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Thread: Red Dwarf Sun

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    Default Red Dwarf Sun

    I am working on a book in which the world has a somewhat otherworldy appearance with a huge blue gas dwarf for a moon (yes, I know, it's a binary companion), for which I calculated the apparent size and length of a month a while back out of curiosity. Now I want to make the sun slightly orange tinged, and since I already did it for the moon I also want to make the unneccesarry effort to get the parameters of the sun reasonably realistic.

    To get the sunlight slightly orange, it's temperature needs to be lowered. Which means it's either a red giant or red dwarf. Since the theme is more eternity than end times, I want to go with the dwarf.

    Now my first question is, how low would the temperature have to be to make the light slightly orange. Let's say like close to sunset orange? I don't want the whole world to be orange, but the majority of the day lit light sunset.
    Second, what would the size of the star be as a red dwarf of that temperature.

    And now the tricky part, at what distance would the total radiation reaching an Earth-size planet be the same as for Earth, and finally how long would a year at that distance be?
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    Default Re: Red Dwarf Sun

    Well, you can ahve clearly "Orange Stars without them actually being Red (or other) Dwarfs.

    An Orange Type K Star of middling Temperature would hve around 4400-4500 Kelvin Surface Temperature compared to the around 5800 of our sun, be around 80% its size and 70% its mass, and around 20% of its luminosity.

    So assuming everything else is similar, its habitable Zone Ideal Point would be around 0.55 AU, with a year shortened accordingly (assuming its path is not more elliptic than our suns). So the year would be roughly 55 % of our year, ergo 200 days.




    I hope this helps.
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    Default Re: Red Dwarf Sun

    Things to watch out for:

    Your planet could need to be so close to the red dwarf that it ends up tidally locked, and a "binary companion" less likely.

    Red dwarf stars are more often magnetically hyperactive, leading to more erratic and intense flare activity, which is bad for life (compounded by how close a planet needs to be to get sufficient insolation) -- see, recent news about Proxima Centauri.
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    Default Re: Red Dwarf Sun

    A star hot enough to be orange wouldn't actually be a red dwarf? Such a star would be a K type star, whereas red dwarfs are type M. There are supergiant stars of this class, but the regular variety you'd be interested in would be around 0.45 to 0.8 solar masses and would really just be a slightly cooler version of our own Sun (G type). In fact, this type of star is the one that scientists looking for extra-solar life are most interested in, because they are longer lived than G type stars, emit less harmful UV radiation, and don't have the problems M type stars do where the habitable zone is so close in that any planet in it would almost certainly be tidally locked.

    A typical K type star would have a surface temperature of 4500K (compared to 6000K for our Sun), a radius 0.8 times Sol, and about 0.2 times its radiation output. Given such a star, and bearing in mind the inverse square law, you'd expect a similar level of radiation to reach an Earthlike planet at a distance of 41 million miles. Orbital period would be approximately four Earthly months. The star would appear rather larger in the sky than the Sun does for us because you're so much closer in, even though the star itself is smaller.

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    Default Re: Red Dwarf Sun

    There's this, at least.

    http://phl.upr.edu/library/media/sun...abitableworlds

    K or M-type stars are good candidates for "noticeably redder, especially around sunset."

    It's possible that when high up, it would be harder to spot.

    Red dwarf stars vary enormously in size - the bigger ones tend to be more stable- less of a problem with tidal locking, less of a problem with flares, etc.

    Gilese 581 is fairly large by red dwarf standards, at 30% the diameter (and mass) of our Sun - its temperature is 3480 K.

