Nuclear fusion supplies the stars with their energy, allowing them to generate light.
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Stars, like our sun, are powered by nuclear fusion. (Image
credit: DrPixel/Getty Images) |
Nuclear fusion is the process of forcing together two light
atomic nuclei and creating a heavier one, in the process taking a tiny amount
of matter and turning it into massive amounts of energy.
It is nuclear fusion that supplies the stars — including
the sun — with their energy, allowing them to generate light. The vast majority
of energy that Earth receives comes from the sun, and without it, life itself
on our planet would be impossible.
This energy is directed at our planet from what can loosely
be described as our star's surface, the photosphere. This layer of the ball of
superheated plasma we call the sun is heated by the star's core, where the
majority of nuclear fusion takes place. This source of energy is so ubiquitous
and so vital here on Earth, that it's little wonder that physicists are
desperate to emulate it in reactors on our planet. A future powered by fusion
could mean humanity's growing power needs are met by clean and highly
efficient fusion energy.
NUCLEAR FUSION POWERS THE STARS
The key to understanding how fusion generates energy is
Albert Einstein's infamous equation explaining how energy equals mass times the
speed of light squared (E=mc²). This tells us that matter and energy are
interchangeable, while the term c² tells us that a little mass creates a lot of
energy.
When matter particles fuse, the particles going into the
process have slightly more mass than the daughter particles that are created,
with the difference in mass 'liberated' as energy.
Even with the considerable mass-to-energy yield of fusion,
each incidence of fusion releases only a tiny amount of energy. Fortunately,
stars offset that by having a lot of raw material to power fusion, and these
processes run at incredible rates.
The main fusion process that provides the vast majority of
the sun's energy is the proton-proton I (PPI) chain. There are two other
branches of the PP chain (II and III) but these only account for around 15
percent of the thermonuclear fusion in the sun.
The PPI chain process involves four hydrogen atoms smashing
together and creating a helium atom, two electrons, two neutrinos, and two
highly energetic gamma-ray photons.
An illustration of
the process of nuclear fusion, specifically the creation of helium from
hydrogen. Four protons (hydrogen nuclei) are combining on the left, releasing
in the process two protons and two neutrons (a helium nucleus). (Image credit: Mark Garlick/Getty Images) |
While some of the energy is carried away as the kinetic
energy of the daughter particle, the majority is carried by the two gamma-ray
photons. These photons will struggle to escape the star's dense interior,
however — taking over 30,000 years to move from the core to the surface.
During this time the photons are undergoing a series of collisions,
absorptions, and re-emissions, which 'downgrade' their energy to photons of
visible light eventually radiated out by the photosphere.
Each occurrence of the PPI radiates about 0.0000000000044
Joules, which means — ignoring the other fusion process going on in the sun
— our star has to complete this process about 9x10³⁷ (9 followed by 37
zeroes) times every second to maintain its luminosity!
If four grams of hydrogen were converted to helium through
this process, only 0.0028 grams would escape as energy. That equates to about
260 billion Joules, enough energy to power a 60-watt light bulb for about 100
years.
Because of its tremendous hydrogen content, the sun has
maintained this fusion rate for around four and a half billion years and will
continue to do so for a further four and a half billion years until the
hydrogen in its center is exhausted.
This hydrogen-burning helium forging phase is what
astrophysicists call the main sequence lifetime of a star. But, helium isn't
the only chemical element being forged in the sun.
HOW DOES NUCLEAR FUSION FORGE THE CHEMICAL ELEMENTS?
Astronomers describe stars as containing hydrogen, helium
and everything else (with elements heavier than helium described as 'metals' by
astronomers) and these other elements also play a role in fusion.
The PPI isn't the main fusion reaction in more massive stars
than the sun, however. Instead, most of these stars' energy comes from the
carbon-nitrogen-oxygen (CNO) cycle which requires the higher temperatures of
more massive stars to get started.
The CN cycle begins with the nucleus of a carbon-12 atom
using it as a catalyst — an element that speeds up a reaction but is
unchanged at the end of it — for fusion. Carbon-12 through proton capture
goes through various stages until a helium atom is emitted and carbon-12 is
recovered. The NO cycle is similar but uses nitrogen-14 as a catalyst.
The energy generated by fusion serves a vital purpose within
stars, providing the outward pressure that balances the ball of plasma against
the inward force of gravity. That means that when fusion ceases, so goes the
outward pressure; this results in the collapse of the star and the swelling and
loss of its outer layers.
For stars more massive than the sun — which will end its
life as a smoldering white dwarf — this gravitational collapse creates enough
pressure to trigger the nuclear fusion of helium created by the main sequence
lifetime in its core, fusing it to create carbon, neon and oxygen.
When helium is exhausted, collapse occurs again triggering
the fusion of even heavier elements. As this continues, the star develops an
onion-like structure with lighter elements fusing in its outer layers and
subsequently heavier elements being created towards the core.
A close-up of the sun depicting solar surface activity and
the corona. (Image credit: DrPixel/Getty Images) |
This progression of nuclear fusions ends even for the most
massive stars when iron dominates the stellar core. This is because iron is an
extremely stable element and stars aren't massive enough to trigger its fusion.
When all nuclear fusion ceases, the star undergoes a final
and catastrophic gravitational collapse. This triggers a supernova that flings
the elements the star has forged during its lifetime out into the universe.
This material from these dead stars becomes the building
blocks of the next generation of stars, the planets, and everything around us,
including our own human bodies.
Additionally, shockwaves from the compressing iron core —
which will eventually birth a neutron star or even a black hole — hit gas
shed by the supernova triggering further nuclear fusion creating elements
heavier than iron and radioactive materials as well as blasting out x-rays and
gamma-rays.
Reference: "DOEExplains...Nuclear Fusion Reactions." U.S. Department of Energy Office of Science (2022).
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