[Europan Ocean (OP)]

If we to see the Sun in the sky just after the impact of Theia with the Earth, when the moon was still hot and gaseous, would it have been the same bright yellow, or a dimmer orange or even red?

[Origin]

I assume after the impact there would not be a moon for quite a while.  There would be a debris field in orbit around the earth that would gradually coalesce into the moon.  Further I assume that there would be so much dust and debris in the atmosphere that you would not see the sun at all, the surface of the earth would be in darkness.

[Europan Ocean (OP)]

From that time, millions of years as you say after the impact when the moon was still hot, if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

[Halc]

From that time, millions of years as you say after the impact when the moon was still hot, if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

The debris would have cooled quickly. The moons, as they grew, might have generated their own heat. There were at least two main moons for a while which eventually collided at very slow speed, thus forming the current moon and defining the month.

If all the debris blocked the sun, I can't imagine that lasting for too long as most of the parts either combined or collided with Earth. Most of what would exit the system probably did so quickly in a day or so.

Any discoloration of the sun would probably be due to dust in the atmosphere. Plenty of metors coming in to constantly replace that, but I don't think there would have been much atmosphere left after the impact, so I can't see the dust being suspended for long.  Still, Mars with its thin atmosphere mananges to hold onto enough stuff to color everything, so maybe it looked a bit like a sunset, which turns red because I think red light penetrates the thick (thousands of km) atmosphere better than the higher frequencies, just like red light reaches us from the center of the galaxy when the visible wavelengths all find something to hit.

[Europan Ocean (OP)]

From that time, millions of years as you say after the impact when the moon was still hot, if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

The debris would have cooled quickly. The moons, as they grew, might have generated their own heat. There were at least two main moons for a while which eventually collided at very slow speed, thus forming the current moon and defining the month.

If all the debris blocked the sun, I can't imagine that lasting for too long as most of the parts either combined or collided with Earth. Most of what would exit the system probably did so quickly in a day or so.

Any discoloration of the sun would probably be due to dust in the atmosphere. Plenty of metors coming in to constantly replace that, but I don't think there would have been much atmosphere left after the impact, so I can't see the dust being suspended for long.  Still, Mars with its thin atmosphere mananges to hold onto enough stuff to color everything, so maybe it looked a bit like a sunset, which turns red because I think red light penetrates the thick (thousands of km) atmosphere better than the higher frequencies, just like red light reaches us from the center of the galaxy when the visible wavelengths all find something to hit.

From outside the atmosphere, would the young cooler Sun be yellow like it is today, or dimmer, red or orange?

[Europan Ocean (OP)]

I read from another source, the sin in space is white, and it's our atmosphere which makes it appear yellow. From a carbon dioxide atmosphere, it would be red, true?

[Bored chemist]

There's a contribution from blue light being scattered out of the beam, leaving it more yellow.

A star is a pretty good approximation to a  black body and so the temperature pretty much defines the spectrum.

[Eternal Student]

Hi.

From outside the atmosphere, would the young cooler Sun be yellow like it is today, or dimmer, red or orange?

   and this question....

if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

Seems to have been overlooked.

    The sun is basically a fairly standard star right in the middle of the main sequence on the  Hertzsprung-Russell diagram.    It hasn't changed it's brightness, temperature, colour or anything very much for a few billion years.    This is because its mass (which isn't thought to have changed very much) basically determines the hydrostatic equilibrium that will be achieved, which determines the temperature which in turn determines everything else (colour and brightness) because as @Bored chemist  has just mentioned,    the sun is almost exactly a black-body.

    The only complication is that the Theia impact could have been a very, very long time ago.   4.5 billion years by one source  ( https://en.wikipedia.org/wiki/Theia_(planet) ).   That does put the sun right at the earliest moments of its evolution and where it might have been just slightly off the main sequence.

      Using this source of information,  "The Suns evolution" - Northwestern University, U.S.  ( https://faculty.wcas.northwestern.edu/infocom/The%20Website/evolution.html  ) , it is suggested that the sun could have been about 30% dimmer 4.5 billion years ago.
      Using the Stefan-Boltzman law for blackbodies   we have     (Luminosity)  α  (Temperature)⁴,   so that the temperature would have been about  91.4%  of what it is now,    i.e. almost exactly the same.    This means the spectrum of radiation from the sun would have been almost exactly the same.    Using Wien's displacement law,  the peak wavelength would have been just slightly higher.   To say that another way,  the sun's light would have been slightly redder.    How red?   I'm not much of an artist,  I'd say it was slightly orangey.

    There are online calculators you can use to determine the peak wavelength and show that on the spectrum of visible light  BUT  it's better if you use something that considers all the wavelengths that are emitted and attempts to show you the colour you perceive with human eyes on the screen.    I don't know how reliable this website is but this one attempts to do exactly that:    https://sciencedemos.org.uk/color_blackbody.php?temperature=5300  .   In my opinion what that website shows looks a bit pink at times to my eyes ....  I'm really not sure how reliable it is or how good my eyes are but it illustrates the point if nothing else.   Let's just say the early sun was a bit more orange, if you squint and stand on one leg. 

Put the temperature in at 5800 k  (which should be the sun today) and you get this overall colour and brightness:

5800k.JPG (9.96 kB . 262x120 - viewed 2495 times)
Put the temperature in at 91.4% of 5800  =  5301k  and you get this colour and brightness:

5301k.JPG (9.87 kB . 232x122 - viewed 2516 times)
So you would see something like that if you were above the atmosphere of the planet earth and looking at the sun.   I hope that helps.

