Nuclear Astrophysics(a Cosmic Cookbook)Nuclear Astrophysics(a Cosmic Cookbook)

Dr Chris Ruiz - TRIUMFDr Chris Ruiz - TRIUMF

Nuclear Astrophysics

Atomic nucleus 1 x 10-15 mAtomic nucleus 1 x 10-15 m

The everyday star ~1 x 109 mThe everyday star ~1 x 109 m

1,000,000,000,000,000,000,000,000 x smaller!

We are interested in….We are interested in….

X-ray burstsX-ray burstsClassical novaeClassical novae

supernovaesupernovae

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…

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…

We’ll find out how…

=

+

(Don’t try this at home!)

Helium

Hydrogen

Periodic table of the chemical elementsPeriodic table of the chemical elements

Need some physics..

No equations!

Need some physics..

No equations!

What is a star? What is a star?

A big ball of hot gas, right?

Stars in history….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!

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!

Eratosthenes 276-194 B.C

Calculated circumference of Earth, distance to Moon and Sun, using only trigonometry and careful observations

Within 1%!

Eratosthenes 276-194 B.C

Calculated circumference of Earth, distance to Moon and Sun, using only trigonometry and careful observations

Within 1%!

Aristarchus 310-230 B.C

Heliocentric theory

Aristarchus 310-230 B.C

Heliocentric theory

Burning of Alexandria, 391 C.EBurning of Alexandria, 391 C.E The Dark AgesThe 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)

• 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

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

The Renaissance (rebirth) - and heliocentrism

Copernicus: heliocentric theory (again!)

Copernicus: heliocentric theory (again!)

Kepler: laws of planetary orbital periods

Kepler: laws of planetary orbital periods

Galileo: telescope + more support for heliocentric

theory!

Galileo: telescope + more support for heliocentric

theory!

Isaac Newton 1643-1727Isaac Newton 1643-1727

16861686

• Universal Gravitation

• Derived Kepler’s laws

• Last doubts of Heliocentrism removed

• Universal Gravitation

• Derived Kepler’s laws

• Last doubts of Heliocentrism removed

Josef von Fraunhofer 1787-1826

Josef von Fraunhofer 1787-1826

• 1814 invented spectroscope and discovered hundreds of dark lines in solar spectrum

• 1814 invented spectroscope and discovered hundreds of dark lines in solar spectrum

Explained in 1859 by Kirschoff and Bunsen as Atomic Absorption LinesExplained in 1859 by Kirschoff and Bunsen as Atomic Absorption Lines

Planck’s radiation law, 1900Planck’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!

• 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!

1908 Ernest Rutherford Nobel prize for radioactive decay experiments, discovery of the atomic nucleus - proposes Sun has an internal heat source…radioactivity!

1908 Ernest Rutherford Nobel prize for radioactive decay experiments, discovery of the atomic nucleus - proposes Sun has an internal heat source…radioactivity!

• 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?

• 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?

HeHe

HHHH HHHH

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…..?

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…..?

Albert says Yeeeeees!!Albert says Yeeeeees!!

E = mc2E = mc2

(ok, I lied about no equations!)(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…..

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

Electrostatic forces between charged particles

Like charges repelLike charges repel

Opposite charges attractOpposite charges attract

Thus for the constituents of an atom:

Thus for the constituents of an atom:

+ +

+ -

Positively charged protons repel each other

Positively charged protons repel each other

A negatively charged electron feels an attractive

force towards a proton

A negatively charged electron feels an attractive

force towards a proton

A neutron has no net electric charge

A neutron has no net electric charge

Conditions inside a starConditions inside a star

• It’s Hot! distribution of speeds• It’s Hot! distribution 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

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

The hotter a gas is, the greater the average speed (energy) of the particles is,

and the greater the range of speeds

The hotter a gas is, the greater the average speed (energy) of the particles is,

and the greater the range of speeds

So can protons fuse together?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

• 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…….?

• 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 fuseTwo 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

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.

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.

IsotopesIsotopes

Number of neutronsNumber of neutrons

Num

ber

of p

roto

nsN

umbe

r of

pro

tons

0 1 2

0

1

2

1H 2H 3H

4He3He

6Li

3 4

3

Fusion in the sun, gradually converts hydrogen to helium!Fusion in the sun, gradually converts hydrogen to helium!

The are thousands of nuclear isotopes!The are thousands of nuclear isotopes!

uraniumuranium

goldgold

ironiron

carboncarbon

• 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….

• 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….

Star like our sunStar like our sun

A more massive star than our sunA more massive star than our sun

Red Giant (helium burning)

Red Giant (helium burning)

Red Supergiant

Red Supergiant

Massive stars will have several burning stages, resulting in an

‘onion-skin’ structure…

Massive stars will have several burning stages, resulting in an

‘onion-skin’ structure…

SupernovaeSupernovae

• 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!

• 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) (cup of tea ~600 Joules)

= 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!

Energy released = 1046 Joules (in a matter of seconds) (cup of tea ~600 Joules)

= 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!

Classical novaeClassical novaeStellar Binary systemStellar Binary system

Red GiantRed Giant

White DwarfWhite 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!

• 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 burstsX-ray bursts

Neutron star in binary systemNeutron 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!

• 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!

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!

nucleosynthesisnucleosynthesis

Static H,He,C,O,Ne,Si burning in massive star

Static H,He,C,O,Ne,Si burning in massive star

56Fe56Fe

Rapid-proton process (hot

hydrogen fusion)

Rapid-proton process (hot

hydrogen fusion)

(Proton-fusion up, beta+ decay diagonal)

(Proton-fusion up, beta+ decay diagonal)

Slow-neutron process

(cold neutron fusion)

Slow-neutron process

(cold neutron fusion)

Neutron fusion across, beta- decay diagonal

Rapid neutron process (hot

neutron fusion)

Rapid neutron process (hot

neutron fusion)

So we want to study the nuclear reactions…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:

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)

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)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……

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

Measuring fusion reactions: producing exotic nuclei

World’s largest cyclotronWorld’s largest cyclotron

Proton beam

75% speed of light!

Proton beam

75% speed of light!

Solid target - nuclear

reactions

Solid target - nuclear

reactions

Ionizer (strips

electrons off)

Ionizer (strips

electrons off)

Exotic atomsExotic atoms

Magnetic Isotope selector

Magnetic Isotope selector

Particle accelerator

Particle accelerator

experimentexperiment

Beam of exotic nuclei at same energy as in star

Beam of exotic nuclei at same energy as in star

Decay or mass measurement experiments

Decay or mass measurement experiments

Measuring fusion reactions:

The DRAGON

Measuring fusion reactions:

The DRAGON

• With this instrument we measure the strength of the fusion reactions

• The exotic nuclei hit a hydrogen (or helium) gas 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

• With this instrument we measure the strength of the fusion reactions

• The exotic nuclei hit a hydrogen (or helium) gas 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

27Si recoils

27Si recoils

SummarySummary 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

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

“We are all made of star stuff”

- Carl Sagan 1934-1996