Download SCE 18 – Part 10

Life of a star like our sun
The Universe
• What is it?
• How did it start?
• How will it end?
• Is it really a UNIverse?
• Or a (small) part of a MULTIverse?
• “I am very interested in the universe – I am specialising in
the universe and all that surrounds it.”
(Peter Cook)
History of our Universe
• To begin at the beginning……
• We have no means of directly investigating the origin.
• Can measure the speed galaxies move away from earth.
• We can measure speed of galaxies at different distances, and
due to the finite speed of light, thus at different times in the
past when the light set out.
• Result: the universe is expanding now, & has been in the past
• If we run this “backwards” we see that at earlier times,
galaxies must have been closer together.
• Eventually the distance would approach zero.
• Would occur about 13-14 thousand million years ago.
• Colloquially, this event is known as the “Big Bang” after Fred
Hoyle. Georges Lemaître called it a “Cosmic Egg”.
Early theories of the origin of the universe
• The idea of all the mass/energy of the universe, space & time
being created ‘ex nihilo’ was repugnant to many.
• Lemaître – a Belgian Catholic priest – was receptive – he
called it the Primeval Atom or Cosmic Egg, containing all the
matter – which then exploded.
• Eddington, and many other astronomers in the 30s rejected
the idea of a “Big Bang”.
• However, 1940s, Gamow supported the idea.
• Fred Hoyle was strongly opposed and coined the term “Big
Bang” as a term of derision.
• Hoyle, with Hermann Bondi and Tommy Gold proposed a
Steady State Model.
• Continuous creation of matter and energy: the universe has no
beginning and no end
Evidence for the Big bang - by Accident discovery
• 1964 Arno Penzias and Robert Wilson found evidence of
microwave radiation over all directions.
• The Cosmic Microwave background.
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Perfect Planck radiation curve
1992 Data from the COBE satellite showed the radiation to
be a perfect Planck Black Body curve corresponding to a
temperature of 2.73K.
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Smooth featureless universe??
• The “perfect” Black Body curve presented a problem,
though.
• Black Body curves correspond to a small source.
• **Which is OK for the BB model**
• But the microwave intensity was then
measured to be independent of
direction, suggesting a universe which
would grow up to be smooth, with
no chance of developing galaxies,
stars and planets.
•An escape clause: Quantum Fluctuations. Which are
always present (Heisenberg’s Uncertainty principle).
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Quarks
Nucleus of Hydrogen atom
Quarks
Proton + neutron - the nucleus of a Deuterium atom
[or half of a Helium nucleus (2 Protons + 2 Neutrons) (α-particle)]
Very early stages in the birth of the universe
After inflation stopped: a “quark-gluon plasma.”
• Quarks are building blocks of protons & neutrons & are bound
together by the strong nuclear force).
• The two quarks are called “up” and “down”.
•
As the temperature falls, quarks combine to form protons and
neutrons.
• Weak nuclear force has the ability to transform one type of
quark into another.
• So the proportion of up and down quarks depends on the
relative strengths of the two forces.
• The temperature window for transformation of quarks by the
weak nuclear force is very narrow.
• After which, transformation ceases.
Early universe
• Universe consists of “plasma” of H and He nuclei
• (and a bit of Li)
• apart from electrons, which cannot unite with nuclei to form H
or He atoms as temperature is still too high.
• About 380,000 years after BB, temperature falls to ~ 3000K.
• Nuclei and electrons can now form H & He atoms.
• (Note: but nothing else – no C, O, N etc)
• We can now see what the universe looked like then as
background radiation (microwaves).
• This is the “Cosmic Microwave background (CMB) radiation.
• Radiation ”left over” from the Big Bang,
• The earliest radiation we can detect.
Note due to the logarithmic scale, the amount of H is 109 times higher than that of Li
What happened before 380,000 years ?
• We cannot see anything earlier than 380,000 years.
• Earlier universe was opaque:
a “plasma” of nuclei, electrons, photons, neutrinos, dark
matter…
• Light (Photons) are scattered by charged electrons and nuclei.
• Analogy with the sun: we can only see the surface of the sun –
photons from deeper layers are very efficiently scattered.
• Like a dense fog – only more so!
• Estimated times for a -ray photon to emerge from the centre
of the sun (at 13 x106K):
•
10,000 to 170,000 years!
