By Don Lincoln, Fermilab
It was in 1932 that Australian physicist, Mark Oliphant, then a protégé of Ernest Rutherford, discovered nuclear fusion of what are called isotopes of hydrogen. Ordinary hydrogen consists of a single proton, but there are versions, what scientists call isotopes of hydrogen with a single proton, but one or two neutrons. So, which one applies to the Sun? How does fusion work in the Sun?
With the discovery of fusion, combined with British physicist, Sir Arthur Eddington’s speculation, German-born physicist, Hans Bethe, spent the 1930s working out how fusion works in our own Sun. Two protons fuse together into what is called a deuterium nucleus, which is a proton and a neutron. This happens many times. In order for this to happen, a proton has to turn into a neutron, which also means an antimatter electron and neutrino are generated, but those aren’t so important.
These deuterium nuclei are floating around in the Sun, when they are bombarded with another proton, which fuses to the deuterium. The result is a helium nucleus of a special kind called helium-3. It contains two protons and a neutron. This particular fusion process releases energy in the form of gamma rays.
Then the next step is two of these helium-3 nuclei come together. That means there are four protons and two neutrons. When they fuse together, they form a type of beryllium. The beryllium then breaks apart and the result is a nucleus of helium of the form called helium-4, which is two neutrons and two protons. The other two protons are then released, and they can subsequently fuse again to make deuterium or helium-3.
This is the process whereby our Sun generates energy. Other stars, especially heavier or older stars, experience other fusion chains. In fact, inside stars of increasing size and age, fusion of all of the elements up to iron can occur.
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Putting Nuclear Processes into a Mathematical Model
So, how do we know this happens in our Sun? It’s because we can measure these processes here on Earth and we know that they happen and how much energy is released at each step of the way. We can then put these nuclear processes into a mathematical model, combined with what we know of the amount of matter in the Sun, along with fractions of much hydrogen and helium we observe, and we see that the prediction is a mass of hot plasma that has the temperature and characteristics we see in our Sun.
We can go a step farther. We also understand very well how gravity works. We know that gravity is pulling the matter that makes the Sun inward. Without something to overcome that inward tug, the Sun would collapse into a small and dense sphere. The thing that overcomes the gravitational collapse is the heat generated by the fusion that powers the Sun. Scientists can calculate the two effects and the result is a star with Sun-like characteristics.
A Giant Red Star
But astronomers can do more than that. Using the same techniques, they can calculate the temperatures and pressures that will occur in heavier stars, which experience different fusion processes. Some of those stars have hotter cores and the result is that the heat puffs up the star to much larger size. The expansion cools off the outer layers, but the result is a giant red star with a size that would encompass the orbit of the Earth.
And we know that such stars exist, because we can observe them using our telescopes. They have the right temperature and chemical composition, and, with modern telescopes, we can actually measure their size.
For instance, the star Betelgeuse, which is located about 700 lightyears away and forms one of the shoulders of the constellation Orion, is a red super giant, with a radius of about 900 times bigger than the Sun, or a volume of about 700 million times bigger. Betelgeuse is so big that astronomers could measure its size back in the 1920s.
With modern instrumentation, astronomers have measured the size of many stars. We can learn things about distant stars by observing the light they emit, combined with tons of calculations and an intricate understanding of nuclear physics.
Of course, stars are just a piece of the world of astronomy. Carl Sagan is frequently misquoted as pointing out that there are billions and billions of them. He never specifically said that, but the sentiment is right. Stars assemble into huge collections of stars called galaxies, which can have millions, billions, or even trillions of stars in them. We can appreciate its expanse better by remembering that our own Milky Way galaxy has between 200 and 400 billion stars, a tiny speck in the story of the universe.
Common Questions about Fusion
With the discovery of fusion, combined with British physicist, Sir Arthur Eddington’s speculation, German-born physicist, Hans Bethe, spent the 1930s working out how fusion works in our own Sun.
Without something to overcome the inward tug, the Sun would collapse into a small and dense sphere. The thing that overcomes the gravitational collapse is the heat generated by the fusion that powers the Sun.
The star, Betelgeuse, which is located about 700 lightyears away and forms one of the shoulders of the constellation Orion, is a red super giant, with a radius of about 900 times bigger than the Sun, or a volume of about 700 million times bigger. Betelgeuse is so big that astronomers could measure its size back in the 1920s.