Chapter 11: Our Sun: The Nearest Star

For a long time, the energy source of the Sun was a great scientific mystery. It was easy to show that chemical energy from burning fossil fuels could not be responsible for its continued illumination (the Sun would have burned out in 4 to 5 thousand years!). The Sun could be longer-lived if it got its energy from gravitational contraction, but its life expectancy would still be a fraction of a billion years. It turns out the answer to this mystery would require the discovery of a new source of energy. The form of energy Lord Kelvin and other scientists in the 19th century hadn't known about was the amazing efficiency of atomic energy.

The Rutherford model of an atom.

Early in the 20th century, Ernest Rutherford showed that the atomic nucleus was incredibly dense and that it occupied a tiny percentage of the volume of an atom. Blown up to human scales, if the atomic nucleus were a pea at the center of the fifty-yard line of a football stadium, then the electrons would be whirring in the rows of seats. One unusual feature of these compact nuclei is their tendency to, in special cases, split apart and give off energy. Rutherford and Henri Becquerel determined that certain naturally occurring heavy elements could give off radiation and change their properties through radioactive decay. In one elegant experiment, Rutherford sealed a small amount of radioactive material in a glass tube that otherwise contained a perfect vacuum. Several months later, he showed by a careful analysis that there was helium in the tube and that the radioactive material had changed its chemical properties; atoms had actually transformed from one species to another! Here was the transformation of elements that alchemists had sought for centuries.

The early pioneers of radioactivity had little idea of the nature of the radiation they were studying. They dubbed the three types of radioactivity "alpha, beta, and gamma" after the first three letters of the Greek alphabet. Today, we know that these types of radiation are quite distinct. Alpha decay is the emission of helium nuclei, the kind trapped by Rutherford in his experiment, during radioactive decay. Beta decay occurs when a neutron turns into a proton, an electron, and a neutrino. Gamma radiation is energy released from the atom in the form of very high-energy electromagnetic waves, also during radioactive decay.

Marie Curie

The young Polish physicist Marie Curie showed that radioactive material produced millions of times more energy per atom than any chemical process. She and her French husband Pierre were the first to isolate a radioactive element. Marie went on to become the first female professor in the 600-year history of the Sorbonne University in Paris and she was the first person to be awarded two Nobel prizes. Because they knew little about the nature of radiation, these pioneers were also unaware of its damaging effect on human tissue. Many researchers paid with their lives for their research, including Marie Curie.

Many common radioactive decay paths found in nature on Earth start at the semi-stable isotope Uranium-238 (half-life 4.468 billion years). Examples include:

Schematic of the CNO cycle to fuse hydrogen into helium

Radon Gas: Radium-226 (half-life 1,602 years) decays via alpha decay into Radon (half-life 3.8 days). Radon gas is often a concern in basements in areas with a lot of granite (which often contains a background of Uranium-238) as this very dense gas can build up in basements and become a health hazard.

Polonium Poison and Fuel: Polonium-210 (half-life 138.4 days) makes up about 1 part in 10¹⁵ of the Earth's crust. It decays via alpha particle emission into Lead-206 and represents the last step in the Uranium-238 decay chain. Every gram of this radioactive material releases 140 watts of energy. This high potency makes it an effective energy power source (a 0.5-gram capsule can reach temperatures above 500°C and drive an engine) and an effective poison! In 2006, Alexander Litvinenko, a former FSB and KGB officer, was poisoned with Polonium-210 while in England after meeting with two other former KGB officers. He died 3 weeks later.

Radioisotope Thermoelectric Generators: Many spacecraft, including the Pioneer and Voyager missions, Galileo, Cassini, and New Horizons, are powered through heat engines that utilize nuclear as the initial energy source. One of the most common fuels is Plutonium-238 (half-life 87.7 years). Its long half-life, high-energy density, and low shielding requirements (only 2.5mm of lead shielding is needed to protect electronics from this radioactive material).

Our Sun and other stars aren't powered by nuclear decay, which is fission, but rather through nuclear fusion. You may have noticed that all the examples above include the nuclear decay of heavy atoms. It turns out that atoms that are heavier than Iron all give off energy when they break apart (and require energy to be fused), and atoms lighter than Iron give off energy during fusion reactions but require energy in many cases to be broken apart. The reasons for this dichotomy are buried deep in the heart of quantum mechanics, but basically, an atomic nucleus has binding energy and by Einstein's famous relationship E = m c², that energy has a tiny equivalent mass. So the mass of an atomic nucleus is always less than the sum of the masses of the protons and neutrons it's made of. The binding energy varies as the nucleus gets more massive and the binding energy per nuclear particle is highest for iron. That means iron is the most stable atomic nucleus of all the elements in the periodic table. Elements heavier than iron can release energy by fission and elements lighter than iron can release energy by fusion. The common nuclear energy mechanism in main-sequence stars include:

• The Proton-Proton Chain: This series of pathways merges Hydrogen-1 into Helium-4 while releasing over 26 MeV of energy. This fusion chain requires high temperatures (greater than 4.6 × 10⁶K) and densities that are found in the centers of stars like our Sun.

• CNO Cycle: In higher mass stars than our Sun, with hotter cores (greater than 1.3 × 10⁷K), stars are able to use Carbon (if present) to seed a nuclear fusion cycle that confers hydrogen to helium while also creating carbon, nitrogen, and oxygen.

Much more complicated reactions can occur in older stars, and these reactions are discussed in other articles. Via nuclear fusion, stars are able to efficiently generate energy for millions to billions of years. Our own Sun will fuse hydrogen into helium while on the main sequence for roughly 10 billion years. We are at roughly the halfway point in our Sun's life.