Arthur Eddington

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British astronomer Arthur Eddington (18821944) led the 1919 expedition to Principe to prove that Einstein's theory of general relativity was correct (see p.24). He went on to make the first direct measurements of stellar masses in binary stars, discovering the link between mass and luminosity for main-sequence stars. He also correctly suggested , that fusion was the primary source of energy in stars.

THE HYDROGEN CHAiN

The hydrogen or proton-proton chain that produces energy in the cores of Sun-like stars involves a sequence of reactions in which hydrogen nuclei (protons) fuse with each other and then undergo radioactive decay to <r-eventually produce a helium nucleus (two protons and two neutrons).

hydrogen nucleus (proton)

time, this meant that the sun too had been shining for billions of years, but there was no known energy source capable of sustaining it for that long (the previous favorite had been gravitational contraction and heating, which could have sustained the sun for a few million years).

Fortunately, while nuclear physics had revealed the problem, it was also to produce the solution. As knowledge of reactions between atomic nuclei improved, astronomers such as Arthur Eddington began to realize that nuclear fusion (the joining of light atomic nuclei to make heavier ones) was a potential source of immense energy from the destruction of relatively little material. it still took until 1938 for German-born physicist Hans Bethe to work out the precise details of the hydrogen fusion chain that powers stars like the sun. it is now estimated that the sun and stars like it have a sufficient mass of hydrogen to keep shining for about 10 billion years.

mass lost in the reaction is converted into energy in accordance with equation E=mc2

gamma-ray mass lost in the reaction is converted into energy in accordance with equation E=mc2

gamma-ray hydrogen nucleus (proton)

When two protons collide, one changes into a neutron, releasing a positron and a neutrino another proton fuses with the proton-neutron pair, releasing a gamma-ray photon

When two protons collide, one changes into a neutron, releasing a positron and a neutrino another proton fuses with the proton-neutron pair, releasing a gamma-ray photon

/helium nucleus the two groups collide, forming a helium nucleus and releasing two protons

over THE LIMIT

The Crab Nebula supernova remnant shows "ripples" caused by the rapidly rotating neutron star (pulsar) at its heart. Neutron stars are collapsed stars heavier than the 1.4-solar-mass "Chandrasekhar limit" that marks the heaviest possible white dwarf. The limit was actually discovered by Lev Landau.

particles from the sun

Neutrino observatories such as this one in Canada use huge underground detector tanks to record the near-massless neutrino particles that are released by nuclear fusion in stars like the Sun.

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