
F
4.00206
10
Si
P
S
Cl
26.9815 28.0855 30.9736 32.066
22
Sc
23
Ti
V
24
Cr
25
Mn
26
Fe
27
Co
28
Ni
29
Cu
30
Zn
31
32
Ga
Ge
33
34
As
Se
Ar
35.4527 39.948
35
Br
36
Kr
39.0983 40.078 44.956 47.88 50.942 51.996 54.9309 55.847 58.9332 58.69 63.546 65.39 69.723 72.61 74.9216 78.96 79.904 83.80
38
39
41
45
53
54
44
46
47
48
49
50
52
37
42
43
51
40
Rb
5
85.468
Sr
Zr
Y
87.62
88.906
Nb
58
57
La
Ce
Mo
92.906
91.224
Tc
Ru
Rh
Pd
Ag
In
Cd
Sn
Sb
Te
Xe
I
127.60 126.905 131.29
101.07 102.906 106.42 107.868 112.411 114.82 118.710 121.75
(98)
95.94
59
71
69
63
61
65
67
68
60
66
70
64
62
Ho
Lu
Tb
Pr Nd Pm Sm Eu
Er Tm Yb
Dy
Gd
15 140.908 44.24 (145) 50.36 151.965 57.25 158.925 62.50 164.930 67.26 168.934 73.04 174.967
1
1
1
1
1
1
138.906 140.1
6
55
Cs
56
57
Ba
72
to
132.905 137.327
71
88
89
87
73
Hf
Ta
74
W
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
81
82
Tl
Pb
83
Bi
84
Po
85
At
86
Rn
(210)
(222)
178.49 180.948 183.85 186.207 190.2 192.22 195.08 196.967 200.59 204.383 207.2 208.980 (209)
109
111
105
106
107
112
110
104
108
KEY
Rf
Fr
to
Db Sg Bh Hs Mt Uun* Uuu* Uub*
Ra
Atomic number = number of protons
= number of electrons
1
272
277
268
(223) 226.025 103 (261.11) 262.11 263.12 264.12 265.13
269
H
Symbol
1
1
91
93
99
95
97
01
03
1.00794
2
0
Atomic
mass
=
number
of
protons
0
0
98
96
90
92
94
89
1
1
A
Np Pu
Pa
Es Fm Md No
Lr
+ neutrons = number of nucleons in the nucleus
m Cm Bk Cf
Ac Th
U
38 231.036 38.029 237.048 (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)
The figures between brackets indicate
227.028 232.0
2
the mass number of the most stable isotope.
* Names and symbols of these elements are temporary.
According to Handbook of Chemistry and Physics,
7
th
74 Ed. 1993, CRC Press
and Pure and Applied Chemistry, 1997, 69, 2471
•
Abundance table of the elements
H
10 10
A
He
C
Ne
N
10 6
C
Mg
Si
Fe
F
10
2
Ti
P
Li
Ge
B
Se
B
Kr
Kr
Sn
Be
0
Te
Ba
Pb
Pt
Dy
40
The science of the stars and the cosmos
– C the most abundant nuclei after that are
carbon C (with 12 neutrons and protons), oxygen O
(16), neon Ne (20), magnesium Mg (24), silicon Si
(26) and iron Fe (56). These are also the most
stable nuclei in the Universe;
D
Zn
1
10 –2
Ni
Na
10 4
The abundance table of the elements in the solar
system • indicates, for every element in the
periodic table ¶ , the quantity of this element to be
found in the solar system. The reference basis for
this is the abundance of one particular element,
silicon. Its abundance was arbitrarily
set at 106. The abundance of the other elements
is given relative to the abundance of silicon in
powers of ten:
10-2 = one hundredth, 1, 102 = 100, 104 = 10,000,
106 = 1,000,000 (a million),
108 = one hundred million, 1010 = ten billion.
