If you start with a mass of hydrogen gas and bring it together under its own gravity, it will eventually contract once it radiates enough heat away. Bring a few million (or more) Earth masses' worth of hydrogen together, and your molecular cloud will eventually contract so severely that you'll begin to form stars inside. When you pass the critical threshold of about 8% our Sun's mass, you'll ignite nuclear fusion, and form the seeds of a new star. While it's true that stars convert hydrogen into helium, that's neither the greatest number of reactions nor the cause of the greatest energy release from stars. It really is nuclear fusion that powers the stars, but not the fusion of hydrogen into helium. Show David Malin, UK Schmidt Telescope, DSS, AAO All stars, from red dwarfs through the Sun to the most massive supergiants, achieve nuclear fusion in their cores by rising to temperatures of 4,000,000 K or higher. Over large amounts of time, hydrogen fuel gets burned through a series of reactions, producing, in the end, large amounts of helium-4. This fusion reaction, where heavier elements are created out of lighter ones, releases energy owing to Einstein's E = mc2. This occurs because the product of the reaction, helium-4, is lower in mass, by about 0.7%, than the reactants (four hydrogen nuclei) that went into creating it. Over time, this can be significant: over its 4.5 billion year lifetime thus far, the Sun has lost approximately the mass of Saturn through this process. NASA’s Solar Dynamics Observatory / GSFC But the way it gets there is complicated. You can never have more than two objects collide-and-react at once; you can't simply put four hydrogen nuclei together and turn them into a helium-4 nucleus. Instead, you need to go through a chain reaction to build up to helium-4. In our Sun, that involves a process called the proton-proton chain, where:
E. Siegel / Beyond The Galaxy
Sarang / Wikimedia Commons
Uwe W. and Xiaomao123 / Wikimedia Commons So those are the four possible overall steps available to the components that make up then entire "hydrogen fusing into helium" process in the Sun:
And I want you to note something very interesting, and perhaps surprising, about those four possible steps: only step #2, where deuterium and a proton fuse, producing helium-3, is technically the fusion of hydrogen into helium! NASA/JPL/Gemini Observatory/AURA/NSF Everything else either fuses hydrogen into other forms of hydrogen, or helium into other forms of helium. Not only are those steps important and frequent, they're more important, energetically, and a greater overall percentage of the reactions than the hydrogen-into-helium reaction. In fact, if we look at our Sun, in particular, we can quantify what percentage of energy and of the number of reactions in each step is. Because the reactions are both temperature dependent and some of them (like the fusion of two helium nuclei) require multiple examples of proton-proton fusion and deuterium-proton fusion to occur, we have to be careful to account for all of them. Kieff/LucasVB of Wikimedia Commons / E. Siegel In our Sun, helium-3 fusing with other helium-3 nuclei produces 86% of our helium-4, while the helium-3 fusing with helium-4 through that chain reaction produces the other 14%. (Other, much hotter stars have additional pathways available to them, including the CNO cycle, but those all contribute insignificantly in our Sun.) When we take into account the energy liberated in each step, we find:
Wikimedia Commons user Kelvinsong It might surprise you to learn that hydrogen-fusing-into-helium makes up less than half of all nuclear reactions in our Sun and that it's also responsible for less than half of the energy that the Sun eventually outputs. There are strange, unearthly phenomena along the way: the diproton that usually just decays back to the original protons that made it, positrons spontaneously emitted from unstable nuclei, and in a small (but important) percentage of these reactions, a rare mass-8 nucleus, something you’ll never find naturally occurring here on Earth. But that’s the nuclear physics of where the Sun gets its energy from, and it's so much richer than the simple fusion of hydrogen into helium! Does hydrogen fuse in the Sun?Fusion reactions occur when two nuclei come together to form one atom. The reaction that happens in the sun fuses two Hydrogen atoms together to produce Helium.
What element does hydrogen fuse into?The primary source of solar energy, and that of similar size stars, is the fusion of hydrogen to form helium (the proton–proton chain reaction), which occurs at a solar-core temperature of 14 million kelvin.
What does hydrogen fuse into in stars?Fusion is the process that powers the sun and the stars. It is the reaction in which two atoms of hydrogen combine together, or fuse, to form an atom of helium.
What elements will our Sun fuse?The process of nuclear fusion combines hydrogen atoms to produce helium and the energy that keeps the Sun shining. The next three elements heavier than helium — lithium, beryllium, and boron — are sometimes formed as intermediate products during the fusion process.
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