Why is the sun yellow/red/orange but the hottest flame is blue.



My (19F) brother (11) has autism and wants to know why the blue flame that is shown on Bunsen burners is hotter then a yellow safety flame. My mums got involved in the debate but she doesn’t know either lol. We said blues the hottest but he said the sun is yellow therefore the safety flame is the hottest.

In: Physics


The temperature doesn’t determine the color. Different fuels burn at different colors. Some fuels burn invisibly. Also, the coolest part of the sun is far hotter than the hottest part of a lighter by millions of degrees.

Lighters and the sun are different colors because they use different fuels (using *completely* different processes – combustion vs nuclear fusion) to create the light.

Here’s a YouTube video of a guy setting up a rainbow of flames using different fuels for the different colors: https://youtu.be/p2XIMKX3ktg

Here is a video of an invisible fire that broke out years ago in an auto race: https://youtu.be/Ku7TdLeEGsQ

The sun’s light is actually white, as it is a collection of all visible wavelengths. It only looks yellow/orange/red through our atmosphere

The Sun is also blue. As others have pointed out, the sun is actually white – a mixture of all of the visible spectrum. Its *peak* output is in yellow/green, as you can see from [this graph](https://www2.lbl.gov/Science-Articles/Archive/sb/Aug-2004/reflectance_wavelength.jpg) of the Sun’s energy in wavelength. So it’s *mostly* white but just *slightly* more green/yellow.

The sky is blue^^[citation ^^needed] and that energy has to come from *somewhere*. As sunlight filters through the atmosphere it gets refracted a bit (like light going through a prism) and the blue wavelengths get refracted way more than the others. This means the blue light goes all *around* making the sky appear blue, which means there’s less blue light coming directly from the Sun (from our perspective).

Your perception of color is as much a function as which wavelengths you *don’t* see as it is which ones you do. Since you’re seeing less blue light, the green/yellow becomes even more dominant.

And as others have pointed out, flame color is as much a function of what it’s burning as it is the heat of it. The Sun creates a huge range of wavelengths (hence, being white) while, say, a bunsen burner is using a relatively pure fuel and only producing a narrow range.

The Sun isn’t “burning” it is producing heat through nuclear fusion, squeezing hydrogen atoms together to create helium and releasing a huge amount of energy in the process. https://youtu.be/vVE0B6g9F_0

Hot objects do glow, and they’ll generally start red and move up through yellow towards white but you’ll never really get something to blue because that’s not how black body radiation curves work. The sun is basically white because its surface(where the light is made) is at just 5800 Kelvin which is why “Cool White” LEDs have a 5500-6000K color temperature. Higher temperatures mean a bluer white but still very white.

The blue flame from the bunsen burner isn’t because its stupid hot, its because of *what* is stupid hot. When you burn methane it will look blue because its hot enough to excite the electrons on the molecules and when they settle down they give off a blue photon. You can get a similar effect if you burn copper, you’ll end up with a green flame and its very obvious in this case that its green from the copper and not green from the heat.

TDLR – Two different properties can give a flame color, the blue flame is not blue hot, its blue because of what is hot

It’s not that blue is always hotter, it’s that bunsen flames go blue when they are hot.

This is because as the flame gets hotter, more energy can be put into the light. Blue light has the most energy, and red light has the least energy.

But, different things are better at putting energy into light. In a bunsen flame with just methane and oxygen, the light can be turned blue at over 1000 degrees. But add copper chloride, and you can make it go blue at a much lower temperature.

It’s a pretty hard question to answer well; it’s easy to get into some tricky quantum mechanics etc.

One of the few simple facts in all of this is that a photon of blue light has more energy than a photon of red light.

An object will always be emitting thermal radiation in the form of photons, but the maximum energy of these will be determined by its temperature. At room temperature these are all infrared, but as you heat something up it will eventually start emitting visible red photons – but it will still be emitting lots of infrared photons too. The sun surface temperature is around 5600degC (5773K). At this temperature it will be emitting some blue photons*, but it will still be emitting bucketloads of red, infrared, etc. To our eyes the average appears white. This type of cross-spectrum temperature-driven radiation is called ‘Blackbody radiation’.

With things like flame colours, you’re looking at a different process to blackbody radiation. Instead certain specific electrons around an atom are being boosted up to a higher energy state, and then emitting a precise frequency of photon as they drop back down again.

In a gas flame, when the hole in the bunsen is closed, the gas & air can’t pre-mix so it burns quite inefficiently, which leads to specks of carbon (soot). This soot is a big blackbody emitter & overwhelmes everything else. When you open the hole, the gas & air premixes & it burns without soot, so you can now see this blue light that’s being emitted as the electrons around the gas and air molecules drop back to their usual (ground) energy states.

It’s a bit like the combustion reactions are producing some clean notes like playing individual keys on a piano, but these can be overwhelmed by the soot blackbody emissions coming in and hammering handfuls of keys.


*tangent: Some of these blue photons happen to be in tune with an electron in Nitrogen, so they get absorbed & reemitted in random directions, while other visible frequencies pass right by. Hence the sky scatters blue light, and appears blue.