The table was developed from measurements and
observations, and is of great value to
astrophysicists. It shows that:
– B there is a significant gap between helium He
and carbon C, explained by the fragility of the
nuclei of lithium Li, beryllium Be and boron B;
Ar
Ca
Mendeleev’s periodic table of elements ¶ enables
us to classify and name the different chemical
elements discovered to date by the number of
protons in the nucleus, from 1 for hydrogen to 92
for uranium and even higher for laboratoryproduced nuclei that do not exist in the natural
state. It specifies the chemical properties of the
elements, which depend on the number of electrons
in the atom.
– A the most abundant elements are hydrogen H
and helium He (one gram of matter on average
contains 98% of these). This is true for the whole
of the observable Universe;
10 8
O
PERIODIC TABLE AND
ABUNDANCE TABLE
Ne
10.811 12.011 14.0067 15.9994 18.9984 20.1797
13
14
15
16
17
18
6.941 9.0122
12
11
3
6
He
80
Yb Hf
Hg
Th U
– D nuclei heavier than iron (Fe) are much more
rare. Iron is the most stable nucleus in the
Universe.
© PhotoDisc
¶
Relative abundance (Si = 106)
6
A globular cluster is a concentration
of thousands of stars bound by gravitation.
very agitated state. They reach this state when
they are brought to a very high temperature.
So fusion naturally occurs in the extremely
hot environment of stars, like the Sun. At
the Sun’s core the temperature reaches 15
million degrees, making the fusion of the
lightest nuclei such as hydrogen (one proton)
and helium (two protons and two neutrons)
possible.
In more massive stars than the Sun, core
temperatures are even higher, enabling the
fusion of heavier nuclei than hydrogen. These
reactions produce carbon, oxygen and iron nuclei.
120
160
200
240
Number of nucleons (protons and neutrons) in the nucleus
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
7
9
> THE PRINCIPLE OF NUCLEOSYNTHESIS
STARS OPERATE ON THE PRINCIPLE
OF THERMONUCLEAR FUSION.
WHERE DOES ENERGY
COME FROM?
The result of the fusion of hydrogen in the Sun
is that four hydrogen nuclei form a helium
nucleus (see diagram on p.18). This releases
energy. In this reaction the sum of the masses
of the four original nuclei is greater than the
mass of the final nucleus. According to the
equation of equivalence between mass and
energy, E = mc2, known as Einstein’s law, the
missing mass ‘m’ has changed into energy, E.
Where does this energy go? It is mainly emitted in the form of heat and light. The energy
radiated in the form of light is enough to make
the Sun shine, and the energy radiated in the
form of heat is enough to maintain life on
Earth (see also the Nuclear Energy: fusion and
fission booklet). Paradoxically, the power emitted into space by the Sun is very weak, at only
0.2 microwatts per gram, i.e. 10,000 times
less than the energy given off by a human
being, which is a few milliwatts per gram.
THE DIFFERENT TYPES
OF NUCLEOSYNTHESIS
The syntheses of atomic nuclei in different
astrophysical environments may be defined as
follows:
• During the first three minutes of the Universe’s
existence, the primordial nucleosynthesis took
place. This explains the abundance of hydrogen,
its isotope (see inset on p. 5) deuterium
(1 proton, 1 neutron) and the two stable isotopes of helium (helium-3 [2 protons, 1 neutron] and helium-4 [2 protons, 2 neutrons]).
The science of the stars and the cosmos
The stars
“The fusion of
hydrogen in the Sun
releases enough
energy to make it
shine and support life
on Earth.”
• The formation of certain slightly heavier nuclei
such as lithium (Li), beryllium (Be) and boron
(B) is explained by spallation reactions.
• Within stars,
A reaction characterized by the
action of a natural flux of high
fusion reactions
energy particles present in space,
occur that transcosmic radiation. This flux makes
the
heaviest nuclei present in the
form atomic
interstellar environment (carbon,
nuclei in what is
nitrogen, etc.) explode, and the
nuclei produced (lithium,
known as stellar
beryllium, boron) are dispersed.
nucleosynthesis.
• Fusion reactions are not possible for nuclei
heavier than iron, so these elements are more
rare and their synthesis is due to a different
type of nuclear reaction, neutron capture, which
occurs in This is where a nucleus captures one or
supernovae. more neutrons in succession. It then
becomes unstable and disintegrates by
Thus all the beta emission during which one neutron
turns
into a proton. This creates a heavier
chemical
nucleus (one extra proton).
elements in
the periodic table (see p. 6) are present in the
Universe.
