Monday, December 15, 2008

What if... #15

What if the Sun was hotter and more blue, or cooler and more red than it is?

This post is part of the 2008 advent series "What if..."


  1. Earth would be unavailable to support life or evolutionary advance.

  2. In the Red case, I think Superman's powers would be diminished.

    So maybe in the Blue case, we'd all have superpowers.

  3. Cooler and redder is already done - chlorophyll is evolved to work at the red end of the spectrum. Auxilliary chemical antennas harvest shorter wavelengths and protect against UV degradation. A cooler sun requires atmospheric modification to avoid Iceball Earth. It occurs naturally.

    Warmer and bluer is also done - Jungle Earth as the Carboniferous Period (Paleozoic Era 299 to 359 million years back). One imagines the People's Republic of Canada is demanding Ottawa bailouts to accelerate cutting and burning of its forests. Hope springs eternal that Manitoba might be visible from satellite between November and April.

    The two preponderant forms of Terran life, plants and beetles, are happy either way. Get rid of people (e.g., evacuated Chernobyl) and Earth is a nice place to visit.

  4. In the red case, superman's powers would diminish, but if travelled to a world with a yellow sun, then we would have powers.

    Blue suns don't matter.

    And I'm not sure what Al's point is... It seems fairly obvious to me that this is a Superman-related question.

  5. Stefan's questions are too hard for me. ;-)

  6. Blue: life would have sprouted on Mars. Red: on Venus.

  7. If the Sun were a different type of star, we would have been born on a planet orbiting another star instead that does look like our Sun is now.

    If you believe in multiverse theories, then because there always is an exact copy of our solar system, including the internet, this blog and this question, nothing at all would have changed.

  8. Goldilocks would have had to find another planet or even perhaps solar system that was just right on which to live :-)

  9. If we also asssume that the Earth was closer (red sun) or further away (blue sun) then the gross temperature remains the same, and we can look at more interesting questions. In the blue case, one problem is that the Sun would be shorter-lived, so we might not have had time to evolve before the sun went red giant. In the red case, tidal forces are stronger, so the Earth would be more likely to have ended up tidally locked.

  10. I guess it really depends on how much redder/cooler or bluer/hotter things got.

    But if you changed nothing but the sun, and it was sufficiently hotter, then I guess the increase in water vapor, though it might increase the Earth's albedo a bit with extra cloud cover, would increase its greenhouse capacity to a greater degree, and precipitate a runaway greenhouse effect. You wind up with something like Venus.

    If the sun suddenly became sufficiently cooler, then I guess there would be an ice age that perhaps even periods of increased volcanism couldn't supply enough CO2 to reverse, and you get a permanent snowball Earth.

  11. Hm, I am not so sure about my stellar evolution 101 - in general, blue stars are younger stars, while red ones are older? But on the other hand, massive and hence short-lived stars are also hotter and bluer than the Sun. So, it may boil down to the point that the timespan for the life of the Earth - and life on Earth - would be different?

    Anyway, I was inspired to this "what if" by a strange but funny article in the April 2008 issue of the Scientific American, The Color of Plants on Other Worlds. As Uncle has mentioned, light harvesting pigments of plants are quite sensitive to changes in the spectrum, so, in the article, they write:

    Around stars hotter and bluer than our sun, plants would tend to absorb blue light and could look green to yellow to red. Around cooler stars such as red dwarfs, planets receive less visible light, so plants might try to absorb as much of it as possible, making them look black.

    Unfortunately, the illustrations of imaginary alien plants with strange colours aren't available anymore on the SciAm website.

    Cheers, Stefan

  12. With a redder sun, less UV would reach the top of the atmosphere but I wonder what the net result would be on the ground. REM that ozone absorbs UV but is also produced by UV, so the redder sun would produce less O3, and the reverse for a bluer sun.

    It is an interesting question, especially because of the sensitivity of strong absorption to tiny changes in sectional density. The formula is I_out = I_in * exp (-kD) where k is the transmission constant and D is sectional density (like how many 03 molecules/cm^2 in column of air.)

    Here's the weird part: if I have a layer that lets in 10% of some wavelength, but then reduce the density to 75% of previous, now how much gets through? Surprisingly, it's up to 17.8%. (From 0.1^0.75) If the layer had been letting in only 1%, the same change in thickness leads to letting in a whopping 3.16 times more! That's why a mild thinning ("hole") in the ozone layer can be so bad.

    BTW Stefan, (originally) red stars last longer than blue stars since they burn H slower, but how old they are depends on when they formed! Also REM that blue stars turn into red supergiants etc. so current color is not a consistent indicator anyway.

  13. Hello Stefan,
    hot and cool in this context can have two
    different meanings:
    Hot blue star means high surface temperature.
    The temperature on a planet of that star
    is a factor dependent on size of this star, distance and maybe "greenhouse"
    So a cool red sun will burn up earth
    in some billions of years when the sun will turn into a red giant.
    Question of plant colors is rather difficult.
    Our chlorophyll absorbs blue and red to
    convert this in chemical energy.
    But there is a brad region in the green
    part, which is not used. Strange!
    Additionaly there are plants with red
    leafs (beeches and brown algae) here
    on earth.
    Because the color of plants on earth are not
    straightforward "logic", I hesitate to
    predict those colors on other planets.
    Reasons that the colors can not be much
    different were discussed in an other
    thread on vision. Everything said there
    applies for plants as well.

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