The Sun (and its relationship to Fire)

Why is the sun burning without the presence of oxygen in our space?

Click here for the straightforward answer for the layman, "The burning of the sun is not chemical combustion. It is nuclear fusion. Don't think of the sun as a giant campfire. It is more like a giant hydrogen bomb."

While below are some additional well-written comments.

Extract from with some spelling corrections

Go to Related Answer by C Stuart Hardwick in 2022

Lance Berg
Updated May 19, 2019

You are right, the Sun cannot burn in space with no oxygen. Well, not entirely right, it could also burn in the presence of fluorine, or chlorine, or bromine, or well, quite a list of other “oxidizing agents” (which, oddly, do not for the most part contain oxygen)

But “burning” is “combining with an oxidizing agent” and given the absence of that in any bulk… ok, another caveat, there’s actually quite a lot of oxygen in the Sun. Turns out that’s one of the most common elements in space, and the Sun, being a big gravity well, has quite a lot of it gathered together. And it's… well more on the “and it’s…” part later. Anyway, it’s just that there’s a lot more hydrogen and helium there than oxygen, vastly more, and we can ignore the oxygen for the moment.


The Sun isn’t burning.

I know, you’re used to seeing a light source and thinking “well, that’s fire”

But you aren’t really, are you? Because you see light bulbs all the time, and they, at least older ones, glow brightly because they have a little spring inside which has been heated to high temperatures in a vacuum.. deliberately in a vacuum because that means, as you’ve already pointed out, they won’t burn.

Things emit light because they are hot, not because they are on fire. Fire is just a way to make things get hot. But electricity will work too. If you have an electric stove, or a toaster, you can watch this work as well, notice that the elements start to glow when they get hot. Again, they aren’t on fire, they’re just really hot, so they are radiating visible light.

Now, what’s another thing that gets really hot and emits light?

Ah, the atom bomb.

Well, the Sun isn’t on fire, and it isn’t a metal element being heated by electric current. It isn’t even a Fission bomb like the ones used in Hiroshima and Nagasaki and, we hope, never again. They use Uranium or Plutonium or potentially a number of other very heavy and very rare elements, which split into smaller elements releasing energy.

No, it’s a fusion bomb. That uses two atoms, usually in our case Hydrogen, and fuses them together, forming the heavier Helium and releasing energy.

Remember how I mentioned there’s lots of Hydrogen in there? Well, that’s handy, because if you press hydrogen together really really really hard, it fuses and boom, ignition, and since there’s lots and lots, the reaction keeps on running for a few billion years. We’re about halfway through the Hydrogen Fusion Explosion that is called the Sun.

That’s the “and it’s…” I mentioned earlier.

Burning doesn’t make or destroy oxygen. That’s just breaking up molecules like O2 and combining the free oxygen atoms with Carbon or Hydrogen or Nitrogen or Iron etc, to make things like ashes, water, nitric oxides, rust. The oxygen still exists, it’s just bound up with those other elements.

Fusion turns two bits of hydrogen into one bit of helium. The hydrogen is gone forever, and there was, prior to this, no helium. Nothing just moved around like with mere burning, the elements were fused.

And that makes a lot of heat.

And heat… glows.

“And it’s?” Oh, well part of the chain of combinations that result from fusing heavier and heavier atoms together is the formation of Oxygen. “And it’s” making some.

82.9K views 335 upvotes 25 shares

Mati Berman Falk · March 15, 2018
I think light bulbs actually don't have a vacuum in them, as it made them more likely to break. Instead they have a nitrogen atmosphere inside to prevent combustion.

Lance Berg · March 15, 2018
Technically correct, but originally a vacuum was used, and since the Sun is sitting in a vacuum, I was trying to present a simplified picture.

Harley Jackson
Not technically correct. It’s actually correct.

Edward Coulter · April 8, 2020
Besides nitrogen, many light bulbs contain argon gas for longer life. The hot tungsten filament evaporates slowly (which deposits on the inside of the glass bulb causing blackening and reduced light) but this is countered by having the filament surrounded by a gas, preferable a heavy gas. Another neat trick is the addition of a little iodine (tungsten metal halide bulbs) to improve bulb life and brightness. Iodine reacts with tungsten on the glass bulb then carries it back as a vapour to the hottest part of the incandescent filament to decompose and reconstitute the filament.