© Nasa/A. Schaller
8
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
10
> THE STARS
> THE STARS
One aim of nuclear astrophysics is to understand how stars are born, live and die.
WHAT IS A STAR?
Stars are balls of very high temperature gas.
This gas is ionized, which means that the negatively electrically charged electrons are
totally or partially separated from the positively electrically charged nuclei. This gas is
also known as plasma.
With the naked eye or a telescope, we can only
see the bright surface of the stars. In astrophysics, many discoveries have been made over
recent years using ground-based telescopes or
telescopes mounted on satellites. The entire
electromagnetic spectrum is used, from radio
waves to X- and gamma rays (see Radioactivity
booklet), because each spectral range provides
particular information:
– infrared rays tell us where and how the stars
and planets were formed;
– visible light tells us about the different
nuclear reactions produced within the stars
throughout their life;
– radio waves, X-rays and gamma rays show
us the sometimes very violent phenomena
that occur at the end of a star’s life: supernovae, pulsars, neutron stars and black holes.
By interpreting the data from all these types
of radiation, we can work out how much
energy is produced by the star, what the
temperature is on its surface, and its chemical composition.
In stellar gas, hydrogen and helium are of all
the chemical elements by far the most common,
followed by oxygen, carbon and nitrogen. For
every 1,000 billion hydrogen atoms, there are
100 billion helium atoms and approximately
1 billion oxygen atoms.
© Hubble/AURA/STScI/Nasa
WHY DO THE STARS SHINE?
Globular cluster.
The science of the stars and the cosmos
The stars are generally stable objects within the
Universe. A star is an enormous sphere of hot
gas kept in balance by two opposing effects:
– on the one hand, gravitation, which prevents
the gas from dispersing and tends to attract
particles to the center;
– on the other, internal pressure due to thermal
agitation of the gas, which counteracts this
inward force.
11 > Nuclear astrophysics
© D. Malin (AAO)/ROE/UKS Telescope
“Stars are heavenly
nuclear reactors.”
Stars are born from the material left by other stars.
Gravitation depends on the mass of the star
and pressure depends on its temperature. The
core of the star is extremely hot (several
million degrees) but its surface is cooler
(several thousand degrees). The temperature
difference causes the flow of heat, and there-
fore of energy, from the center towards the
surface. It is this heat that is radiated by the
star and makes it look to us as though it’s
shining. Where does the star’s energy come
from? It comes from the nuclear fusion reactions that take place in its center, which are
also known as thermonuclear reactions.
Because these reactions generate energy from
matter – the nuclei that make up the star
(essentially hydrogen) – the nuclei are known
as “fuels”, by analogy with other forms of
energy. Stars evolve by slowly burning up their
fuel (hydrogen).
The heat given off by the nuclear reactions
prevents the star from collapsing, and gravity
prevents it from dispersing.
Stars shine for a long time because the
thermonuclear reactions in their core release
their enormous quantities of energy slowly.
NON-EXPLOSIVE STAGES OF THERMONUCLEAR FUSION
IN A STAR WITH 25 TIMES THE MASS OF THE SUN (25 MJ*)
FUEL
(raw material)
TEMPERATURE
(degrees)
MOST ABUNDANT
PRODUCTS OF FUSION
LENGTH OF STAGE
Hydrogen
20 million
helium, nitrogen
7 million years
Helium
200 million
carbon, oxygen
500,000 years
Carbon
800 million
oxygen, neon, magnesium
600 years
Neon
1.5 billion
oxygen, magnesium, silicon
1 year
Oxygen
2 billion
silicon, sulphur
6 months
Silicon
3.5 billion
iron, nickel
1 day
* Mass of the Sun = M = 1.991 x 1030 kg, compared with the mass of the Earth = 6 x 1024 kg; the mass of the Sun
is 300,000 times greater than the mass of the Earth.