Best Wishes.

[Eternal Student]

Hi again.

Ours is a yellow sun. Sure, it puts out light over all the visible spectrum, but peaks at the yellow range.

   @Halc probably won't mind if I fix that.  Apparently it peaks at the green part of the visible spectrum, not yellow   (reference:  https://www.sciencefocus.com/science/what-colour-is-the-sun/ ).   However the mix of all the other colours in the spectrum getting thrown out at the same time produce something we interpret as mainly white.   This is whitey-yellowy in the daytime as seen from planet earth because of the Rayleigh scattering of the blue light in our atmosphere, leaving the sky where the sun isn't generally blue-ish while the place where the sun is looks more yellow.

Best Wishes.

[Europan Ocean (OP)]

The Earth around the times after Theia, would have an atmosphere of CO2 and Nitrogen, and of course H2O.

[Janus]

Hi.

From outside the atmosphere, would the young cooler Sun be yellow like it is today, or dimmer, red or orange?

   and this question....

if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

Seems to have been overlooked.

    The sun is basically a fairly standard star right in the middle of the main sequence on the  Hertzsprung-Russell diagram.    It hasn't changed it's brightness, temperature, colour or anything very much for a few billion years.    This is because its mass (which isn't thought to have changed very much) basically determines the hydrostatic equilibrium that will be achieved, which determines the temperature which in turn determines everything else (colour and brightness) because as @Bored chemist  has just mentioned,    the sun is almost exactly a black-body.

    The only complication is that the Theia impact could have been a very, very long time ago.   4.5 billion years by one source  ( https://en.wikipedia.org/wiki/Theia_(planet) ).   That does put the sun right at the earliest moments of its evolution and where it might have been just slightly off the main sequence.

      Using this source of information,  "The Suns evolution" - Northwestern University, U.S.  ( https://faculty.wcas.northwestern.edu/infocom/The%20Website/evolution.html  ) , it is suggested that the sun could have been about 30% dimmer 4.5 billion years ago.
      Using the Stefan-Boltzman law for blackbodies   we have     (Luminosity)  α  (Temperature)⁴,   so that the temperature would have been about  91.4%  of what it is now,    i.e. almost exactly the same.    This means the spectrum of radiation from the sun would have been almost exactly the same.    Using Wien's displacement law,  the peak wavelength would have been just slightly higher.   To say that another way,  the sun's light would have been slightly redder.    How red?   I'm not much of an artist,  I'd say it was slightly orangey.

    There are online calculators you can use to determine the peak wavelength and show that on the spectrum of visible light  BUT  it's better if you use something that considers all the wavelengths that are emitted and attempts to show you the colour you perceive with human eyes on the screen.    I don't know how reliable this website is but this one attempts to do exactly that:    https://sciencedemos.org.uk/color_blackbody.php?temperature=5300  .   In my opinion what that website shows looks a bit pink at times to my eyes ....  I'm really not sure how reliable it is or how good my eyes are but it illustrates the point if nothing else.   Let's just say the early sun was a bit more orange, if you squint and stand on one leg. 

Put the temperature in at 5800 k  (which should be the sun today) and you get this overall colour and brightness:

5800k.JPG (9.96 kB . 262x120 - viewed 2495 times)
Put the temperature in at 91.4% of 5800  =  5301k  and you get this colour and brightness:

5301k.JPG (9.87 kB . 232x122 - viewed 2516 times)
So you would see something like that if you were above the atmosphere of the planet earth and looking at the sun.   I hope that helps.

Best Wishes.

If I go into Blender, make a couple of spheres, and give them an emission shader using black body color selection. (there is and option to choose color by directly entering the black body temperature) , this is the result I get.

blackbody.png (320.01 kB . 960x540 - viewed 2396 times)

[Eternal Student]

Hi.

   That does look more plausible than the colours produced by the website I linked to,  @Janus.    White or White (but where we can tell ourselves the last white is a bit more orange),

This bit I mentioned earlier...

Using this source of information,  ".... it is suggested that the sun could have been about 30% dimmer 4.5 billion years ago.
      Using the Stefan-Boltzman law for blackbodies   we have     (Luminosity)  α  (Temperature)4,   so that the temperature would have been about  91.4%  of what it is now,....

... is a simplification, it assumes the surface area of the sun hasn't changed much.   If it was, for example, slightly larger during the Theia impact then we get slightly more Luminosity due to that larger surface area and a hence we would actually need to have a slightly lower temperature.   It won't make a lot of difference.

The Earth around the times after Theia, would have an atmosphere of CO2 and Nitrogen, and of course H2O.

    Yes... I don't know exactly how different the scattering of light and absorption of certain parts of the spectrum might have been in that atmosphere.    Rayleigh scaterring has more to do with particle size rather than anything else.    Those atmospheric components you mentioned are small enough and are still in our modern atmosphere (most of it is Nitrogen) so the Rayleigh scaterring wouldn't be that different.   Outright absorption by the molecules is also minimal,  Nitrogen, Carbon Dioxide and Water Vapour are essentially colourless gases - but the chemists can probably tell you more about what colours they might absorb  -  it should be barely noticeable.
   As @Halc  mentioned earlier, the main difference in colour you would see from planet earth is due to all the dust that would be in the atmosphere after such a collision.

Best Wishes.