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with anything else. In fact, without a helping hand, that is pretty much what would always happen, and the pathway to synthesizing heavier elements inside stars would be blocked. In 1953, when the understanding of the nuclear physics of stars was still in its infancy, astronomer Fred Hoyle realized that carbon had to be manufactured inside stars, irrespective of what the nuclear physicists told him, because he strongly believed that there is nowhere else in the universe to make it. Coupled with his astute observation that astronomers exist, he theorized that this could happen only if a slightly heavier type of carbon nucleus exists such that it can be formed very efficiently as the result of fusion between the short-lived beryllium-8 and a third helium nucleus. For the theory to work out, Hoyle figured out that the heavy carbon should be 7.7 MeV/c2 heavier than ordinary carbon. Once this new form of carbon has been made in the star, the pathway to heavier elements opens up. At the time, no such form of carbon was known but, spurred on by Hoyle’s prediction, scientists wasted no time in hunting for it. It was a matter of days after Hoyle made his prediction that nuclear physicists working in the Kellogg Laboratory at Caltech confirmed his prediction without any shadow of doubt. This is a remarkable story, not least because of the way it helps us build confidence in our understanding of how stars work: There is no better vindication of a beautiful theory than the verification in an experiment of a prior prediction.

Today we have a great deal more evidence that supports the theory of stellar evolution. One striking example comes from the study of the neutrinos produced every time a proton turns into a neutron in the fusion process. Neutrinos are ghostly particles that hardly ever interact with anything, and as such, most of them stream out from the sun as soon as they are produced without hindrance. The neutrino flux is so great, in fact, that around 100 billion of them pass through each square centimeter of the earth every second. This is an easy fact to read but an astonishing thing to imagine. Hold your hand up in front of you and look at your thumbnail. Each second, 100 billion subatomic particles from the core of our star will pass through it. Fortunately for us, the neutrinos nearly always pass through our hands, and in fact the entire earth, as if they did not exist. However, on rare occasions, a neutrino will interact, and the trick is to build experiments that are able to catch these extremely rare events. The Super-Kamiokande experiment, located deep in the Mozumi mine near the city of Hida in Japan, is up to the challenge. Super-Kamiokande is a huge cylinder 40 meters across and 40 meters tall, containing 50,000 tons of pure water, surrounded by over 10,000 photomultiplier tubes that are capable of detecting the very faint flashes of light that are produced when a neutrino collides with an electron in the water. As a result, the experiment is able to “see” the neutrinos streaming from the sun, and the number arriving turns out to agree with expectations based upon the theory that they are produced as a result of fusion processes inside the sun.

Eventually, the star will exhaust its supply of helium and begin to collapse even further. As the core temperature rises past 500 million degrees, it becomes possible for the carbon to burn, producing a variety of heavier elements all the way up to iron. Your blood is red because it contains iron, the end point of fusion in the core of stars. Elements heavier than iron cannot be manufactured through fusion in the core because there is a law of diminishing returns, and for nuclei heavier than iron there is no more energy to be released from fusing with extra nuclei. In other words, adding protons or neutrons to an iron nucleus can only make it heavier (not lighter, as would be necessary for fusion to act as a source of energy). Nuclei heavier than iron prefer instead to shed protons or neutrons, as we saw earlier in the case of uranium. In these cases, the sum total of