J
The science of the stars and the cosmos
11 > Nuclear astrophysics
11
> THE STARS
> THE STARS
Cross-section of a star
Cross-section of the central part of a star, with a mass
25 times greater than the mass of the Sun (25 MJ), about
to explode (according to the American physicist
S. Woosley).
J
l
Helium 3.7 M
J
Carbon 0.2 M
THE BIRTH OF STARS
J
Oxygen 1.6 M
J
Neon 0.06 M
J
lMagnesium 0.02 M
l
Nebulae within galaxies are gigantic clouds of
gas and dust. Gravitation or some external
event can cause part of these clouds to contract. The mass of gas concentrates and the
molecules collide; the temperature rises until
hydrogen fusion occurs. A star is born.
Astrophysicists can see the birth of stars
because of the infrared radiation emitted by
the stars through these clouds.
l
J
l
Silicon-Calcium 0.6 M
J
l
Iron core 1.4 M
EVOLUTION OF THE STARS (HERZSPRUNG-RUSSELL BRIGHTNESS
AND COLOR DIAGRAM)
10+5
10+3
0.008
6
0.08
Red giants
10
+2
10
1
3.2
1.8
1.5
1.3
Main
sequence
Sun
1
0.7
10 -1
30
0.5
White dwarfs
10
0.4
2
4
6
13
70
0.3
-2
3,
00
0
5,
00
0
4,
00
0
6,
00
0
20
,
15 000
,0
00
10
,0
9, 00
0
8, 00
0
7, 00
00
0
10 -3
100
Time spent in main sequence (billions of years)
10+4
Red
supergiants
17
This diagram represents the brightness of a star
according to the temperature on the surface of the
star in our galaxy (the reference basis for
brightness is the brightness of the Sun = 1).
We can see that the stars are grouped into several
regions:
• The central band known as the main sequence
contains the stars in the longest phase of their
life, when they are transforming hydrogen into
helium in their core.
• At the top right are the stars in the most
advanced phases of nuclear combustion: fusion
of helium into carbon and oxygen, and fusion of
carbon into heavier elements, neon, magnesium
and silicon. These are red giants and red
supergiants.
• At the bottom left are the white dwarfs, the final
phase of stars with a small mass, like our Sun.
Surface temperature (K)
The science of the stars and the cosmos
11 > Nuclear astrophysics
“The supernova:
a very bright explosion
marking the death
of a star.”
THE LIFE OF STARS
Stars are self-regulating nuclear reactors.
When a nuclear reaction flares up inside a star,
the flexible gas core dilates slightly, the temperature falls and the reaction calms down.
Conversely, when a reaction dies down, the
core contracts, the temperature rises and
nuclear reactions begin again. The star lasts
as long as its core remains flexible and gassy.
The life of a star is a succession of gravitational
contractions and cycles of nuclear combustion.
A star that is working well shines, and if it
shines, it is slowly burning. The fuel cycle within
a star is particularly efficient: the products of
one combustion cycle are used as fuel for the
next cycle. So helium, the product of hydrogen
fusion, burns to give carbon and oxygen through
thermonuclear fusion, and these are subsequently transformed into silicon. Gradually, the
length of each cycle decreases considerably
as the fuel provides less and less energy.
THE DEATH OF STARS
Stars with only a small mass, like the Sun, can
only burn hydrogen and helium. Then, part of
their outer layer is expelled, and they become
The science of the stars and the cosmos
© FORS Team/VLT/ESO
l
Brightness (Sun = 1)
12
Remains of the supernova 1054 (Crab nebula).
white dwarfs – stars that have exhausted their
nuclear resources.
The most massive stars, between ten and hundreds of times bigger than the Sun, have
higher temperatures at their center. Nuclear
combustion is faster and goes further than
helium combustion. Once the carbon fusion
is over, a huge loss of energy resulting from a
large emission of neutrinos triggered by the
heat, literally
Electrically neutral particles of low
mass, which interact little with matter.
exhausts the
star. Thermonuclear combustion stops at iron, the most stable
nucleus in the Universe. Iron is not combustible, so when it accumulates at the core
of massive stars, it amounts to a death sen11 > Nuclear astrophysics
13
14
15
> THE STARS
THE MAIN ELEMENTS OF SOLAR PHYSICS
ARE NOW UNDERSTOOD.
