Science Question on Class F Star for Fantasy Setting

[Autokrat]

This is my first post/thread here (I try not to begin my life at a forum by opening a brand new thread, but I have a science related question) so I would normally begin by introducing myself, but this Forum doesn't posses a specific sub-forum for that so I guess I'll just say hello, I'm Autokrat.

Before I say anything else I would like to apologize if this is the wrong forum. The question I have is related to both Fantasy and Science; however, so I felt this was the most appropriate place (although it looks like a discussion forum for fan discussion established Fantasy settings, so I could be completely off track.)

I have been working for some time on an original Fantasy setting and among one of the things I try to do is write realistic Fantasy (insofar is that is possible.) In attempt at originality, the world on which my stories take place is not identical to Earth. I wanted to do something different, so have some basic researching (read: appeals to the Google gods) I decided to place my world in orbit around a Class F star (F8 or F9.)

This of course opens up a entire host of questions that needed to be answered; however, I'm scientifically deficient (a puny Sophmore working towards an Undergrad in Philosophy) so I have been unable to answer the following questions.

How far away would the planet have to be to orbit within the habitual zone of a Class F star with a Luminosity roughly say twice that of Sol? I did find the following formula: [(Luminosity of Star)^0.5] x 1 AU = Distance of Habitual Zone in AU. After plugging the numbers I came out with 1.4 AU.

Nevertheless, then I discovered that this was only a rough approximation that may not be accurate. While the specifics aren't important for the stories I have set during the primitive stages of the society living on this world, they are important for the stories I have set in the space age of this Fantasy world. I still consider it Fantasy even though I try to have Hard Sci-fi elements, because I do have a magic system, so its more or less Space Fantasy.

I'm not sure how albedo would effect the required habitual zone, or if the wavelengths of light emitted from a Class F star would be different enough from a Class G2 star to make the above formula invalid. This is important since I'm trying to develop a working calender for the world with all that implies to the plot and character ages.

Another question I have is how would the different wavelengths of light from a Class F star effect the ecosystem? I found one source that mentioned something about plants on a planet orbiting a Class F star might be more orange or red. This is particularly important since I want descriptions of the environment to be realistic and this applies through all phases of my world's history.

Again, I'm sorry if this is the wrong forum. I suppose this might belong in the science forum, but since it is a Fantasy related question I placed it here.

[CaptainChewbacca]

If specifics aren't important, don't sweat specifics. Albedo matters a bit, but not much since habitable zone governs whether or not liquid water will exist on the surface. Different wavelengths of light will affect the colors of your plants and how life on that world sees, but that's about it.

[Mayabird]

Yeah, I'd say you'd probably get more answers in SLAM. Fantasy would be if you had questions about a magic system in this world or something like that.
*mod ninja kick*

[Ariphaos]

The primary concern for habitable zones in stars much more massive than Sol is mostly one of how fast it moves - life needs to evolve and sentience needs to develop before (or closely before) the planet moves from liquid water to boiling.

Something with only twice the luminosity of Sol at a roughly equivalent point in its evolution is only going to be about 1.2 solar masses, however, so the Goldilocks window is still going to be billions of years wide. One thing I do in settings on similar worlds is consider the role that plankton have in cloudforming, so that rainstorms occur by day and clear skies by night, to help keep the planet as a whole cool. This is only going to be important if your life has spent a similar amount of time evolving, however.

http://webphysics.davidson.edu/Applets/ ... fault.html

Compare the spectrum of our sun (~5,800 Kelvin) with your star (~6,900 Kelvin) to see the spectrum difference. The Yellow-Green centered light of our Sun shifts to Blue-Cyan for yours, and while there is an increase in the ultraviolet spectrum, it's not terribly significant (though it would promote more ozone formation).

Different colors of plants might dominate, but I'm not really sure. Part of that is going to be chemistry, and part of the reason for our green is due to cooling, which might not need to be in the blue range in order to work.

