Power and element generation in stars

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Power and element generation in stars

• Astronomy 115

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Energy transfer

• As the names of the layers imply, it is not the
  composition of the sun that is interesting, but
  the manner in which energy is transmitted from
  layer to layer.
• This difference in manner of energy transfer will
  be a direct result of the lessening density of
  the Sun outwards in fact, the outer edge of the
  convective zone (the photosphere) is far less
  dense than the Earths atmosphere!

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The Suns energy is generated by
thermonuclearreactions in its core

• Thermonuclear fusion occurs at very high
  temperatures
• Hydrogen fusion occurs only at temperatures in
  excess of about 107 K
• In the Sun, hydrogen fusion occurs in the dense,
  hot core

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Proton-Proton Chain Reaction

• The Suns energy is produced by hydrogen fusion,
  a sequence of thermonuclear reactions in which
  four hydrogen nuclei combine to produce a single
  helium nucleus called proton-proton chain
  reaction

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Proton-Proton Chain Reaction Step 1
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Proton-Proton Chain Reaction Step 2
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Proton-Proton Chain Reaction Step 3
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Proton-Proton Chain Reaction
4 H ? He energy neutrinos Mass of 4 H gt Mass
of 1 He

• In every second, 600 million tons of hydrogen
  converts into helium to power the Sun
• At this rate, the Sun can continue hydrogen
  fusion for more than 6 billion years.

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Solar neutrinos

• How do we know about the interior of the sun and
  how it produces power?
• One answer is neutrinos.
• We, on Earth, can measure neutrinos produced
  within the solar core. This is because neutrinos
  almost never interact with matter.

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Neutrino detection
Neutrinos DO interact with matter, but their
cross- section is small, meaning they dont hit
other matter very much.

• 7 107 neutrinos pass through your thumbnail
  (which is an area about 1 cm2) each second. But
  your body interacts with a neutrino only about
  once in 70 years. This length is jokingly
  referred to as the....

Neutrino Theory of Death! (human lifespan and
all, heh, heh)
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Neutrino detection
The first actual detection of a neutrino was made
by Frederic Reines and Clyde Cowan.
They didnt actually measure a neutrino, just the
by product of its reaction with a proton (1 in
1018 chance of occurring).
ne p ? n e
e e- ? 2g
In 1956 they measured these gamma rays from a
nuclear reactor at Hanford in E. Washington and
(conclusively) Savannah River in South Carolina.
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Why do we care about neutrinos?
Reason 1 Neutrinos are produced in the core of
the Sun in HUGE amounts (about 1038 neutrinos/s).
Reason 2 Most neutrinos escape the Sun without
interacting with the Suns matter, so they reach
the Earth in 8 minutes ! They travel at very
close to the speed of light.
Reason 3 Neutrinos are produced by several
reactions in the proton-proton chain and depend
on solar core composition, pressure, and
temperature.
Reason 4 They provide another boundary
condition for the standard model (i.e., the way
we describe subatomic particles).
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Complete fusion process in the solar core
(colored boxes show neutrino production)
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The solar neutrino spectrum
neutrino reactions in the Sun
p p ? D e n
p e- p ? D n
7Be
7Be
8B
(1MeV 1.6 x 10-13 J)
The relative contributions of the different
neutrino reactions depend on conditions in the
solar core.
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First detection of solar neutrinos
Homestake Mine experiment led by Ray Davis in
South Dakota 1.5 km underground 1965-1987
378,000 liters of cleaning fluid (ultra-pure
carbon tetrachloride). When neutrino interacts
argon is produced. 37Cl n ? 37Ar e- En
0.8 MeV Measures one neutrino every 2 days.

(17p 20n)
(18p 19n)
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The solar neutrino problem

• Standard Model of the Sun says that Homestake
  should detect 1.5 2 neutrinos per day, but it
  only detects 0.5 per day. Factor of 3 to 4
  difference.
• Either we dont understand the sun like we
  thought we did, or something else is going on.
  Hopefully not the first thing, because then the
  Standard Model would be hopelessly wrong.

