So, I know the sun is getting brighter over it's lifespan and I'm wondering how that affects its emission spectrum. The reason I'm asking this is because I find it weird that plants reflect green light, when it's the most abundant wavelength. Was it always the most abundant wavelength? Or has that changed since photosynthesis evolved?
Nice question! Sun's spectral peak wavelength is currently 483 nm which falls under the category of green.
Sun's wavelength changing
In it's early days, the Sun was a lot cooler than it is today. So when it was a zero age main sequence or ZAM, it would be more 25 to 30 per cent cooler than it is today, which amounts to 89% of the solar temperature which I solved via the Luminosity-Radius-Temperature Relation. The reason this happens is because as proton-proton chain fusion occurs, due to nucleosynthesis, helium is produced which has more density, so it sinks down to the core (it is possible since the sun is a plasma, since the density is higher), then that helium cannot fuse at such temperature so it shrinks, this gravitational contraction in turn focuses the core more and more until the temperature has increased to sustain thermonuclear fusion and be in hydrostatic equilibrium i.e internal pressure which is fusion = external pressure which is gravity. So we understand that the Sun was cooler in the past Than it is today
Let's consider the Sun as a non-ideal blackbody, if we heat it, the frequency will increase due to Wien's law ($λpeak = b/T$) or Planck's law (for objects with extreme temperature like black holes accretion disk). Since the frequency is inversely proportional to wavelength, if the heat would be more, the frequency would be higher. So, it technically means that the Sun had a much longer wavelength in the past, so it would be a slightly redshifted, perhaps slightly more greener or it would be more like chartreuse. Specifically based on the temperature I obtained, using Wien's law, it would be 562 nm
The reason plants do not absorb the green wavelength
Plants reflect (Little is absorbed too) green light because absorbing green light would lead to saturation because that's the peak wavelength thus absorbing too much of it would be harmful. While the Sun was slightly redder in the past, I don't believe that this minor shift in wavelength (of just over 79 nm) is the primary reason why chloroplasts absorb red light and reflect green as it still falls under the category of green..
Thank you, Hope it helps you!
No, but that's not why plants reflect green light
The Sun, as well as the light of nearly all stars in the universe, had their peak wavelengths shift at some point during their life.
In the case of the Sun, it definitely had a different peak wavelength from green. The Proto-sun, or shall we say, the Sun during it's birth, was practically the equivalent of a low mass star, an red dwarf star, as it had a low mass itself and further it was still in it's deuterium fusing phase. It took millions of years for the Sun to shift from the deuterium-fusing phase to the protium one. And that also means that the star got more hotter, as it started to ignite protium at its core, as soon as it settled into a more stable state.
This also meant that assuming the Sun as a blackbody, the peak wavelength began to shift from being either red/orange (Proto-Sun) to a greenish-blue (Current Sun) as it began to get hotter and hotter. Today the Sun's peak wavelength is between 483-504 nm, which falls roughly inside the green spectrum. This is a reason why we can sometimes see a "green flash", before the Sun goes down.
However the reason the Sun doesn't look green, and rather white, is because besides emitting green light, it also emits a copious amounts of red and blue light as well, along with some ultraviolet and infrared. The merger of these 3 colours causes the white light that we see.
However shifting to the main reason.
The reason why plants reflect green
Basically, plants can't absorb too much light. Yes, too much light can actually damage plants. Absorbing too much light spells DOOM for any plant, as the photons now knock off atomic bonds in a way reminiscent of how high energy radiation destroy DNA. Just like gamma rays and microwaves, visible light can also cause damage to organic chemicals by either imparting heat to the plant tissues (in the case of microwaves and infrared) or ionizing the proteins and other chemicals binding plant cells together (for gamma rays and UV).
So in other to prevent too much light from entering the plants, they have designed the pigment chlorophyll, so that they do not absorb the peak wavelength of sunlight (green). Too much of a good thing can be bad, yk. If plants absorbed all of the light that fell on it, it would look completely black, sort of like this:
Kinda looks elegant, but that's just before visible light imparts a b*ttload of energy onto the leaves and kills them off.
So that's why plants on Earth are greenish in color. If they didn't reflect green light, they would have been long dead.
I say "On Earth" because that might not be the case for plants, or more accurately, phototrophs in the universe. If you have a star that's dim like a red dwarf, then you might get black leaves to absorb as much light as possible, and if your star is brighter than the Sun (Like Sirius), then you would definitely want to deflect away as much light as possible, and thus you would get a very pale (but not white) plants on planets orbiting bright stars. The reason why plants cannot be white no matter how bright they get can be found in this answer.
One more reference for why plants are green is Quieting a noisy antenna reproduces photosynthetic light harvesting spectra (2020). They compute the optimal absorption frequencies for a noise-cancelling antenna system (bottom), and find this optimum closely matches what the organisms are doing (middle).