The Sun
tence for them. Their core, having reached a
great density, rebounds, producing a shock
wave that sweeps through the surrounding
matter. The implosion of the core is coupled
with the explosion of the star. This star, now
a supernova, emits an intense light that can
be seen by astrophysicists when they observe
the galaxy to which the star belongs. A neutron star or black hole remains at the center
of the supernova. The stars in our galaxy are
divided into two different populations distinguished by their chemical composition and
their distribution in space:
The science of the stars and the cosmos
Supernova.
• stars that contain heavy elements in the same
proportions as the Sun,
• stars that have a low abundance of elements
heavier than helium (they may contain a thousand times less iron, for example). These constitute the globular clusters traveling around
the galaxy.
© PhotoDisc
Globular cluster.
© DigitalVision
© PhotoDisc
“There are two
star populations
in our galaxy.”
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
> THE SUN
> THE SUN
“The Sun is the closest
and best studied of the stars.”
Internal structure of the Sun
WHAT IS THE SUN MADE OF?
The science of the stars and the cosmos
Core
Neutrino
Photon trajectory (not to scale)
Radiation flux
Force of pressure (outwards)
Gravitational force (inwards)
Temperatures and densities
within the Sun
SOLAR NEUTRINOS
The Sun’s oscillations are superficial movements of its
surface. The areas moving closer to the observer are shown
in blue; the areas moving away are shown in red.
11 > Nuclear astrophysics
The science of the stars and the cosmos
Density (g/cm3) 0
0.07
1.3
2.0
344,000
135
0
The neutrino is a particle in the same family as
the electron. It carries energy and has a very low
mass. Neutrinos are produced when protons
change into neutrons. The neutrino flux detected
on Earth, which is much smaller than the
calculated neutrino flux, has concerned
astrophysicists for a long time. But from precise
measurements made by underground detectors
(Gallex experiment in Europe, Superkamiokande
experiment in Japan and SNO in Canada), we now
know that the Sun is not responsible for the
neutrino deficit. It is explained by the fact that
these particles oscillate between three states, and
that detectors, which are sensitive to only one of
these states, can only capture a third of them.
160,000
The relative proportions of the various
chemical elements in the solar system and
the Sun are known from two main sources:
• Analysis of the radiation emanating from
the Sun’s photosphere. We cannot observe
the inside of the Sun
The bright surface of the star
and the only observable part.
directly. We analyze
visible light, but also
radiation that is invisible to the naked eye:
radio waves, infrared, ultraviolet, X-rays and
gamma rays. All these types of radiation
make up the Sun’s spectrum.
• Laboratory analysis of meteorites enables
us to determine the isotopic composition of
matter in the
Composition in terms of isotopes,
solar system.
i.e. chemical elements whose
atoms have the same number
O v e r a l l , o n e of
protons and a different number of
gram of matter neutrons, e.g. carbon-12 (6 protons
and 6 neutrons) and carbon-14
from the Sun (6 protons and 8 neutrons) are
is made up two carbon isotopes.
o f 0.70 g o f
hydrogen, 0.28 g of helium and 0.02 g of all
the other chemical elements in the periodic
table (see p. 6).
But observation is not enough. It must also
be accompanied by theoretical studies of
physics.
the neutrino flux that transports energy from
the center of the Sun to its surface, which physicists are now able to detect and study. This
flux is the indicator that the solar nuclear reactor
is operating correctly.
As the Sun is a sphere of gas in perpetual
movement, it oscillates. These oscillations are
being studied, particularly with the help of
the Golf satellite to which CEA made a major
contribution. When the model has been finely
adjusted and agrees with the observations,
the internal properties of the Sun can be
deduced.
Stars whose initial mass and composition is
different, can also be modeled in this way (by
computer simulation). Computer simulation has
a special place among tools for studying nucleosynthesis in the Big Bang and the stars.
Depth (km)
690,000
Researchers are developing a physical model
of the Sun. This enables us Conforms to the laws
to determine the physical of physics.
parameters (density, temperature, pressure,
chemical composition, energy level, etc.) of
the Sun at all depths, from the surface through
to the core.
