Transcript Nuclear Astrophysics (a Cosmic Cookbook)

Nuclear Astrophysics
(a Cosmic Cookbook)
Dr Chris Ruiz - TRIUMF
Nuclear Astrophysics
Atomic nucleus 1 x 10-15 m
The everyday star ~1 x 109 m
1,000,000,000,000,000,000,000,000 x smaller!
We are interested in….
X-ray bursts
Classical novae
We can never touch these
stellar events, only observe
them from afar.
But, we can do experiments on
Earth which can help us to
understand them…
supernovae
We’ll find out how…
Helium
+
Hydrogen
(Don’t try this at home!)
=
Periodic table of the chemical elements
Need some physics..
No equations!
What is a star?
A big ball of hot gas, right?
Stars in history….
Babylon, 1,800 B.C
Astronomical records for agriculture
Babylonians invented 360º in a
circle 60 minutes in an hour, 60
seconds in a minute, the
sexagecimal (base 60) system!
Also, days of the week, signs of the
zodiac,….
…but thought Sun was a planet!
Aristarchus 310-230
B.C
Heliocentric theory
Eratosthenes 276-194 B.C
Calculated circumference of
Earth, distance to Moon and
Sun, using only trigonometry
and careful observations
Within 1%!
Burning of Alexandria,
391 C.E
The Dark Ages
• Science of the Greeks ‘heretical’
• Only Monks could read & write
• All literature dedicated to glory of God
and the literal interpretation of the
gospels
• Motions in the heavens unquestionable
facts of creation
• Enquiring minds had to do so in secret
(Alchemists)
However, Arabic kingdom of Al-Andaluz (Andalucia) in
Spain carried on much Greek mathematics and astronomy
Christian monks were known to have visited this kingdom
and accessed Greek texts…undercurrent of things to come
The Renaissance (rebirth) and heliocentrism
Copernicus: heliocentric
theory (again!)
Galileo: telescope + more
support for heliocentric
theory!
Kepler: laws of planetary
orbital periods
Isaac Newton 1643-1727
1686
• Universal Gravitation
• Derived Kepler’s laws
• Last doubts of Heliocentrism removed
Josef von Fraunhofer 17871826
• 1814 invented spectroscope
and discovered hundreds of
dark lines in solar spectrum
Explained in 1859 by Kirschoff and
Bunsen as Atomic Absorption Lines
Planck’s radiation law, 1900
• 1895 Helium isolated and shown to
be in solar spectrum
• Also Silicon, Carbon, Sodium,
Oxygen,…
• Kelvin-Helmholtz mechanism: Sun is
a hot liquid body, cooling, contracting,
and generating heat from gravitational
energy. Problem:
• 20 million year lifetime!
• 1920 Arthur Eddington shows that treated as
a hot gas, Sun’s gravity must balance outward
internal pressure…Sun must be millions of
degrees!
• Also proposes Sun converts hydrogen to
helium with a net release of energy!
How can this be?
1908 Ernest Rutherford
Nobel prize for radioactive
decay experiments,
discovery of the atomic
nucleus - proposes Sun
has an internal heat
source…radioactivity!
4 hydrogen atoms are heavier
than 1 helium atom, so if you
can somehow ‘join’ all the
hydrogen together, perhaps the
difference in mass will come out
as energy…..?
He
H HH
H
Albert says Yeeeeees!!
E = mc2
(ok, I lied about no equations!)
So now all we need is a
preponderance of hydrogen in the
sun…
check: Cecilia Payne 1925
Now for the Nuclear physics…..
Electrostatic forces between
charged particles
Thus for the constituents of
an atom:
Like charges repel
+
+
Positively charged protons repel
each other
+
-
Opposite charges attract
A negatively charged electron
feels an attractive force
towards a proton
A neutron has no net
electric charge
Conditions inside a star
• It’s Hot!  distribution of speeds
The hotter a gas is, the
greater the average speed
(energy) of the particles is,
and the greater the range of
speeds
In addition: electrons are ripped off nuclei because of
collisions - this is called a plasma
So have protons of many different energies in a sea of
electrons
So can protons fuse together?
• According to classical physics, we would
expect the proton to fuse with another if it
has enough energy to overcome the
electrostatic barrier (imagine a marble
‘rolling’ up the curve)
• But the average energy of a proton in the
sun is much lower than the barrier
• But according to quantum physics, there is a small probability that the particle
‘tunnels’ through the barrier
• Although this probability is miniscule, just think how many protons are in the
sun!
• Need to know how to make helium (4 particles) - probability of 4 protons
coming together are same time is almost zero
• So what happens when just 2 protons fuse…….?