[Autokrat]

Something with only twice the luminosity of Sol at a roughly equivalent point in its evolution is only going to be about 1.2 solar masses, however, so the Goldilocks window is still going to be billions of years wide. One thing I do in settings on similar worlds is consider the role that plankton have in cloudforming, so that rainstorms occur by day and clear skies by night, to help keep the planet as a whole cool. This is only going to be important if your life has spent a similar amount of time evolving, however.

So, the plankton release lots of cloud forming compounds on days when the son's ultraviolet rays are stronger than normal. Now, since they don't do that all the time here on Earth, are you suggesting that with a Class F star they would due to the increase in the ultraviolet spectrum?

In terms of time spent evolving, I'd say it is similar to Earth. Also, this planet is roughly identical to Earth in: radius, mass, orbital properties (distance not withstanding) and atmosphere.

http://webphysics.davidson.edu/Applets/ ... fault.html

Compare the spectrum of our sun (~5,800 Kelvin) with your star (~6,900 Kelvin) to see the spectrum difference. The Yellow-Green centered light of our Sun shifts to Blue-Cyan for yours, and while there is an increase in the ultraviolet spectrum, it's not terribly significant (though it would promote more ozone formation).

Different colors of plants might dominate, but I'm not really sure. Part of that is going to be chemistry, and part of the reason for our green is due to cooling, which might not need to be in the blue range in order to work.

Just to make sure I got all of this down.

1. The greater increase in ultraviolet light would instigate plankton to generate more atmospheric compounds to generate reflective clouds which in turn would lead to greater amounts of rain during the day.

2. The increase in ultraviolet light would promote more ozone formation. (How would that effect atmospheric pressure and would it tie into what the plankton are doing?)

3. While different colors in plants could be promoted (perhaps with a dominance in the red/orange), there would still be a decent mix of colors.

[Bakustra]

Another note is that the sky would be darker thanks to a higher prevalence of violet light to scatter. I'm not sure what the star would look like from the planet in terms of color, especially since the reason why we perceive the sun as yellow is not quite understood, but it would probably be white or maybe bluish-white.

[Ariphaos]

So, the plankton release lots of cloud forming compounds on days when the son's ultraviolet rays are stronger than normal. Now, since they don't do that all the time here on Earth, are you suggesting that with a Class F star they would due to the increase in the ultraviolet spectrum?

I'm saying it would shift from a biological survival mode to an ecological survival mode, turning into a reverse greenhouse effect to cool the planet as it begins to exit the habitable zone.

In terms of time spent evolving, I'd say it is similar to Earth. Also, this planet is roughly identical to Earth in: radius, mass, orbital properties (distance not withstanding) and atmosphere.

Not to be picky, but why? You want to make it unique, do so.

Just to make sure I got all of this down.

1. The greater increase in ultraviolet light would instigate plankton to generate more atmospheric compounds to generate reflective clouds which in turn would lead to greater amounts of rain during the day.

It would probably spread to terrestrial life as well.

Deserts would be even deader places, and civilizations more keen on stopping desertification and its causes.

2. The increase in ultraviolet light would promote more ozone formation. (How would that effect atmospheric pressure and would it tie into what the plankton are doing?)

It's reasonable to assume that more UVB will reach the ground on a cloudless day, but it's not going to be linear with the stellar increase. The sun is not death, white skin might either be less common or have other defenses against radiation, etc. and so on.

3. While different colors in plants could be promoted (perhaps with a dominance in the red/orange), there would still be a decent mix of colors.

No, I mean: http://en.wikipedia.org/wiki/File:Chlor ... pectra.png

The most common types of chlorophyll are going to have similar overall efficiency in this scenario - less red light being compensated for by more blue light.

Our vision is predominantly suited to pick out vegetation from blood (ever hear a hunter bitching about finding out a companion was colorblind?). So we have a different receptor for green light than red light, but not so far into the red range that the chlorophyll's near-infrared reflection and fluorescence dominates.