    The biggest stars still called "red dwarf stars" are on the order of 60% of the diameter of our Sun (and still only 10% as bright). HIP 12961 has a mass 70% of our Sun's - still fairly low temperature - 3838K.

    https://en.wikipedia.org/wiki/HIP_12961

    EDIT: Interestingly, it's stated here:

    http://epod.usra.edu/blog/2009/02/sk...exoearths.html

    that the sky will still be blue for red-dwarf stars significantly above 3300K, white for ones close to 3300K, and orange for ones below 3300K.
    Last edited by hamishspence; 2018-03-30 at 10:23 AM.
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    Default Re: Red Dwarf Sun

    Starting with the Earth around the sun and shriking it until it turns orange seems like the best approach. Keeps things from getting too different. So Type K it is.

    That illustration of a K type sunset looks great. Can easily be described as "a large orange sun". 200-300 days long year shouldn't have a dramatic impact on seasons and ecology. At least with the moderate amount of scientific accuracy I care to put into it.
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    Default Re: Red Dwarf Sun

    An alternative way of changing the apparent colour of the sky and star - give the planet's atmosphere lots of particles. If the atmosphere is semi-permanently full of dust - the star may look orange or even red, at almost all times.
    Last edited by hamishspence; 2018-03-30 at 11:03 AM.
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    Default Re: Red Dwarf Sun

    Quote Originally Posted by hamishspence View Post
    There's this, at least.

    http://phl.upr.edu/library/media/sun...abitableworlds

    K or M-type stars are good candidates for "noticeably redder, especially around sunset."

    It's possible that when high up, it would be harder to spot.

    Red dwarf stars vary enormously in size - the bigger ones tend to be more stable- less of a problem with tidal locking, less of a problem with flares, etc.

    Gilese 581 is fairly large by red dwarf standards, at 30% the diameter (and mass) of our Sun - its temperature is 3480 K.

    The biggest stars still called "red dwarf stars" are on the order of 60% of the diameter of our Sun (and still only 10% as bright). HIP 12961 has a mass 70% of our Sun's - still fairly low temperature - 3838K.

    https://en.wikipedia.org/wiki/HIP_12961

    EDIT: Interestingly, it's stated here:

    http://epod.usra.edu/blog/2009/02/sk...exoearths.html

    that the sky will still be blue for red-dwarf stars significantly above 3300K, white for ones close to 3300K, and orange for ones below 3300K.

    Great links -- thank you.
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    Default Re: Red Dwarf Sun

    Quote Originally Posted by hamishspence View Post
    An alternative way of changing the apparent colour of the sky and star - give the planet's atmosphere lots of particles. If the atmosphere is semi-permanently full of dust - the star may look orange or even red, at almost all times.
    I was thinking of that too. But I also like the effect of having the sun look bigger.

    Quote Originally Posted by GrayDeath View Post
    An Orange Type K Star of middling Temperature would hve around 4400-4500 Kelvin Surface Temperature compared to the around 5800 of our sun, be around 80% its size and 70% its mass, and around 20% of its luminosity.

    So assuming everything else is similar, its habitable Zone Ideal Point would be around 0.55 AU, with a year shortened accordingly (assuming its path is not more elliptic than our suns). So the year would be roughly 55 % of our year, ergo 200 days.
    With some work I got Universe Sandbox 2 working. For a star with a mass of 0.8 Suns and a surface temperature on Earth of 15°C, it gave me an orbital period of 163 days. Indeed at 0.542 AU orbital radius. Giving an apparent size that is 1.38 times that of the Sun. Which if I recall correctly is a bit lower than twice the area.
    Last edited by Yora; 2018-03-30 at 12:27 PM.
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    Default Re: Red Dwarf Sun

    Don't forget that for all our sun is a "Yellow Dwarf" it actually gives out white light - it just looks yellow because the way light passes through the atmosphere (the blue is scattered out causing the atmosphere to look blue and the sun yellow).

    So, an alternative way to change the sun colour is to have something else in the atmosphere...