The physical characteristics of the observable
surface of the Sun calculated by the model
– radius, brightness, and temperature – are
compared and adjusted to match the actual
observed characteristics of the Sun.
Other quantities are used to check that the
model is correct: one of the most relevant is
515,000
A MODEL OF THE SUN`
The Sun is just one of the hundred billion
stars in our galaxy, but it is the star closest to
us, at about one hundred and fifty million kilometers away, and therefore the one we have
observed best.
© CEA/National Solar Observatory
16
Temperature (°K)
15 million
7 million
3 million
1.5 million
6,000
11 > Nuclear astrophysics
17
> THE SUN
> THE SUN
It can be used to reconstruct and trace the Sun’s
evolution from its birth through to the moment
when it will shed its outer layer and a compact,
dense object, a white dwarf, will remain.
The transformation of hydrogen into helium in the Sun
Stage 1
LIFE EXPECTANCY OF THE SUN
AND STARS
Deuterium
Neutrino: ν
e+
Hydrogen fusion is enough to fuel the Sun
throughout most of its luminous life. To
determine its life expectancy, we need to
compare its energy reserves with its energy
consumption.
Calculations using the model of the Sun indicate that the total nuclear energy available will
run out after about ten billion years. This is
twice the age of the oldest rocks on Earth and
Stage 2
Helium-3
Deuterium
Gamma radiation
Stage 3
Helium-3
Helium-3
“The Sun, aged
4.6 billion years,
is half way through
its life.”
the Moon and of meteorites (4.6 billion years).
It is believed that all the bodies in the solar
system were born at around the same time. So
the Sun, aged 4.6 billion years, is half way
through its life.
CALCULATION OF THE SUN’S
LIFE EXPECTANCY
Helium-4
More than 10% of the Sun’s mass has been
converted into helium. As a first approximation,
we can assume that it originally consisted
of pure hydrogen. As 0.7% of the mass of
the hydrogen will be converted into energy
by the formation of helium nuclei, the total
quantity of energy the Sun
has is
J
Enuc = 0.1 x 0.007 x M c2 = 2.10 x 1033 Mev.
Like any other star, the Sun is a huge nuclear reactor. Nuclear fusion reactions take place in its core, during
which hydrogen is changed into helium, releasing energy. The temperature in the center of the Sun is fifteen
million degrees and the density is one hundred and fifty times that of water (150 g/cm3).
The transformation of hydrogen into helium is complex, and takes place in three stages:
• Stage one: two protons interact to form deuterium. During this process, one proton is changed into a neutron
by emitting a positron (or positively charged electron) and a neutrino, a particle from the same family as the
electron, which carries energy and has a very low mass.
• Stage two: one deuterium nucleus combines with a proton to form helium-3, releasing energy in the form
of a gamma ray (or photon).
• Stage three: two helium-3 nuclei combine to form helium-4 by ejecting two protons.
When all the hydrogen in the Sun’s core has transformed into helium, the Sun will contract under the force
of gravitation. The temperature at its center will increase until it triggers the nuclear fusion of helium.
The lifespan of the Sun is obtained by dividing the
energy remaining by its consumption. We obtain
the figure of approximately 10 billion years.
An electron volt is a unit of measurement
= 1.602 x 10-19 joules (1 MeV = 1 million
electron volts).
© Eso/ANTU/UT1
18
J
M = solar mass = 1.991 x 1030 kg (see p. 11).
Death of a star like the Sun.
The science of the stars and the cosmos
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
19
20
> SUPERNOVAE
SPECTACULAR BUT RARE, SUPERNOVAE ARE
CATACLYSMIC EXPLOSIONS
OF CERTAIN TYPES OF STARS.
Supernovae
From generation to generation of stars, the
galaxy is becoming richer in heavy elements.
The most powerful driving forces behind the
chemical evolution of galaxies are supernovae.
and of their spectrum. Supernovae can be classified by comThe spectrum is the analysis of
emitted light; it can be used to
pa ring these
determine which chemical elements
are present and their abundance.
different data.
WHAT IS A SUPERNOVA?