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The solar neutrino problem
Adding up all the neutrinos does not get the
amount predicted in the Standard Model,
regardless of the detection method used.
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Solution to solar neutrino problem neutrino
oscillations
There are three flavors of neutrinos electron
neutrino (?e), muon neutrino (??), and the tau
neutrino(??) MSW Effect neutrinos oscillate
between flavors as they travel through space.
This is effect is strongly enhanced when
neutrinos pass through matter (Mikheyev, Smirnov,
and Wolfenstein, 1986) Homestake Mine could only
detect electron neutrinos Neutrino oscillations
require that the neutrino has mass (changes the
Standard Model of particle physics)
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How do we know if neutrinos oscillate?
Using very large omni-directional sensors of
water and heavy water (D2O). Measure a lack of
?e and overabundance of other flavors
Water Based SuperKamiokande in Japan, 50,000
tons of ultra-pure water Able to detect ?e above
7.5 MeV ?e scatter with e- in water, producing e-
that travel faster than c in water (called
Cherenkov radiation) which produces radiation
detected by thousands of photomultiplier tubes
(PMT) Measured lack of ?e (like
Homestake) Confirmed that neutrinos can
oscillate, but were unable to detect all the
solar neutrinos
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The solar neutrino observatories
Neutrinos are hard to measure, so the detectors
are large and omni-directional.
Neutrino observatories are defined mainly by the
energy range and flavors they can sample.
Heavy Water Sudbury Observatory (SNO) in Canada
1000 tons of D2O (UW Physics main US
participator) Can detect all flavors of
neutrinos (?e, ? µ ,and ? t) above 5
MeV Measured lack of ?e and abundance of ? µ
and/or ? t Best evidence for neutrino
oscillations and thus massive neutrinos
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Solar neutrino problem solved!

• In June of 2001, the SNO team reports that the
  neutrino deficit is solved

• Our model of the solar core is correct
• Neutrino mass needed to be added to the
  Standard Model

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Neutrino astrophysics
SN 1987A (supernova) Three hours before
observing light, neutrinos were detected in a 13
second burst. Kamiokande II 11
antineutrinos IMB 8 antineutrinos Baksan
5 antineutrinos Dark Matter One candidate
for DM is the sterile (truly non-interacting)
neutrino. Cosmic Neutrino Background Big Bang
Nucleosynthesis, constraints on matter
distribution
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Nucleosynthesis Triple Alpha reaction
How are elements heavier than helium produced?
The triple alpha reaction (3 Hes are
involved)


Carbon is formed in an excited state, originally
predicted before it was known that this could
happen. Requires temperatures on the order of
.
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Results of nucleosynthesis the cosmic abundances
of the elements (not all due to stellar processes)
Figure Shu, The Physical Universe
Abundance relative to hydrogen
Mass number (number of baryons in nucleus)
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Hotter fusion and heavier elements

• Could stars in principle live forever simply by
  contracting gravitationally and increasing their
  temperature to ignite the next heavier source of
  nuclear fuel whenever they run out?
• No. The strong interactions range is smaller
  than the diameters of all but the smaller nuclei,
  but the range of the Coulomb interaction still
  covers the whole nucleus.
• If nuclei get large enough the increase in
  electrostatic repulsion of protons becomes
  greater than the increase in binding energy from
  the strong interaction.
• Thus there is a peak in the binding-energy-per-bar
  yon vs. atomic mass number relationship, that
  turns out to lie at iron (Fe).

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Hotter fusion and heavier elements (continued)
Implication Once a stars core is composed
completely of iron, it can no longer replenish
its energy losses (from luminosity) by fusion.
Stars therefore must die, eventually. In other
words, you get energy by fusion all the way up to
production of iron but not beyond.
Binding energy per baryon
Figure Shu, The Physical Universe
Atomic mass number
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The high mass track
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HIGH MASS TRACK
1) Proto Star
2) Main sequence

• While on the main sequence what do high mass
  stars burn in their cores?
• Hydrogen
• What fusion process?
• CNO

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The CNO cycle

• Low-mass stars rely on the proton-proton cycle
  for their internal energy
• Higher mass stars have much higher internal
  temperatures (20 million K!), so another fusion
  process dominates
• An interaction involving Carbon, Nitrogen and
  Oxygen absorbs protons and releases helium nuclei
• Roughly the same energy released per interaction
  as in the proton-proton cycle.
• The C-N-O cycle!