History from 10 minutes to 380,000 years
• The temperature continued to fall (as expansion
continued) but at a lower rate.
• Electrons and nuclei are still NOT united as atoms.
• But after about 379,000 years, the temperature became
sufficiently low that protons, neutrons and electrons could
combine to form atoms.
• Still highly energetic atoms, but no longer coupled to
photons.
• Universe now becomes transparent.
Satellite observations reveal minute fluctuations
• Later, detailed measurements have shown changes in the
microwave intensity in different directions in space.
• Correspond to minute temperature fluctuations:
about 0.0003 K around an average value of 2.725K.
• This indicates very small concentrations of microwave
energy, which, when it was emitted about 360,000 years
after the Big Bang would have been about 1K around an
average value of 3000K.
• Thus (E=mc2), there are slight variations in mass
which then grow to form clouds of gas, stars and galaxies.
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Cosmic microwave background
(WMAP satellite)
CMB (Planck satellite) 2013
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Cosmic Microwave Background
• The CMB – about 380,000 years after the Big Bang
is the Afterglow of the B.B. – not the B.B. itself.
• At 3000K, light emitted would be in the Ultraviolet.
• Expansion of space “red shifts” this light to microwave
frequencies where WMAP detects it today.
• From ~ 400,000 y to 400 million y, the Universe was dark:
The “Dark Age”.
• Before 400,000,000 years (counting from the big Bang)
clouds of gas were too small & too dilute to warm up
sufficiently.
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The life of the universe – so far
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Stars: birth and death
• Radiation is almost uniform, in all directions,
but with very slight variations corresponding to regions of
more and less dense matter.
• Force of gravity causes more dense regions to attract
matter from less dense regions.
• Matter (H and He) becomes sufficiently compressed to
cause temperature to rise and start nuclear reactions.
• In stars bigger than our sun, nuclear reactions produce
nuclei of heavier elements up to iron.
• Very big stars can synthesise Uranium (our heaviest
element.)
• In death, stars eject these elements into space (often
very violently).
Solar System
• The sun is not a first generation star (BB + 400 million years) –
otherwise it could not contain the heavy elements.
• The solar system – and us – are recycled nuclear waste!
• Our star, the sun, was formed from cosmic gases and dust
about 6 billion years after BB.
• Probably formed in a rotating cloud of gas and dust
• Like those seen in tips of Eagle nebula.
• Rotation could then define orbits of planets, asteroids etc. in
the solar system.
• Nuclear reactions in sun involve “burning” of H to He with
evolution of Heat energy. Strong force in action again!
• But temperature too low for synthesis of heavier elements.
Eagle nebula
Possible star nursery
Power station of the sun
2 protons react to give Deuterons (one
proton transforms to a neutron (via
Weak Force) with emission of neutrinos
and positrons.)
Further protons react to form
He3 : nucleus of He isotope
Two He3 nuclei fuse to give He4
nucleus with ejection of 2 protons
At each stage, energy is emitted as p and n
are more stable bound than unbound.
(Can also consider E = Δm x c2, where Δm
is change in mass.
Energy eventually appears as light and
neutrinos (1015 per m2).
He nucleus
Early history of the Solar System
• Solar System formed
about 4.6 billion years ago
from a giant molecular cloud.
• - the debris from a previous
generation of stars.
• Earth’s atmosphere originally contained no free O2, although
oxygen is third most abundant element of the universe.
• Most locked up as H2O, CO2, or in minerals.
• Cyanobacteria, appeared ∿ 2.5 billion years ago.
• This lead to the Great Oxygenation event, when anaerobic lifeforms rapidly became extinct and were replaced by aerobic
forms.
• The earth became the “little blue planet” due to oxygen
• and Ozone O3, in the atmosphere.
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Photosynthesis
Photosynthesis
• Cyanobacteria, “..arguably the most successful group of
microorganisms on earth”,
• use light to power growth by splitting O2 from H2O
• The process involves absorption of a solar photon, say by a
chlorophyl-like molecule:
• Ferrying an excited electron
(“exciton”) to a (distant) “Reaction
Centre”, where further synthesis
CHLOROPHYL
to proteins occurs.