THE DIFFERENT TYPES
OF SUPERNOVAE
A supernova is a star that explodes after the
implosion, or collapse, of its core. It becomes
as bright as billions of stars. Supernovae are
the result of spectacular but rare events;
there are about three per century in galaxies
like ours.
Supernovae are important for understanding
our galaxy. They heat the interstellar environment, scatter heavy elements within it and
accelerate cosmic rays (see pp. 23 and 24).
The observation of supernovae is based on their
light curve, i.e. the evolution of their brightness over time, of their maximum brightness
Previously, the presence or absence of hydrogen
in the spectrum was used to classify a supernova as one of two different types: I (absence of
hydrogen) or II (presence of hydrogen).
However, this traditional spectroscopic classification has recently been replaced by a
physical distinction characterizing the way
they exploded: thermonuclear or gravitational.
THERMONUCLEAR SUPERNOVAE
When two stars are in close proximity, they
orbit around one another in a binary system.
© Hubble/Aura/STScI/Nasa
Explosion of a supernova
Pre-explosive star
The science of the stars and the cosmos
11 > Nuclear astrophysics
Collapse of the core
The science of the stars and the cosmos
Interaction of the
shockwave with the
outer layer falling
towards the core
Explosive ejection
of the envelope
Expanding envelope
11 > Nuclear astrophysics
21
> SUPERNOVAE
Thermonuclear supernovae occur in binary
systems when one of the stars is a white dwarf.
The matter from the first star falls on to the
white dwarf, increasing its mass to 1.4 times
that of the Sun. It collapses and explodes. All
the matter is dispersed into space, and nothing remains at the center of the supernova.
GRAVITATIONAL SUPERNOVAE
A gravitational supernova occurs when a star
explodes at the end of its life. It explains the
formation of the heaviest elements in the Uni-
> SUPERNOVAE
verse. The same amount of energy is released
in a single day as the Sun has released in the
last three million years. It ejects vast quantities of gas and dust.
When the core of a massive star implodes and
it immediately sheds its outer layer, a fantastic
amount of energy is released, essentially in
the form of neutrinos. Only one ten thousandth
of the total energy appears in the form of visible light.
Depending on the initial mass of the star that
exploded, the implosion of the iron core of a
© Hubble Heritage Team/W. Blair/D. Malin/Nasa
© Dr C. Burrows/ESA/STScI/Nasa
Shockwave generated by a supernova.
The science of the stars and the cosmos
succession of neutron captures and disintegrations.
The two types of supernova do not produce
different elements in the same proportions,
nor do they explode at the same rate (one
thermonuclear supernova appears for every five
gravitational ones). Gravitational supernovae
effectively produce a number of elements
between carbon and calcium, oxygen being
the most abundant, while thermonuclear
supernovae provide iron and its neighboring
elements. According to estimates, approximately 50% of iron comes from this type of
supernova.
massive star leaves behind a dense object that
can be identified as either a neutron star or a
black hole.
SUPERNOVAE, THE SOURCE
OF HEAVY ELEMENTS
SN1987A, THE SUPERNOVA
OF THE
CENTURY
A supernova can be
visible to the naked
eye, from Earth, if it
explodes within the
perimeter of our own
galaxy or in the
Magellanic Clouds,
our satellite galaxies.
The last time this happened was on 24 February
1987, when the supernova named SN1987A
appeared in the Large Magellanic Cloud. Because of
its proximity, it meant that a vast array of scientific
results could be collected. Observatories and
satellites all over the world immediately pointed
their instruments and detectors in its direction.
Several types of radiation it emitted could be
observed: visible light, radio waves, ultraviolet and
infrared. And for the first time, a neutrino flux could
be detected and measured. It was a gravitational
supernova.
“The study of
supernovae tells us
more about the rate
of formation
of stars at all stages
in the evolution
of the Universe.”
11 > Nuclear astrophysics
At the center of massive stars that are set to
become supernovae, the density is such that
protons change into neutrons by capturing an
electron. The ball of neutrons measuring about
thirty kilometers in diameter that remains
after the explosion, in place of the supernova,
is a neutron star.