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    Default Re: Red Dwarf Sun

    Quote Originally Posted by GrayDeath View Post
    So assuming everything else is similar, its habitable Zone Ideal Point would be around 0.55 AU, with a year shortened accordingly (assuming its path is not more elliptic than our suns). So the year would be roughly 55 % of our year, ergo 200 days.
    Quote Originally Posted by Yora View Post
    With some work I got Universe Sandbox 2 working. For a star with a mass of 0.8 Suns and a surface temperature on Earth of 15°C, it gave me an orbital period of 163 days. Indeed at 0.542 AU orbital radius. Giving an apparent size that is 1.38 times that of the Sun. Which if I recall correctly is a bit lower than twice the area.


    Thing is, objects orbiting closer, orbit quite a lot faster, all other things being equal. Hence 163 days for the 0.542 AU object. The star being a little lighter might increase that, compared to the same orbit around a 1 solar mass star, but not very much.
    Last edited by hamishspence; 2018-03-31 at 10:52 AM.
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    Default Re: Red Dwarf Sun

    I think the orbital period changes as the inverse square root of the star's mass, all other things being equal, so if you halve the mass you increase the orbital period by approximately 1.4x.

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    Default Re: Red Dwarf Sun

    Quote Originally Posted by hamishspence View Post
    Thing is, objects orbiting closer, orbit quite a lot faster, all other things being equal. Hence 163 days for the 0.542 AU object. The star being a little lighter might increase that, compared to the same orbit around a 1 solar mass star, but not very much.
    Quote Originally Posted by factotum View Post
    I think the orbital period changes as the inverse square root of the star's mass, all other things being equal, so if you halve the mass you increase the orbital period by approximately 1.4x.

    I should ahve remebered that, but then again, its been ages since I was at the University dealing with that kind of formulae....^^
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    Default Re: Red Dwarf Sun

    Quote Originally Posted by hamishspence View Post
    Thing is, objects orbiting closer, orbit quite a lot faster, all other things being equal. Hence 163 days for the 0.542 AU object. The star being a little lighter might increase that, compared to the same orbit around a 1 solar mass star, but not very much.
    You already get this in the simple pi times r squared. Even small decreases in radius get you an increasingly shrinking of area with each step. At work we're using flower pots with various diameters, and while there is little apparent difference between 13, 12, and 11 cm, things suddenly get very small very quickly which each additional 1 cm step down.

    Universe sandbox doesn't let me fiddling with greenhouse effects, but I think if you moderately adjust the levels of various atmospheric gases, the 15°C distance becomes a good bit larger, leading to a significantly longer year, long enough to assume seasonal ecological cycles similar to what we have on Earth.

    Which makes me wonder why I bother with star types, gas dwarfs, and angular diameter in the first place. I guess I just like the knowledge that such a planet could actually exist. Except the magic, of course.
    Last edited by Yora; 2018-03-31 at 03:44 PM.
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    Default Re: Red Dwarf Sun

    Quote Originally Posted by Yora View Post
    Universe sandbox doesn't let me fiddling with greenhouse effects, but I think if you moderately adjust the levels of various atmospheric gases, the 15°C distance becomes a good bit larger, leading to a significantly longer year, long enough to assume seasonal ecological cycles similar to what we have on Earth.
    I think the actual temperature of the Earth's surface without the existing greenhouse effect would be about 6C--not sure if that makes any difference to your estimates there, just providing it as a data point.

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    Quote Originally Posted by Yora View Post
    You already get this in the simple pi times r squared.
    Yup - but circumference is pi x d (pi x 2r) not (pi x r^2). If you halve the diameter of a circle then (velocity being equal) you halve the time taken to circumnavigate that circle.


    Quote Originally Posted by factotum View Post
    I think the actual temperature of the Earth's surface without the existing greenhouse effect would be about 6C--not sure if that makes any difference to your estimates there, just providing it as a data point.
    The figure is -18C.

    https://en.wikipedia.org/wiki/Earth

    Atmosphere

    Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C, in contrast to the current +15 °C,[145] and life on Earth probably would not exist in its current form.[146]
    Tidal locking might not rule out Earth-like conditions.