So unless there is some specific reason for different sorts of chlorophyll to dominate, the lower end visual range might be almost identical for a hunter-gatherer sentient species. They might be more sensitive on the higher frequency range, however.

[starslayer]

Another note is that the sky would be darker thanks to a higher prevalence of violet light to scatter. I'm not sure what the star would look like from the planet in terms of color, especially since the reason why we perceive the sun as yellow is not quite understood, but it would probably be white or maybe bluish-white.

No, the sky would be brighter, assuming the atmospheric makeup is roughly identical to Earth's and the sentient life has eyes roughly equivalent to humans, because the blackbody peak of the star will move towards the blue, as Xeriar mentioned. This means there is more blue and violet to scatter, making the sky a brighter, deeper blue. It wouldn't be very much different from our Sun, though, given he picked such a late F star, which is only be about three or four tenths away from the Sun, a G2.

As for the reason the Sun looks yellow, that's pretty simple: blue light is preferentially scattered in our atmosphere, so that when the Sun is lower in the sky, it looks yellower. When it's high in the sky, though, it's not yellow at all; it's white, and also appears white from high altitudes, where there is much less air in the way to scatter away the blue light.

[Autokrat]

I'm saying it would shift from a biological survival mode to an ecological survival mode, turning into a reverse greenhouse effect to cool the planet as it begins to exit the habitable zone.

Similar to how Earth cooled down?

I suppose my confusion here is when you say “exit the habitable zone.” I thought the habitable zone was the distance from the star that water could exist at. When you say exiting the habitual zone are you referring to conditions on the planet itself changing as opposed to distance from the Star? (I'm assuming the former.)

Not to be picky, but why? You want to make it unique, do so.

I'm trying to make it unique but similar. Changing the radius, age, orbital properties etc, will inevitably involve math and, the more math is involved the greater chance I will make a glaring mistake. You could consider this adhering to the “write what you know” rule.

It would probably spread to terrestrial life as well.

Deserts would be even deader places, and civilizations more keen on stopping desertification and its causes.

From what you are saying here, I'm getting this:

Deserts would be deader because of the lack of vegetation and biology to release cloud forming chemicals to protect against UV. And so civilizations would do their best to halt desertification because that would remove the important cloud forming biology.

No, I mean: http://en.wikipedia.org/wiki/File:Chlor ... pectra.png

The most common types of chlorophyll are going to have similar overall efficiency in this scenario - less red light being compensated for by more blue light.

Our vision is predominantly suited to pick out vegetation from blood (ever hear a hunter bitching about finding out a companion was colorblind?). So we have a different receptor for green light than red light, but not so far into the red range that the chlorophyll's near-infrared reflection and fluorescence dominates.

So unless there is some specific reason for different sorts of chlorophyll to dominate, the lower end visual range might be almost identical for a hunter-gatherer sentient species. They might be more sensitive on the higher frequency range, however.

As I understand it, even though the wavelength would be different, because of how eyesight could evolve, the sentient species would still differentiate between the red and green (even though the plants would technically be using chlorophyll B?) so the hunter/gathers could spot blood (which would be difficult if the majority of plants looked red and orange.)

Does that mean that on Earth planet are also "red" but we just evolved to see them as green?

[Ariphaos]

Similar to how Earth cooled down?

I suppose my confusion here is when you say “exit the habitable zone.” I thought the habitable zone was the distance from the star that water could exist at. When you say exiting the habitual zone are you referring to conditions on the planet itself changing as opposed to distance from the Star? (I'm assuming the former.)

In stars capable of fusing helium, after the initial burst of heat from collapse and burning of primordial deuterium and lithium ceases, the fusion process increases in efficiency over time, moving from the proton-proton chain to the CNO cycle. This makes the star grow hotter over time, eventually fueling its expansion into a red giant.