• Ferrying is achieved by passing the
•
exciton “parcel” via densely-packed chlorophyl complexes,
with a requirement that
•
this must be done quickly, (less than nano second
1/1000,000,000 s) before the exciton decays.
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“Quantum Light Harvesting”
• What’s the most efficient route for exciton parcel to reach
“Reaction Centre” in less than a nano-second ?
• Mixture of classical & quantum mechanical physics proposed:
the “parcel” is passed by a “Quantum Walk” rather than a
Random Walk.
• Parcel’s wave travels in all directions simultaneously.
• (Where have we heard this before??).
• Feynman’s analysis of simple optical experiments : reflection
from mirrors, Diffraction gratings....
• Feynman linked this with the principle of “Least Time”.
• There are problems relating to the degradation of quantum
waves: ”decoherence”.
• Solution might require some “classical”assistance.
• But photosynthesis is almost 100% energy efficient.
• This process is similar to those being developed for Quantum
Computing
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Time taken for photon to
goto the mirror
and then
to the photomultiplier.
Least time
Addition of contributions
from each part of the mirror
Major contribution comes
from arrows E to I,
because the timing of their
paths is nearly the same
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Summary - life of stars like the sun - Red Giants
• All stars contract due to gravity, which causes the core
temperature to rise with nuclear reactions.
• For small stars (0.5 - 4 times the mass of the sun)
reactions in the core “burn” H to He, counteracting
gravitational pressure.
• When all H in core is burned, heating in core ceases & H
to He conversion continues in an outer shell.
• Outer layers then expand and spread over a huge volume
creating a “Red Giant”.
• Sun will become a Red Giant in about 5 billion years and
will engulf Mercury, Venus and possibly Earth.
• But not to worry!
• After 1 billion years, the sun will be so hot that all oceans
will have evaporated.
• For stars less than 2.5 times solar mass, the elements
are recycled as Planetary Nebulae.
• For more massive stars, the He core continues to be
squeezed becoming dense enough (107 x normal
densities) for electrons to be squashed – which they
strenuously resist...
• ..... Pauli’s Exclusion Principle.
• Now core heats up (T = 108 K) so He “burning” to C
starts.
• Elements formed by nuclear reactions are now recycled.
• Core remains as a very dense “White Dwarf”.
Ring Nebulaa typical planetary
nebula.
The gaseous shroud is
material expelled from
a dying star.
Violet – atomic H
Red – molecular H2.
Cat’s Eye Nebula
Planetary Nebula
Discovered by William Herschel
In 1786
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Formation of elements beyond Helium
• The “Triple alpha process” could work:
•
3 4He -> 12C + Energy ( rays).
• But the high energy simultaneous collision of 3 alpha
particles is very unlikely.
• Edwin Salpeter suggested an unstable isotope of Be
might accelerate the process as the reaction can now go
in stages.
• 2 4He + Energy of motion -> 8Be
•
8Be
+ 4He -> 12C + Energy ( rays).
• Hoyle realised this process could only work if energies of
reacting particles = a “resonant oscillation” of the C12
nucleus.
• If the “resonance” does not exist then C (and C-based
life) could not exist either.
Fred Hoyle
• Since carbon does exist, where is this “resonant
oscillation” ?
• Not found in the data books
•Hoyle said let’s go and find it!
•He travelled to US and persuaded a team of
experimentalists (with difficulty) to change their scheduled
experiments and do the measurements.
•They found it. Lead to a Nobel prize -
•But not for Hoyle!
Hoyle’s predicted closely defined energy match
• No resonance had been observed at the energies Hoyle
had predicted: 7.65 MeV.
• Hoyle persuaded Fowler’s lab to interrupt the
experimental schedule to look for the missing
resonance.
• It did exist at 7.655 MeV!
• A further “ fortunate coincidence”: the next step in the
chain:
•
12C
+ 4He -> 16O + Energy ( rays),
• Does not involve a favourable resonance otherwise the
Carbon, once formed, would be quickly converted to
Oxygen.
Triple alpha process
Further elements can be formed this way
• A third “fortunate coincidence”: the 8Be isotope has a
sufficiently long half life (10-17 s) to allow further alpha
particles to collide.
(These “coincidences” have provoked many Philosophical
interpretations!)