    TRAPPIST-1d and TRAPPIST-1e are among the most Earth-like planets yet discovered - and they're orbiting one of the smallest red dwarf stars known, pretty much guaranteeing tidal locking:

    https://en.wikipedia.org/wiki/TRAPPIST-1d
    https://en.wikipedia.org/wiki/TRAPPIST-1e

    The star has flares- but fairly weak flares. UV and X-ray radiation might be a problem though.

    For a viewer on one of the two Trappist planets, their star would look a lot bigger, and a lot "duller" than our Sun does to us. Due to most of its light being infrared, it would be something like 1% as bright (which is still over 3000x as bright as the full moon), it would appear much larger, and the redness (with a bit of averted vision glances to minimise eye damage) would be perceptible. As shown earlier, the sky would be downright orange instead of blue, assuming Earthlike oxy-nitro atmosphere.

    If the chosen planet's not conventionally tidally locked, but in a special resonance the way Mercury is, seasons, and a day-night cycle, might be possible.

    However, since the TRAPPIST-1 system is not really suited to any of the planets having moons:

    https://en.wikipedia.org/wiki/TRAPPIST-1

    this may rule out using a "alternate version of it" for your purposes.
    Last edited by hamishspence; 2018-04-01 at 03:42 AM.
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    Default Re: Red Dwarf Sun

    Tidal locking does not seem to have as big an impact on climate as people used to think. If you have a sufficiently thick atmosphere, both conduction and convection of heat become important factors. The habitable area would be much wider than just a thin strip going around the planet.
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    Default Re: Red Dwarf Sun

    Quote Originally Posted by Yora View Post
    Tidal locking does not seem to have as big an impact on climate as people used to think. If you have a sufficiently thick atmosphere, both conduction and convection of heat become important factors. The habitable area would be much wider than just a thin strip going around the planet.
    It would seem to make a moon or a binary pair almost impossible, however, and has some pretty deep worldbuilding consequences ("night" is a place, not a time, for example).
    Last edited by Max_Killjoy; 2018-04-02 at 08:50 AM.
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    Default Re: Red Dwarf Sun

    Re: Greenhouse effect
    In addition to the concentrations of the gasses, a higher total atmospheric pressure will also raise your temperature. And the planet's surface features would also make a large difference, both directly by how much heat they absorb from sunlight and indirectly by how they affect your water cycle.

    Basically, don't worry too much about the math, you have enough fudge factor to vary by several degrees.


    Re: Tidal Locking and Moons
    There are a few ways this could go.
    Option A - Planet is tidally locked to the sun, and the moon is too light to affect much.
    Option B - Planet is tidally locked to the moon, and its day-night cycle is driven by the moon's orbit.
    Option C - The moon's orbit is mostly perpendicular to the planet's orbit, and the planet's rotation is on the same plane as the moon's orbit. One pole of the planet faces a spot near the sun, but there are days and seasons near the boundary.
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    Default Re: Red Dwarf Sun

    In Option C, you have the axis of rotation continually pointing towards the sun? That's pretty extreme precession. Ordinarily, with that kind of inclination, a pole would point towards the sun at the summer solstice, away from the sun at the winter solstice, and perpendicular to the sun at the equinoxes.

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    Default Re: Red Dwarf Sun

    Quote Originally Posted by DavidSh View Post
    In Option C, you have the axis of rotation continually pointing towards the sun? That's pretty extreme precession. Ordinarily, with that kind of inclination, a pole would point towards the sun at the summer solstice, away from the sun at the winter solstice, and perpendicular to the sun at the equinoxes.
    See Uranus for an example of this.
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    Default Re: Red Dwarf Sun

    Also, I don't know how accurate it is, but Larry Niven's "We Made It" is a planet whose axis of rotation points through its primary twice a year, and at those times of year you get 1500mph winds across the surface--not too sure what the mechanism is supposed to be there, though.

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