Why are red stars red, despite being hotter than a blue flame?


At 1500 C, iron glows “[dazzling white](https://en.wikipedia.org/wiki/Red_heat)”.
At roughly 2000 C, [propane](https://en.wikipedia.org/wiki/Gas_burner) burns [blue](https://en.wikipedia.org/wiki/Gas_burner#/media/File:Propane-burner.jpg).
At 3551 C, [Mu Cephei](https://en.wikipedia.org/wiki/Mu_Cephei) earned the name “Herschel’s Garnet Star”, and this is not a reference to uvarovite.


In: 37

Flames colors are far more dependent upon their composition than their temperature

There’s no green hot but if you add copper sulfate to a fire its gonna be very green

The color you seem from flames at non-astronomical temperature (<3000 C) isn’t determined by blackbody radiation but instead by the electrons of the exhaust hopping between fixed levels. As you add more energy, the CO2 in the smoke hops between higher energy levels causing it to give off higher frequency (bluer) photons

Stars on the other hand have a photosphere(the part that actually emits light) that is at crazy high temperatures which cause it to emit enough blackbody radiation in the visible spectrum to actually be visible. As you crank the temperature of stars up it again shifts the main spike towards bluer wavelengths so you get red, yellow, white, then blue stars.

Flames glow depending on the elements being burned. Copper for example makes a green flame, sodium a blue one. There are thousands of chemicals you can burn to make colors. Thats how we color fireworks.

Stars glow according to black body radiation, which will dictate the color anything will glow at a given temperature.

[Here are some graphs to help explain](http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html) basically the hot stars let off all sorts of wavelengths, but the combinations of these wavelengths add together to.make a certain color in our eye. The sun, a yellow star, actually emits a peak wavelength that is green, but since it’s still emitting a ton of red wavelengths, it averages out to a yellow color

I think this turns out to be more of a question of human vision than physics.

The emission spectrum of molten iron really is much redder than even the reddest star. The problem is that our eyes are [very bad at judging absolute color](https://en.wikipedia.org/wiki/The_dress), they constantly make assumptions about what “white” is and adjust the color from there. Daylight (5500 K color temperature) seems to be white on its own, and so do the soft white light bulbs in my house (2800 K color temperature), but if you compare them side by side, daylight looks shockingly blue and the light bulbs a deep orange.

Molten steel is typically seen in an indoor environment, so you don’t get a good chance to compare it with daylight, and it’s so bright that your eyes adjust to it and you can’t see much else.

A propane flame, though, is different. The gas is too thin to make a strong blackbody spectrum: what you see instead is emission lines from various molecular bonds (C-H, C-C) vibrating.


Mu Cephei is a red supergiant star, the reddest star visible to the naked eye. It has an estimated surface temperature of 3,551 K (3,278 °C; 5,932 °F). This temperature is hot enough to cause the star to emit a reddish-orange light, which is why it has earned the nickname Herschel’s Garnet Star. The color is caused by the absorption of blue and green light by the star’s atmosphere, which is composed mostly of hydrogen and helium.