•
Continuation of this process of Nuclear Fusion
• with evolution of energy at each step
• can continue as far as iron, Fe.
because the Binding energy increases with each
nuclear
transformation.
So stars cannot produce the elements heavier than iron
directly.
Nucleosynthesis in a massive star
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Supernovae
• Stars more than about eight solar masses can burn as far as
56Fe.
• Then reaction in core ceases and gravity causes it to collapse
inwards – it implodes.
• Core temp. soars to 1010K & nuclei break up giving p & n again.
• This absorbs energy (heat) so core is less able to resist gravity.
Protons & electrons are crushed together:
• p + ε -> n + neutrinos
• With only n left - neutrinos escape carrying energy, so despite
Pauli’s Exclusion principle trying to prevent n-n contact, core
density rises to x1012 normal densities.
• Then core bounces back sending shock waves which blast the
outer layers into space.
Supernovae generate the (missing) heavy elements
• In this highly explosive phase which lasts only a few
seconds, the light generated can outshine an entire
galaxy.
• Over the few months before it fades away, a supernova,
can radiate more energy than the sun is expected to
generate over its entire life.
• The extreme conditions of the explosion generate the
elements heavier than iron.
• The debris is ejected into space and then is then recycled
by other stars.
• What remains may be a (small) neutron star – very, very
dense.
• It could be a “Pulsar”.
• Or, if VERY dense, a “Black Hole”.
Crab nebula
Supernova remnant.
Exploded within the Milky Way
Reported by Chinese
astronomers on 4 July 1054
Visible in daylight for 630 days.
Europeans didn’t record it.
But observed later by John
Bevis, amateur astronomer in
Wiltshire.
Expanding at 1800 km/s.
The life of the universe – so far
What do we not understand......
• Some History.....
• 1925 Paul Dirac came to St John’s College to study for a
PhD.
• Produced an elegant solution to a problem that had
baffled Heisenberg.
• 1928 Attempted to produce the first theory of the electron,
fully consistent with Einstein’s (1905) theory of General
Relativity.
• He started “playing with equations” until one fitted.
• But instead of one equation describing the electron, he
found 4, intertwined in 2 pairs.
• One pair he recognised as the 2 spins of an electron.
• The other pair were very mysterious.
Antimatter
• Dirac’s equation was hailed as a masterstroke.
• But predicted a particle with negative energy.
• This lead to suspicion - particularly from Heisenberg &
compatriots in Heidelberg.
• Only later, after Dirac’s failed attempts at explanation,
experiment discovered an electron with positive charge.
• An anti-electron dubbed a “positron”.
• Now recognised that every particle has an antiparticle:
antiprotons, antineutrons etc.
• Particles and antiparticles annihilate producing pure
energy.
• Similarly, energy creates particle-antiparticle pairs.
What happened to antimatter after the Big Bang?
• The enormous energy released at the B.B. must have
produced particle and antiparticles in equal numbers.
• A major unanswered question concerns the fate of the
antiparticles.
• One possibility that particles and antiparticles are not exactly
equal and opposite.
• As energy in the form of photons converts to particles +
antiparticles and back again:
•
Photon <----->Particle + Antiparticle
• As matter is very slightly favoured (1 part in a billion), then
eventually it will dominate completely,
• and antimatter will no longer exist in the Universe.
• This is a Major unsolved problem.
The Universe - data from analysis of the CMB
Constitution of the universe
today.......
We understand less than 5% ....
Dark energy implies that
the expansion of the universe
is accelerating.
Constitution of the universe 13.7
billion years ago when the Universe
was only 380,000 years old.
FINAL THOUGHTS
• “The last century was defined by physics. From the minds of the
world’s leading physicists there flowed a river of ideas that would
transport mankind to the very pinnacle of wonder and to the very
depths of despair. This was a century that began with the certainties
of absolute knowledge and ended with the knowledge of absolute
uncertainty.”
•
Jim Baggott: “The Quantum Story”
• The Dirac Equation : “ In a few squiggles of this pen, he had
described the behaviour of every single electron that had ever existed
in the universe.”
•
Graham Farmelo: “The strangest Man”
• “The theory of Quantum Electrodynamics describes Nature as absurd
from the point of view of common sense. And it agrees fully with
experiment. So I hope you can accept Nature as She is - absurd.”
•
Richard Feynman: “Q E D”
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