Proton-Proton Fusion

This is the nuclear fusion process which fuels the Sun and other stars which have core temperatures less than 15 million Kelvin. A reaction cycle yields about 25 MeV of energy.

Proton-Proton Cycle

The fusion of hydrogen in lower temperature stars like our Sun involve the following reactions yielding positrons, neutrinos, and gamma rays.

The solar neutrino problem 太阳中微子问题

which can be followed by either

The latter of these reactions is part of what is usually called the proton-proton cycle, which yields about 25 MeV and can be combined to the form

Step 1. Proton Fusion

Next step 下一步

The fusing of two protons which is the first step of the proton-proton cycle created great problems for early theorists because they recognized that the interior temperature of the sun (some 14 million Kelvins) would not provide nearly enough energy to overcome the coulomb barrier of electric repulsion between two protons. 两个质子的融合是质子-质子循环的第一步，这给早期理论家带来了巨大困难，因为他们认识到太阳内部温度（约1400万开尔文）远不足以提供足够的能量来克服两个质子之间的电斥力库仑屏障。

With the development of quantum mechanics, it was realized that on this scale the protons must be considered to have wave properties and that there was the possibility of tunneling through the coulomb barrier.

Eddington and his fusion critics

Arthur Eddington thought that nuclear processes must be involved to account for the radiant energy of the sun, but was criticized because the temperature was seen to be not hot enough when considered by classical physics alone. His tongue-in-cheek reply to his critics: "I am aware that many critics consider the stars are not hot enough. The critics lay themselves open to an obvious retort; we tell them to go and find a hotter place."

Step 2. Deuterium formation

Next step 下一步

The second step of the proton-proton cycle . This step involves the weak interaction because it involves the transmutation of one of the protons to a neutron in order to form deuterium. This process requires energy and produces a positron and an electron neutrino. 质子-质子循环的第二步。这一步涉及弱相互作用，因为它涉及将其中一个质子转变为中子以形成氘。这个过程需要能量，并产生一个正电子和一个电子中微子。

In the proton-proton fusion process, deuterium is produced by the weak interaction in a quark transformation which converts one of the protons to a neutron. The neutrinos quickly escape the sun, requiring only about 2 seconds to exit the sun compared to perhaps a million years for a photon to traverse from the center to the surface of the sun. The neutrino flux can be calculated, but earlier measurements of the neutrino flux measured only about a third of the expected number. This is called the solar neutrino problem. It is now presumed to be solved with the evidence for neutrino oscillation at the Sudbury Neutrino Observatory and at the Super Kamiokande neutrino detector. 在质子-质子融合过程中，通过弱相互作用在夸克转变中将一个质子转化为中子，产生氘。中微子很快逃逸出太阳，相比光子从太阳中心到达表面可能需要数百万年，中微子仅需约2秒即可逃逸。中微子通量可以计算，但早期的中微子通量测量仅得到预期数量的三分之一。这被称为太阳中微子问题。如今，这一问题已通过苏必利尔中微子天文台和超 Kamiokande 中微子探测器的中微子振荡证据得到解释。

Step 3. Deuterium-proton fusion

Next step 下一步

The third step of the proton-proton cycle . 质子-质子循环的第三步

Step 4. Helium-3 fusion

Next step 下一步

The fourth step of the proton-proton cycle . 质子-质子循环的第四步

Step 5. Alpha particle formation