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?

• neutrinos

  • 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

• helioseismology
  • 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

Fission

• 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

Fusion

• 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 (movie)

• He₃ + He₃ --> 2He + 2P (movie)

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 of Neutrinos

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