The core is thought to be as hot as 28,080,000°F (15,600,000°C) and to sustain pressures as high as 2,961,000 atm (300 GPa).The pressure at the center of the Earth is thought to be between 3,553,000 atm (360 GPa) and 3,750,000 atm (380 GPa) but temperatures are far lower. The Sun's temperatures and pressures allow the nuclear reactions that provide the energy for the Sun to proceed.
In the core, the Sun creates its energy by a process called hydrogen burning. In this process, four hydrogen nuclei are fused to create one helium nucleus. Since the hydrogen nuclei each consist of one proton, while the helium nucleus consists of two protons and two neutrons, several hydrogen nuclei are required to make one helium nucleus.The process takes several steps because the positive electrical charges of the protons repel each other, and it would be virtually impossible to cause four to collide at the same instant.
The process of creating the helium nucleus is called the protonproton chain because it uses the protons one at a time. Hans A. Bethe, professor of physics at Cornell University, and Charles Critchfield, then a graduate student of George Gamow at George Washington University, first demonstrated in the late 1930s how a series of nuclear reactions could make the Sun shine. Bethe won the Nobel Prize in physics in 1967 for their work (Critchfield had since died).
The several steps of hydrogen burning convert hydrogen nuclei into helium, thus creating new atoms and releasing the heat that drives the Sun and provides energy for the planets.
In the first step of the proton-proton chain, two protons (the stripped nuclei of hydrogen atoms, denoted !H) come together to form a deuteron (D), which is a nucleus with one proton and one neutron. In the process, they emit two subatomic particles: a positron and an electron neutrino (these particles are defined in the sidebar "Elementary Particles," on page 16). Another proton (!H) then collides with the deuteron (D) to form the nucleus of a rare isotope of helium, consisting of two protons and one neutron (denoted 3He),
Two 'H (protons) combine to make a deuterium nucleus that combines with another 'H
to make a 3He that combines with another 3He to make a He.
Further reactions create heavier elements, starting with beryllium and lithium.
and in the process, they emit a gamma ray. Finally, two of these 3He nuclei fuse to create a 4He nucleus, and in the process return two 1H to the plasma in the core. The steps in the proton-proton chain are shown in the figure on page 20.
The basic law of physics that allows radiation energy to be produced from matter is the famous equation that Albert Einstein first stated:
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