Hey I kinda hate all these replies: there ARE blue stars, and those in fact have a hotter surface temp than the sun. Also, methane does burn blue, but yeah it’s still hot. Show your brother pictures of blue stars. They’re huge and hot and burn all their fuel up in like a hundred million years, and explode! Normal stars last billions of years or more. https://www.discovermagazine.com/the-sciences/massive-blue-supergiant-challenges-theory-of-how-big-a-star-can-be

The sun is not the hottest thing around. It’s only around 6,000C at the surface–we can get temperatures far higher than that right here on Earth (particle accelerators routinely create temperatures into the billions of degrees, albeit in a tiny area). There are plenty of stars that are hotter than the Sun.

It’s also worth noting that the flame from a match is also yellow–does he believe that must be hotter than the blue flame of a Bunsen burner?

Much of this has to do with the refraction of light as it passes through our atmosphere. The sun as you see it from earth is not the same as you would see it from a spaceship outside of earth. Refraction of light is what makes our sky blue and why the sun seemingly changes its shade.

Additionally, the sun emits many waves that we CANNOT see, which include almost the entire spectrum beyond that which is visible, such as ultraviolet, microwaves, and even gamma radiation through solar flares.

Bunsen burners do not get anywhere close to releasing enough energy to emit light from almost every spectra. What we see is not the entire truth. Hope this helps!

First this (non-ELI5 disclaimer): There are 14 physical causes of color. An ELI5 answer will have trouble with this. . .

Things burn in different colors.

* Your stove burns natural gas, which glows blue when it burns; I forget why.
* Candles burn orange because the flame contains tiny particles of partially-burned wax – mostly carbon. These particles glow orange because they are hot, and hot things emit light. The temperature of the particle determines the color.
* Fireworks come in all kinds of colors because they contain various metal salts. These burn when the firework goes off. Different metals burn with different (characteristic) colors. For example, sodium is orange.
* Metals in any fire that is hot enough will glow a specific color. When they are just hot, they will glow red, orange, or yellow. Light them on fire and they will give off their own color of light. So, there can be two effects working in combination to make the colors of something hot and on fire.

TL;DR: It’s complicated.

EDIT: typo. Added clarification.

I will try to make this a more condensed version of the ELI5.

What we are looking at is that there are different gasses “burning” here.

The Bunsen burner flame, at its hottest is blue because of the gasses used, is burning at the most efficient rate. Which is applicable for here on earth.

The sun is also “burning” but with different gasses,for it’s size and compression. The surface glows at its own optimal peak performance.

I’m leaving out which colours might be “optimal”, because that is something I can’t answer.

What an amazing question. Your brother has stumbled onto a clever observation.

Basically the answer is a fire is not just the flame! There’s lots of parts of a fire you can’t see for various reasons.

When you turn a busen burner you add or remove oxygen. The safety flame has the vent closed, the least amount of oxygen. It burns but not efficiently! There’s too much fuel and not enough oxygen, so things turn to smoke and soot. You’ll notice how “dirty”a safety flame looks. The soot particles are being heated up and they are starting to glow. It’s called incandescence. Things that are hot glow! Start red then yellow then white and finally blue when hottest.

Well that’s why the Bunsen burner turns blow right? No! There are actually different ways to turn a flames colour. That blue colour is because you’ve mixed the oxygen to the perfect point. The mixture of fuel and oxygen burns and doesn’t create any soot matter. So there’s nothing to glow, no little particles. Instead you’re seeing a chemical reaction! And in fact that blue flame is always there, hidden under the yellow caused by the soot of the safety flame.

That blue is caused by molecular bonds breaking and reforming and not the temperature.

There’s another way to change a flames colour too. Adding certain chemicals also will release certain colours.

So the tldr is that the blue Bunsen flame Isnt blue because of its temperature, but because the oxygen fuel mix is perfect and no soot is produced, so nothing is heated up and glows. Instead the blue is the oxidation of hydrocarbons and is actuslly always hidden under the yellow of flame.

I just wanna add a quick fun fact in between all the answers (with quite different explanations to this phenomenon to be frank): our eyes are adapted to our sun and can detect and process these electromagnetic waves which the sun emits most of (400-800 nm). In human eyes these are the rainbow colours (400 nm as blue all the way to 800 nm as red). The sun also emits uv that we cannot see, but to a lesser extent important to see for our survival thus we’ve not adapted to perceive it. These are all wavelengths however. When we talk about temperature/energy we need to consider BOTH wavelengths and frequency. A bigger star will appear more blue-ish because it’s massive gravitational field leads a more aggressive fusion process giving higher frequency of low wavelength light (blue) which gives a high temperature. Small red dwarfs are less warm and emits higher wavelengths of light. Our sun is yellow and in the middle in terms of temperature (and life span).

The sun is putting out white light. All the flavours of the visible spectrum and then some. We see red/orange because the blue light gets scattered out by the atmosphere (partly creating the blue sky), leaving more of the red end of the spectrum for you to see.

Sun viewed from space [images](https://www.google.com/search?q=sun+viewed+from+space&rlz=1C1GCEB_enCA934&sxsrf=ALeKk03oKOdhUUwaruatFt7KyfG2qbZHoQ:1609630797085&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjPwoT_tf7tAhUDgp4KHcdtAdYQ_AUoAXoECB0QAw&biw=1450&bih=902).

The most simplified way that I can explain is that the color of the flame is determined more by what is burning/the chemical reaction occurring to cause the fire.

The Sun doesn’t burn. I know, its counter intuitive to our experience with fire on earth, but burning things in an oxygen atmosphere is an exothermic chemical reaction – completely different from the nuclear fusion reactions taking place at the core of the Sun. In fact there’s barely any oxygen in the Sun at all so it cannot possibly shine from chemical energy.