Monday, December 09, 2013
Part I -- Physics
The Sun is mostly a big ball of hydrogen, a million miles in diameter, held together by gravity.
A hydrogen atom is a positively charged proton and a negatively charged electron, held together by electrical attraction. The proton (the nucleus of the hydrogen atom) is 2000 times more massive than the electron.
It is so hot at the center of he Sun -- that huge weight crushing down -- that the atomic electrons and protons can't hang together. So instead of atoms, there is a furious sea of electrons and protons careening about.
Two protons approach each other, and what happens? They swerve apart. Particles of the same electrical charge repel. But the hotter it is, the faster they move, and the closer they get before they are repelled. If they get close enough, something dramatic happens.
There is another force in nature, called the strong nuclear force, that holds protons and neutrons together. It is stronger than the electrical force, but has a very short range. If two protons get close enough, instead of being mutually repelled, they are clamped together. This happens at the center of a star where the temperature reaches 10 million degrees.
When two protons bind, one throws off its positive charge as a positron -- an anti-electron -- and becomes a neutron. The positron soon meets its anti-matter partner, an electron, and the two annihilate each other in a flash of energy.
Meanwhile, the proton-neutron pair meets another proton and the strong nuclear force welds them together -- two protons and a neutron. This entity meets another of the same, sheds two protons, and becomes a helium nucleus -- two protons and two neutrons.
Hydrogen into helium. Here is the diagram (from Biography of a Planet).
Now let's do the bookkeeping. Weigh the six protons and two electrons that go into the process. Weigh the helium nucleus and two protons coming out. The latter weigh slightly less than the former. Mass has vanished from the universe. In its place: energy. A lot of energy. E=mc2, where c is the speed of light.
Every second, at its hot core, the Sun converts 657 million tons of hydrogen into 653 million tons of helium, by nuclear fusion. The missing 4 million tons of mass are converted into energy. How much: 4 million tons times the speed of light squared. The energy makes its way to the Sun's surface where it is hurled into space as heat and light. The Earth intercepts about one two-billionth of this energy, or about four pounds worth of the Sun's vanished matter every second. The Sun never misses so slight a fraction of its huge bulk, but for the Earth it is the difference between day and night. And winter and spring.