A. L. Hern · March 17, 2018
Fire is the incandescent gas produced by the breaking of chemical bonds and release of their energy as they recombine with oxygen,
Flames consisting primarily of carbon dioxide, water vapor, oxygen, and nitrogen-Editor

whereas the sun glows through release of energy created by the fusing of hydrogen nuclei into helium atoms via immense gravitational pressure produced by the star’s mass.

The terms “fire” or “burn,” when applied to a star, are purely metaphorical — easy points of poetic reference — but they have no direct scientific application to the processes involved.

Lance Berg · March 17, 2018
All true, but a reply to the OP’s question, not to my answer.

A. L. Hern · March 17, 2018
There’s only one “comment” button, and clicking on it sends one’s posting wherever the website wants. You may have already noticed that this is not a well-designed site.

Florens de Wit
That is why you need to press the "answer" button not the "comment" button. The site is not designed very well though.

Robin Brackman · June 21, 2020
Why do these questioners never ask how a nuclear powered submarine spends months submerged and underway without accessing the atmosphere? They even “make” their own oxygen (by electrolyzing seawater).

Nuclear reactions are not a combustion reaction.

Graham Fountain · September 13, 2020
Actually, the process of fusion in the sun is a little different to what you described.

It starts with 2 hydrogen atoms, but instead of forming helium, they form deuterium. A normal hydrogen atom has 1 proton. Deuterium is a form of hydrogen that consists of 1 proton and 1 neutron. In the fusion process, 1 proton is turned into a neutron by emitting a positron.

Then another fusion takes place where another hydrogen atom is fused to the deuterium atom. This now creates Helium-3. A form of helium that has 2 protons and 1 neutron. Normal helium (Helium-4) has 2 protons and 2 neutrons.

The final step of the Hydrogen-Helium fusion process is that 2 atoms of Helium-3 are fused into an atom of Helium-4, releasing 2 atoms of hydrogen.

The net result is that 4 hydrogen atoms create 1 helium atom. While 4 hydrogens are used, the actual process requires 6 hydrogen atoms, with 2 being used in interim stages then released.

The process in the sun is slightly different to the fusion reaction that takes place in a fusion nuclear bomb. In that reaction, deuterium and tritium are fused into Helium-4. Tritium is Hydrogen-3, that is 1 proton and 2 neutrons. This fusion releases one neutron.

Steve Seiver · August 24, 2019
Since the Sun started its fusion reaction 600 million tons of hydrogen are turned into 596 million tons of helium every second. It has been doing this for billions of years and as you say it has fused (not burned) half of its hydrogen. The difference of four million tons of mass loss per second in this fusion process is what gives us all the light and heat that radiates to space. The tiny sliver of this energy that strikes the Earth is huge but is so small in comparison to the ubiquitous broadcast of energy released everywhere and all points around the Sun. The super heated plasma that’s created is not fire. Oxygen plays no role in this process.

Joshua Schroeder · May 10, 2020
What a great way to picture fusion vs conventionally burning something. Thank you for that.

Arthur Wolters · January 6, 2020
This is the most UNTRUE answer I’ve ever seen on Quora! The reactions in the Sun are unrelated to chemistry (reactions with oxygen or halogens that don’t exist on the Sun). The only significant process in the Sun is the nuclear fusion of hydrogen to helium — plus lots of energy.

Lance Berg · January 6, 2020
Weird, it’s almost as though that’s what I said, but since you seem to object, I guess I must have… no, no, I’m sorry, that’s what I said.

Try reading it again.

Or perhaps you’ve accidentally commented on a different answer than the one you’re objecting to.

Harley Jackson · August 20, 2020
You commented on Lance’s answer, without bothering to actually read it?

Gopal Das · June 22, 2020
Simple thing is that stars and our sun run on hydrogen fuel, through fusion hydrogen is converted to helium, helium to heavier metal etc. Due to cosmic eddy huge gases of hydrogen cloud concentrate in a whirlpool out of gravitation, intense heat generated and fusion starts, a new star is born.

Abhinaya Mary Koshy · March 3, 2018
Fusion of hydrogen to form helium…doesn't that violate the law of conservation or energy, though?

Edward Leahy · March 3, 2018
No. E=MC2 and when you add all the factors in the fusion of Hydrogen to form Helium there is energy left, which is emitted in various forms of electromagnetic energy(visible light being the one we see). For a detailed explanation you can peruse Wikipedia.

