Nuclear Reactions in the Sun
core (15 million oK!), photosphere (visible surface, 5700 oK, photons no longer collide, can escape), chromosphere
(10,000 oK), corona (2 million oK, low density, but high temperature due to magnetic fields and
violent convective motions of lower layers, source of X-rays)
Why are chromosphere and corona hotter than photosphere
even though they are further from core?
How do we know about Sun's interior?
- interact only weakly with normal matter
- leave Sun within few seconds
- opportunity to observe product from thermonuclear core
- 10 trillion neutrinos pass through you every second!
- suggest that fusion model is mostly correct
- vibrations of Sun tell us about interior structure
- just as seismologists use seismic waves to study Earth's structure
- volumes of gas rise and fall on Sun's surface with fixed frequencies
- indication that energy is flowing outward through Sun
What is the Sun's energy source?
- chemical energy from burning of fossil fuels (a few thousand years)
- gravitational contraction, gravitational potential energy ---> kinetic energy ---> heat, radiation
into space (< 500 million years)
Einstein's beautiful logic...
... if speed of light constant, leads to funny effects!
- increase in mass with velocity: closer to speed of light, more energy goes into increasing particle's mass than into increasing its speed
- can work in reverse too: conversion of mass into energy, matter is ``frozen'' energy
- E = mc2
- heats the Earth's core
- when a nucleus splits, the components have slightly less mass
than the original
- difference in mass is converted to energy (via E=mc2)
- chain reactions
- require high enough concentration
- neutrons released as decay product
- each release triggers another decay
- continue until stable element in middle of periodic table, like iron, is reached
- lighter elements converted to heavier elements as lighter nuclei merge
- merged nucleus has less mass than starting pieces, so energy is released
- even so it requires tremendous particle energies to overcome electric
repulsion of protons
- unlike fission, cannot occur spontaneously --
extreme physical conditions required, such as tens of millions of degrees
- goal of a controlled fusion reaction, but reactor materials melt at a few thousand degrees!
How the Sun generates energy... and converts itself from hydrogen to helium
Proton-Proton (pp) chain (high energy protons released, which then participate in new reactions) is one way:
- P + P --> D + positron + neutrino (D or 2H = deuterium, which is hydrogen with 1 proton and 1 neutron)
(movie: source University of Oregon, Prof. Greg Bothun)
- D + P --> He3
- He3 + He3 --> 2He + 2P
And there is the CNO cycle...
At the end, products are 0.7 percent less massive than the initial matter.
Every second, Sun converts 4 million tons of hydrogen to energy and radiates it into space!
It would take another 4 billion years to consume all the Sun's hydrogen.
Fusion only happens in the core (107 K).
Detection and Mass
Click here for
some FAQ's about neutrinos.
- neutrinos (``little neutral one'') predicted by Pauli to explain some nuclear
reactions that did not appear to conserve energy
Wolfgang Pauli, physicist extraordinaire
- not detected for a long time because they only weakly interact with other matter (Earth is
transparent to neutrinos)
- neutrino detectors (telescopes) consist of huge, isolated purified water tanks (examples include a deep mine
in Japan and a chamber under Lake Erie) surrounded by several hundred light sensitive detectors
- incoming neutrinos interact with water to produce positrons and electrons, which move rapidly
through water and emit deep blue light that is observed with sensitive detectors
- lots of water needed because probability that they will interact with water is very slight
- purified water needed because false detections might result from impurities
- isolated detectors needed because it is essential to block other particles from hitting detector
- sensitive detectors needed because light level is approximately that of light visible on
Earth from a candle at distance of Moon!
- neutrinos come from nuclear reactions in stars, the Big Bang, supernovae...
- Supernova 1987A probably emitted 1058 neutrinos! but Japanese system
detected only 11 and Lake Erie only 8 in 13 seconds
Anatomy of a Supernova
Supernova 1987A in a Nearby Galaxy
(observed in optical wavelengths of light in 1987)
- because neutrino telescopes in Northern Hemisphere and SN1987A in Southern Hemisphere,
detected neutrinos had already passed through Earth!
- about one million people actually experienced a SN1987A neutrino interaction with their bodies!
- types of neutrinos we can detect are changing into types we cannot detect (``neutrino oscillation'')
- neutrino oscillations imply that different types of neutrino can't have same mass, therefore
even if one type is massless, another type must have mass
- if neutrinos have mass, the fact that there are so many suggests that they could contribute
to dark matter