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High mass stars the end

• Onion structure of the core

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Beyond helium nucleosynthesis

• As you saw, the triple alpha reaction makes
  carbon nucleus.
• Two carbon nuclei can fuse to make an oxygen
  nucleus.
• Two carbon nuclei can fuse to make a magnesium
  nucleus.
• To fuse heavier elements generally require higher
  temperatures (5 108 K and higher)
• Energy is released all the way up to the
  formation of iron, usually as gamma rays.

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Higher temps, more massive nuclei

• Nuclei are fused at higher and higher
  temperatures in the core of a massive star until
  an iron core forms.
• If the star doesnt reach high enough
  temperatures in its core then it can stop at
  triple alpha process (lower mass stars).
• Eventually stars cannot burn anything more. So
  how are very heavy elements made in the universe?

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Nucelosynthesis summary

• For the majority of stars (95, corresponding to
  stars with initial masses of less than 8 M-Sun),
  direct nuclear fusion does not proceed beyond
  helium, and carbon is never fused.
• Most of the nucleosynthesis occurs through slow
  neutron capture during the asymptotic giant
  branch (AGB), a brief phase (106yr) of stellar
  evolution where hydrogen and helium fuse
  alternately in a shell.
• These newly synthesized elements are raised to
  the surface through periodic "dredge-up"
  episodes, and the observation of short-lived
  isotopes in stellar atmospheres provides direct
  evidence that nucleosynthesis is occurring in AGB
  stars.

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Supernovae

• A Type I supernova is a massive explosion of a
  star that occurs under two possible scenarios.
  The first is that a white dwarf star undergoes a
  nuclear based explosion after it reaches its
  Chandrasekhar limit (1.44 solar masses) from
  absorbing mass from a neighboring star (usually a
  red giant).
• A Type II supernova (more common) occurs when a
  massive star, usually a red giant, reaches iron
  in its nuclear fusion (or burning) processes.

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Supernovae

• All nuclear fusion reactions beyond iron are
  endothermic (require energy) and so the star
  doies not produce energy from these reactions.
• The star's gravity then pulls its outer layers
  rapidly inward. The star collapses very quickly
  the in-rushing matter compresses at the center of
  the star such that the degenerate matter
  (electrons and protons, principally) combine into
  neutrons.
• Neutrons are not compressible, so the rest of the
  in-falling matter rebounds, which is the
  supernova explosion.

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Composite image of Kepler's supernova from
pictures by the Spitzer Space Telescope, Hubble
Space Telescope, and Chandra X-ray Observatory.
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After the supernova

• Supernova remnant includes the remains of the
  star plus the nebula of material thrown outwards
  by the explosion
• The material remains very hot (and glowing) for
  millions of years (even if it becomes too dim for
  us to see)
• The neutron star is very small, very hot, and
  distorts space (and time) around it due to its
  extremely high density
• A black hole is a neutron star whose escape
  velocity exceeds the speed of light

art
reality
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Black hole structure
The event horizon is the distance at which the
escape velocity is greater than the speed of
light. Note that the singularity is where the
actual star is.
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Black holes arent forever

• Hawking radiation can be emitted by a black hole,
  which reduces its mass. Once enough mass is lost,
  the star becomes an ordinary neutron star.

Gamma rays can spontaneously generate a
positron-electron pair (the reverse of what
occurs during hydrogen fusion). Usually, the
electron and positron annihilate within 1035
seconds, but if the pair production occurs near
the event horizon, one particle may be trapped
within the event horizon so the recombination
cannot occur the other particle is emitted as
Hawking radiation and reduces the mass of the
black hole.