Abhinaya Mary Koshy · March 4, 2018
Oh yes. I used to know that. Especially since I studied Physics.

Stuart Schiffman · December 28, 2019
Lightbulbs don’t have vacuums surrounding the filaments. The glass is too fragile and would break. They’re filled with an inert gas, usually argon.

IM Niemann · December 30, 2019
The sun is at most 5,000 miles high in the atmosphere of the earth. It’s not 93,000,000 miles away from earth as the pseudo-scientists lies about it.

Pete Burry · January 11, 2021
What model tape measure did you use to come up with this, I might want to buy one, lol

Related Answer C Stuart Hardwick, Scifi author and science nerd. Answered Jan 8 If the Sun is made out of fire then why doesn't the oxygen used to keep it burning fly off into space? “Fire”, as the word is commonly used, means “the incandescent gaseous product of the rapid combustion of some oxidizable chemical fuel in an oxygen atmosphere.” Commonly, that fuel might be almost anything containing carbon such as paper, plastic, gasoline, or firewood. This is not what the sun is made of. The sun is a star, and its resemblance to fire is metaphorical. Like fire, the sun’s visible surface is an incandescent gas—that is a gas heated to the point it emits light. That’s about where the similarity ends. The sun is not powered by chemical oxidation, but by nuclear fusion. Chemical oxidation involves the outermost electron shells of oxygen and some other atom joining together into some more stable configuration, releasing a small amount of energy that is converted to heat. In the environment around us, fires usually burn at around 750–1,600 °C (1,382–2,912 °F), and in special circumstances when fed lots of extra oxygen, can reach 3,480 °C (6,300 °F) or more. That seems hot if it comes into contact with your soft, squishy, ocean life-derived fleshiness, but in the grand scheme of things, it’s nothing. The sun is powered by nuclear fusion. So much matter is sitting in one place, smooshed together by its mutual gravitational attraction, that the electrons are cast aside and the atomic nuclei are forced to join into heavier nuclei:

  1. hydrogen into helium
  2. helium into carbon
  3. Carbon plus helium into oxygen.
  4. Oxygen plus helium into neon.
  5. Neon plus helium into magnesium.
  6. Magnesium plus helium into silicon.
  7. Silicon plus helium into sulfur.
  8. Sulfur plus helium into argon.
  9. Argon plus helium into calcium.
  10. Calcium plus helium into titanium.
  11. Titanium plus helium into chromium.
  12. Chromium plus helium into iron.
At each stage, tremendous amounts of energy are released right up to the synthesis of iron. Beyond that point, fusion consumes more energy than it releases, which is why elements heavier than iron are mostly made only by the final collapse of stars and in other extremely high-energy cosmic events. But in a star like our sun, nuclear fusion of light elements into heavy elements provides the power, and there is a LOT of power. Inside the sun, temperatures reach tens of millions of degrees. The surface is much, much cooler, and in fact we can only see the very tipy top of the sun’s atmosphere (the photosphere) where temperatures fall to a mere 5,500 °C (10,000 °F). We only see the coolest part at the surface where it glows rather like…wait for it…fire. The sun doesn’t consume oxygen, but in fact, manufactures it. And it does indeed lose comparatively large quantities of oxygen, magnesium, iron, and all the other elements listed above into space. Together, all this lost material in the form of a vast, outrushing plasma, makes up the solar wind (which is also metaphorically named, and very different from the gaseous winds of Earth). 1.2K views 28 upvotes 11 shares Roy Wilson · January 8 It’s interesting to note that one of the early hints of nuclear activity came from chemists calculating how long the sun could burn if it had a perfect supply of coal and oxygen with perfect combustion conditions and they realized the sun would have gone out long ago. They concluded that something beyond chemistry was going on. Jordan Shank · January 9 The sun doesn’t consume oxygen, but in fact, manufactures it. Well, not yet. The Sun won’t produce carbon and oxygen using the triple-alpha process until it’s at the end of its lifespan in a few more billion years. Otherwise, yes, stars are what produce oxygen. C Stuart Hardwick · January 9 The steps do not suddenly stop and start but transition. The sun currently produces, for example, oxygen, magnesium and iron, all of which (and more) are ejected in the solar wind.

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