+
+
Two protons tunnel and fuse
+
+

At the moment of fusion, one proton
turns into a neutron via the emission
of a positively charged electron
(positron) and a neutrino, a small, very
light particle
This new nucleus is called a deuteron. It is a type of hydrogen (because it has one
proton).
Each type of chemical element (hydrogen, helium etc), has several versions of itself that
contain more or less neutrons than usual. These are called isotopes.
Isotopes
Number of protons
3
6Li
3He
4He
1H
2H
3H
0
1
2
2
1
0
3
4
Number of neutrons
Fusion in the sun, gradually converts hydrogen to helium!
The are thousands of
nuclear isotopes!
uranium
gold
iron
carbon
• Hydrogen burns to helium
• When run out of hydrogen, star shrinks a bit, getting dense enough to fuse three helium
nuclei together to make carbon
• carbon can fuse with helium to make oxygen, and so on….
Red
Supergiant
A more massive
star than our sun
Massive stars will have several
burning stages, resulting in an
‘onion-skin’ structure…
Red Giant
(helium
burning)
Star like our sun
Supernovae
• When the evolved star on the previous slide can burn no
more, it collapses
• The core compresses until it becomes the most dense
substance known: a neutron star
• The rest of the star violently explodes, inducing even
more nuclear burning and spreading all the elements into
space
• This material can then form a new solar system, like
ours!
Energy released = 1046 Joules (in a matter of seconds)
= 30,000 trillion, trillion x Annual U.S Energy Consumption
Can outshine the entire Galaxy it occurs in!
More energy than our sun will generate in its lifetime!
(cup of tea ~600 Joules)
Classical novae
Stellar Binary system
Red Giant
White Dwarf
• In binary systems, one star can evolve to a compact
white dwarf while the other can become a red giant
• hydrogen-rich material is transferred (or accreted )
onto the white dwarf surface
• the temperature and density get so high that
hydrogen starts to fuse with carbon, nitrogen, oxygen
and heavier stuff
• this thermonuclear runaway blasts the material off
the star into space at very high speeds!
X-ray bursts
Neutron star in binary system
• neutron star has very strong
gravitational field
• temperatures and densities
reached much higher than novae
• different set of nuclear reactions
occur in thermonuclear runaway
• material cannot escape!
X-ray bursts are very interesting:
the observations don’t quite
agree with the theory yet; they
are extremely regular; no-one
quite knows what the
composition of the neutron star
is!
nucleosynthesis
Slow-neutron process
(Proton-fusion up,
beta+ decay
diagonal)
Neutron fusion
across, beta- decay
diagonal
(cold neutron fusion)
Rapid-proton
process (hot
hydrogen fusion)
56Fe
Rapid neutron
process (hot
neutron fusion)
Static
H,He,C,O,Ne,
Si burning in
massive star
So we want to study the nuclear reactions…
Most involve radioactive nuclei, with short lifetimes
 can’t make a ‘sample’ of them
So create nuclei (using nuclear reactions) continuously, i.e. create a beam of them
Then either:
Quickly study how they decay, or how they
respond to magnetic & electric fields …..(tells you
about their structure, how heavy they are etc)
Accelerate the nuclei up to stellar speeds and fuse
them with hydrogen or helium (or other stuff)
At the TRIUMF facility ‘ISAC’, we do
exactly this: create exotic, short-lived
radioactive nuclei only found in stars,
and we either watch them decay, trap
them and measure how heavy they
are, or accelerate them and fuse them
with other nuclei……
Measuring fusion reactions:
producing exotic nuclei
experiment
World’s largest cyclotron
Beam of
exotic nuclei
at same
energy as in
star
Particle
accelerator
Decay or mass
measurement
experiments
Magnetic
Isotope
selector
Solid target nuclear
reactions
Ionizer
(strips
electrons off)
Proton beam
75% speed of light!
Exotic atoms
Measuring fusion reactions:
The DRAGON
• With this instrument we measure the strength of
the fusion reactions
27Si hit a hydrogen (or helium) gas
• The exotic nuclei
recoils
target
• If fusion occurs, the product nuclei will be
heavier than the original nuclei
• We use electric and magnetic fields to separate
out these heavy nuclei and count, one-by-one,
how many we make!
• We give the results to other astrophysicists who
make computer models of
exploding stars
Summary
 The laws of physics allow the existence of stars via the
transformation of lighter elements to heavier ones
 All we need is hydrogen, and a little helium then gravity and the
other forces of nature take care of the rest
 Massive stars make massive elements
 Explosions of dying stars spread synthesized material around
the galaxy
 This star-enriched material can form a new star system
 Then the chemists, geologists, and evolutionary biologists can
take over
“We are all made of star stuff”
- Carl Sagan 1934-1996