The Sun is a mass of incandescent gas
A gigantic nuclear furnace
Where hydrogen is built into helium
At a temperature of millions of degrees.
—from “Why Does the Sun Shine?”
[The song quoted above is probably best known in its cover version by They Might Be Giants, but it was written for an album of science-themed songs for children in 1950s. It was co-wrtten by Hy Zaret, best known for writing “Unchained Melody”. This is your totally random trivia for the day.]
What makes a star a star?
The answer is simple, though it leads to some deep science. A star is an astronomical object that glows due to nuclear fusion in its interior. Some objects may emit a lot of light — pulsars are a good example — while others such as brown dwarfs glow simply because they are warm and large. (That’s nothing special: humans glow because we are warm! We just don’t emit much light, and it’s all infrared, which our eyes can’t see.) Planets like Jupiter emit a little infrared light, but appear bright because they reflect sunlight.
Stars — including the Sun — are distinguished by their ability to generate their own light by a self-sustaining nuclear chain reaction. Fusion is a tricky thing for humans to manage: it requires a lot of energy to make things work, and so far we haven’t managed to get more energy out than we put in. Stars solved the problem by growing very large: the gravity of all the stuff in them keeps the temperature and pressure high in the core, compressing the material enough to let fusion happen naturally.
So how does a star like the Sun shine? The Sun is known as a main sequence star: one which is fusing hydrogen into helium in its core. Stars spend most of their lives on the main sequence, but the length of time hydrogen fusion goes on depends on the mass of the individual star. Huge, massive stars burn through their hydrogen relatively quickly because the environment in their cores is more extreme; smaller stars may take trillions of years, much longer than the current age of the Universe. The Sun is a moderate-sized star, and is about halfway through its 10-billion-year lifetime.
Fusion in brief
Hydrogen fusion requires very high temperatures and pressure because its raw material consists of protons, which repel each other. Hydrogen, after all, is just one electron orbiting one proton; at high temperature, the atoms become ionized, where the electrons are separated from the protons. That makes hydrogen plasma.
However, if protons can be brought together close enough, the nuclear forces kicks in, fusing them together. But two protons can’t make a nucleus! Instead, upon fusion, one proton changes into a neutron, creating deuterium, which is a hydrogen nucleus consisting of a proton and a neutron. As illustrated above, the basic process is the conversion of a proton into a neutron, releasing a positron — the antimatter version of an electron — a neutrino, and a gamma ray photon. Here’s the equation describing the reaction:
with representing hydrogen and representing deuterium; the letter is the same as the symbol for the element on the periodic table, while the number tells us the total number of protons and neutrons in the nucleus. Meanwhile, stands for the positron (a normal electron would be ), is the neutrino (with the “e” meaning this is specifically an electron neutrino), and is the Greek letter gamma, representing the gamma ray.
We’re obviously not done yet! First, we have tons of electrons in the core from the ionized hydrogen, so the positrons almost immediately run into those, annihilating and creating more gamma rays. Also, thanks to the presence of its neutron, deuterium has a lower energy barrier to fusion than hydrogen, so we get another link in the fusion chain:
We have achieved helium! Specifically, this step produces helium-3 (), which consists of two protons and one neutron, along with another gamma ray.
Helium-3, like deuterium, is relatively easy to fuse together in the conditions inside the Sun’s core: two helium-3 nuclei fuse into helium-4, which has two protons and two neutrons.
The byproducts of this last step in fusion include more gamma rays, but also two hydrogen nuclei (protons)…allowing the whole process to start over! However, if you’re keeping track of the total numbers, you’ll find that it takes six protons to make one helium-4 nucleus, with two protons left over. That means the Sun is slowly using up the supply of hydrogen in its core; when that is exhausted (in about 5 billion years! oh noooooes!), it will leave the main sequence and swell into a red giant star.
This particular set of steps by which hydrogen is converted into helium is called the “p-p chain” (because it starts with two protons), possibly named by a person with young children. Higher-mass stars, with their higher core temperatures, may follow a different set of steps. Fusion reactors built by humans generally start with deuterium, tritium (, an unstable isotope of hydrogen with two neutrons and a proton), or even some heavier elements like lithium. But that’s another subject entirely!
The birth of sunlight
The neutrinos produced in the p-p chain escape from the Sun very quickly, but theirs is a story for another day. We’re interested in what makes the Sun shine. The light produced by nuclear fusion is in the form of gamma rays, but that’s not the kind of light we see coming from the Sun’s surface: most of that is visible light, which is far less energetic? So how does that happen?
Gamma rays are the highest-energy form of light, but the interior of the Sun is dense enough to create a kind of photon pinball machine. Instead of escaping, gamma ray photons bounce off electrons, transferring some of their energy. That both keeps the Sun’s interior hot and (after many collisions) reduces the energy of the photons until they primarily fall into the visible light regime. Even though the speed of light is very high, the large size of the Sun and the number of collisions mean it can take roughly 180,000 years for a photon to travel from the core to the surface. From there, it only takes about 8 minutes for a photon to get from the Sun to Earth. It hardly seems fair.
However, this tells us why the Sun’s core is 15 million degrees Celsius — hot enough for fusion to happen — while its surface is “only” about 5500° C. This is a good thing for us: gamma rays are quite dangerous for life, but visible light…visible light is gentle, soothing light. Nuclear fusion makes the Sun shine, but the far more moderate surface temperature emits light that makes life as we know it possible.
And that, my friends, is why the Sun shines.