The matter projected into space during the
explosion is subject to a very large flux of neutrons escaping from the neutron star. The heaviest nuclei in nature (up to uranium) are formed
in this way, through the rapid capture of neutrons by the nuclei originating in the different
phases of combustion of the star, in the external
layers of the supernova that is exploding. This
phenomenon is known as explosive nucleosynthesis. For example, studies have shown
how gold was produced in the Universe by a
The science of the stars and the cosmos
© J. Hugues/Nasa/CXC/SOA
22
Emission of X-rays from the Cassiopeia supernova.
11 > Nuclear astrophysics
23
25
> SUPERNOVAE
THE INTEGRAL SATELLITE OPENS UP
A NEW GOLDEN AGE OF NUCLEAR
ASTROPHYSICS.
Life cycle of stars
Birth of a star
Cosmic radioactivity seen by the
Integral satellite
Red giant
Nebula
White dwarf
Supernova
Neutron star
Black dwarf
Pulsar
Black hole
SUPERNOVAE, THE SOURCE
OF COSMIC RADIATION
The shock waves produced by supernovae stir,
shake and heat the interstellar environment.
As they pass they accelerate atomic nuclei and
The science of the stars and the cosmos
electrons and are the source of cosmic radiation, which, through the nuclear reactions it
triggers as it passes, is itself responsible for
generating light nuclei, lithium, beryllium and
boron.
11 > Nuclear astrophysics
© CEA
24
The science of the stars and the cosmos
11 > Nuclear astrophysics
26
> COSMIC RADIOACTIVITY SEEN BY THE INTEGRAL SATELLITE
The hot radioactive remains of star explosions
emit X-rays and gamma rays. This is what
astronomers observe, because the most energetic part of the electromagnetic spectrum,
known as gamma, provides the clearest indicators of the synthesis of atomic nuclei in the
Universe.
The study of radioactivity from the Milky Way
and from neighboring galaxies by the Integral
satellite (International Gamma-Ray Astrophysics
Laboratory) has opened up a new golden age
for nuclear astrophysics. The satellite is the
result of a European collaboration run by ESA
(European Space Agency).
Launched in October 2002 at the Baikonur
space center in Kazakhstan, using a Proton
rocket, the satellite is in its data acquisition
phase.
The study of the radioactivity of the remains
of supernovae and of stellar winds, using Integral, should enable us to fine-tune our star
models and gain a better understanding of the
dynamic processes that govern their evaporation and explosion. The aim of this scientific
> COSMIC RADIOACTIVITY SEEN BY THE INTEGRAL SATELLITE
mission is essentially to detect the gamma rays
emitted within stars by short-lived radioactive
elements such as aluminium-26, medium-lived
radioactive elements like titanium-44 and longlived radioactive elements such as cobalt-56.
It also provides an opportunity to locate the
places in the galaxy where the greatest amount
of nucleosynthesis is occurring. Measuring these
should enable the identification of the isotopes
emitting radiation, the estimation of their abundance and the physical conditions of their
source environment.
“CEA is using X- and
gamma ray astronomy
to study the
explosions of stars
and their radioactive
remains.”
NUCLEAR EVOLUTION
© CEA
© CEA
Our galaxy is still evolving. Indicators
of recent nucleosynthesis have been
obtained by the observation of
radioactivity in the disc of our galaxy.
Detection of the disintegration of
aluminium-26, in different directions, has
allowed us to create a map of the galaxy.
Aluminium-26 is a radioactive nucleus
with a lifespan of 1 million years (whilst
our galaxy has a lifespan of approximately
10 billion years). So nucleosynthesis has
been analyzed to study with accuracy
the mechanisms that, in particular,
lead to the formation of aluminium-26.
The aim of this study is to understand how
this isotope can be produced by stars
and ejected into the interstellar
environment before it disintegrates,
i.e. in less than a million years. It seems
that its main sources are the Wolf-Rayet
massive stars and the supernovae.
Spectrometer during ground-based scientific calibration.
The science of the stars and the cosmos
11 > Nuclear astrophysics
The science of the stars and the cosmos
11 > Nuclear astrophysics
27