While the Sun is powered by nuclear fusion, less than half of its energy comes from the fusion of hydrogen into helium. The proton-proton chain reaction involves multiple intermediate steps, with only one - the fusion of deuterium and hydrogen into helium-3 - technically converting hydrogen to helium. In the Sun, deuterium-hydrogen fusion accounts for 39.5% of energy, while helium-3 fusion into helium-4 accounts for the majority of energy at 39.3%. So nuclear fusion powers the Sun, but the fusion of hydrogen directly into helium is neither the greatest source of reactions nor the primary energy producer.

1. By:dr.Hamidreza Jalalian
The Sun's Energy Doesn't Come From
Fusing Hydrogen Into Helium (Mostly)


2. Starts With A Bang
The Universe is out there, waiting for you to discover it Opinions expressed by Forbes Contributors
are their own.
Ethan Siegel , Contributor

3. public domain
The Sun is the sources of the overwhelming majority of light, heat, and energy on Earth's
surface, and is powered by nuclear fusion. But less than half of that, surprisingly, is the fusion
of hydrogen into helium.
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.

4. David Malin, UK Schmidt Telescope, DSS, AAO
A portion of the digitized sky survey with the nearest star to our Sun, Proxima Centauri,
shown in red in the center. While sun-like stars like our own are considered common, we're
actually more massive than 95% of stars in the Universe, with a full 3-out-of-4 stars in
Proxima Centauri's 'red dwarf' class.

5. 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
A solar flare from our Sun, which ejects matter out away from our parent star and into the
Solar System, is dwarfed in terms of 'mass loss' by nuclear fusion, which has reduced the
Sun's mass by a total of 0.03% of its starting value: a loss equivalent to the mass of Saturn.
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

6. 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:
 Two protons fuse together to form a diproton: a highly-unstable configuration where
two protons temporarily create helium-2,
 A tiny fraction of the time, one-in-10,000,000,000,000,000,000,000,000,000 times,
that diproton will decay to deuterium, a heavy isotope of hydrogen,
 And it happens so quickly that humans, who can only view the initial reactants and
the final products, the diproton lifetime is so small that they’d only see two protons
fuse either scatter off of each other, or fuse into a deuteron, emitting a positron and a
neutrino.
E. Siegel / Beyond The Galaxy
When two protons meet each other in the Sun, their wavefunctions overlap, allowing the
temporary creation of helium-2: a diproton. Almost always, it simply splits back into two
protons, but on very rare occasions, a deuteron (hydrogen-2) is produced.

7.  Then that deuteron can easily combine with another proton to fuse into helium-3, a
much more energetically favorable (and faster) reaction,
 And then that helium-3 can proceed in one of two ways:
o It can either fuse with a second helium-3, producing a helium-4 nucleus and
two free protons,

9. Sarang / Wikimedia Commons
The most straightforward and lowest-energy version of the proton-proton chain, which
produces helium-4 from initial hydrogen fuel. Note that only the fusion of deuterium and a
proton produces helium from hydrogen; all other reactions either produce hydrogen or make
helium from other isotopes of helium.
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o Or it can fuse with a pre-existing helium-4, producing beryllium-7, which
decays to lithium-7, which then fuses with another proton to make beryllium-
8, which itself immediately decays to two helium-4 nuclei.

11. Uwe W. and Xiaomao123 / Wikimedia Commons
A higher-energy chain reaction, involving the fusion of helium-3 with helium-4, is
responsible for 14% of the conversion of helium-3 into helium-4 in the Sun. In more massive,
hotter stars, it can dominate.
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:
1. Two protons (hydrogen-1) fuse together, producing deuterium (hydrogen-2) and other
particles plus energy,
2. Deuterium (hydrogen-2) and a proton (hydrogen-1) fuse, producing helium-3 and
energy,
3. Two helium-3 nuclei fuse together, producing helium-4, two protons (hydrogen-1),
and energy,
4. Helium-3 fuses with helium-4, producing beryllium-7, which decays and then fuses
with another proton (hydrogen-1) to yield two helium-4 nuclei plus energy.
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!

13. NASA/JPL/Gemini Observatory/AURA/NSF
Only brown dwarfs, like the pair shown here, achieve 100% of their fusion energy by turning
hydrogen into helium. Because deuterium fusion (deuterium+hydrogen=helium-3) occurs at
temperatures of just 1,000,000 K, 'failed stars' that don't reach 4,000,000 K get their energy
exclusively from the deuterium they're formed with.
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
The classification system of stars by color and magnitude is very useful. By surveying our
local region of the Universe, we find that only 5% of stars are as massive (or more) than our
Sun is. More massive stars have additional reactions, like the CNO cycle and other avenues
for the proton-proton chain, that dominate at higher temperatures.
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:
1. Proton/proton fusion into deuterium accounts for 40% of the reactions by number,
releasing 1.44 MeV of energy for each reaction: 10.4% of the Sun's total energy.

14. 2. Deuterium/proton fusion into helium-3 accounts for 40% of the reactions by number,
releasing 5.49 MeV of energy for each reaction: 39.5% of the Sun's total energy.
3. Helium-3/helium-3 fusion into helium-4 accounts for 17% of the reactions by
number, releasing 12.86 MeV of energy for each reaction: 39.3% of the Sun's total
energy.
4. And helium-3/helium-4 fusion into two helium-4s accounts for 3% of the reactions by
number, releasing 19.99 MeV of energy for each reaction: 10.8% of the Sun's total
energy.
Wikimedia Commons userKelvinsong
This cutaway showcases the various regions of the surface and interior of the Sun, including
the core, which is where nuclear fusion occurs. Although hydrogen is converted into helium,
the majority of reactions and the majority of the energy that powers the Sun comes from other
sources.
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!

15. Astrophysicist and author Ethan Siegel is the founder and primary writer of Starts With A
Bang! His books, Treknology and Beyond The Galaxy, are available wherever books are sold