First, all active stars generate energy through thermonuclear fusion: the “fusion” of lighter elements to produce heavier ones. For instance, in the Sun -and in all stars that are proceeding through the first phase of such reactions- the lightest element hydrogen fuses into helium.
Through a series of steps, hydrogen fuses into helium. The diagram above shows the steps involved in Branch I of the proton-proton chain sequence, which is responsible for 83.3% of all energy produced within the Sun’s core. Image: Image credit: Wikimedia Commons user Borb, via File:FusionintheSun.svg - Wikimedia Commons.
Thermonuclear fusion generates energy because a minute amount of the material is transmuted into energy in accordance to the famous physics equation
Where E = energy; m = mass and c = speed of light.
Although these atoms are unfathomably small -trillions of them could snuggle within the period at the end of a sentence- and the percentage of material transmuted into energy is slight (0.7%), 4 × 10^26 W of energy are produced every single second. To put that value into context, that energy release is equal to that produced by the detonation of 1 trillion megaton bombs. If we could somehow capture one second’s worth of energy, we could fulfill the energy demands of the entire globe for 500,000 years. (The tricky bit is in the capture.) To create this prodigious energy, 4 x 10^38 protons fuse every second! While one would think that the Sun couldn’t possibly sustain such reactions over the long term, one should remember that the Sun consists of 10^57 particles, about 10% of which are located in the core.
Such fusions occur because the Sun’s core temperatures and pressures are enormous. Core temperature: 15 million degrees C (27 million F); pressure = 265 billion bar (3.84 trillion pounds per square inch.) The unrelenting gravitational contraction of the Sun creates these high temperature-pressure conditions conducive to thermonuclear fusion reactions.
However, none of this should work at all.
When one calculates the number of protons that should fuse under such conditions, one finds that NONE of them should. Not one…ever! The problem is that the positively charged protons experience an electrostatic repulsion because their charges are the same. In fact, as one reduces the space between protons, this repulsion increases in accordance with the inverse square law. (One notices the same relation between massive objects due to their gravitational attraction.) The closer the protons approach each other, the more they struggle to separate. Although the prevailing conditions in the core are extreme, they shouldn’t be sufficient to ignite and sustain thermonuclear fusion reactions. Yet, they are.
The fusion works because the protons exhibit both particle and wave-like behavior. The proton doesn’t just occupy a specific, rigidly defined volume. Instead, its location can be described as a probability function: it could occupy a range of locations. Because of the probability functions of protons can overlap in regions of high temperature and pressure, they can fuse. One refers to this phenomenon as quantum tunneling.
The protons fuse, incidentally, because the strong nuclear force, the strongest of all the fundamental physical forces (gravity, electromagnetism, weak nuclear and strong nuclear) overwhelms the weaker electrostatic repulsion that would otherwise keep them apart.
The Sun literally shines because of the workings of the eerie, counterintuitive workings of the quantum realm. The next time you feel the Sun’s warmth on your face -or, if, like some of us, you harbor pleasant memories of the Sun’s warmth- remember that it is only possible because of quantum physics.