So over any large (not a red dwarf) star's lifecycle, the habitable zone (where liquid water can exist) is slowly pushed outwards. So where your star is twice our Sun's luminosity 'now', when the planet first coalesced, it was perhaps 120% of Sol's current luminosity (just a guess, do not quote me on).

Metallicity is going to factor into this as well - don't sweat the details too hard.

I'm trying to make it unique but similar. Changing the radius, age, orbital properties etc, will inevitably involve math and, the more math is involved the greater chance I will make a glaring mistake. You could consider this adhering to the “write what you know” rule.

At the same time, there's the issue of 'writing, period'. I've spent years researching for my own settings. It's fascinating, but there's a point at which you just need to start and damn the consequences. You're going to be wrong about something. Trying to be a master of Astronomy, Biology, Chemistry, Geology, Linguistics, Metallurgy, Sociology, Psychology...

What I was pointing out, though, is that making such things the exact same is going to stand out more than giving them slight variations. If your world is 20% more massive, put down a few more active volcanoes, have some more basalt flows and give the world a bit more water cover. Or give it less - just because our current understanding suggests that larger worlds will tend to have more water does not mean that that is going to be the case for every world we find larger than Earth.

From what you are saying here, I'm getting this:

Deserts would be deader because of the lack of vegetation and biology to release cloud forming chemicals to protect against UV. And so civilizations would do their best to halt desertification because that would remove the important cloud forming biology.

They'd be deader because more UVB hits the ground and less life can live, period. But - see above. Seriously. Xenobiology is only going to be a respected field after we encounter multiple extrasolar worlds harboring complex life.

Until then it's all hypothesis and theory.

As I understand it, even though the wavelength would be different, because of how eyesight could evolve, the sentient species would still differentiate between the red and green (even though the plants would technically be using chlorophyll B?) so the hunter/gathers could spot blood (which would be difficult if the majority of plants looked red and orange.)

Does that mean that on Earth planet are also "red" but we just evolved to see them as green?

Plants reflect rather intensely in the near infrared. Our vision does not extend into the near infrared however (thus the name), but is rather fine tuned to see iron oxide as red and pick up the greener light that plants also tend to reflect with a completely separate receptor.

All I was saying with that, though, is that I'm not convinced that there is a need for some alternate pattern of plant colors, and that if a hunter-gatherer species on your world had similar low-wavelength vision to humans, it wouldn't snap my suspension of disbelief. I might be surprised if they were not more sensitive to blue light than we are (our blue vision is rather poor), etc. It's mostly an issue of there being an extremely strong evolutionary reason for the way our red-green vision is constructed.

[Junghalli]

Mars seems to have had plentiful liquid water at one time, when the sun was dimmer than today, so going by that the habitable zone of a star like our sun is at least .5 AU wide. For a star twice as bright I calculate a habitable zone with an inner edge at 1.4 AU and an outer edge at 2.1 AU.

If I remember right the sun's luminousity has increased 30% since it formed. How much faster would a star twice as bright evolve?

If a star twice as bright as the sun doesn't leave time for complex life to develop in any orbit the safest and easiest thing would probably be to just make the sun a bit smaller.

[starslayer]

In stars capable of fusing helium, after the initial burst of heat from collapse and burning of primordial deuterium and lithium ceases, the fusion process increases in efficiency over time, moving from the proton-proton chain to the CNO cycle. This makes the star grow hotter over time, eventually fueling its expansion into a red giant.

So over any large (not a red dwarf) star's lifecycle, the habitable zone (where liquid water can exist) is slowly pushed outwards. So where your star is twice our Sun's luminosity 'now', when the planet first coalesced, it was perhaps 120% of Sol's current luminosity (just a guess, do not quote me on).

Metallicity is going to factor into this as well - don't sweat the details too hard.

Uh, wait, what? That's not how dwarf stellar evolution works at all. All stars brighten over their lifetimes, even the smaller fully convective red dwarfs, and it's not because the hydrogen fusion process moves from the p-p chain to the CNO cycle. That doesn't even begin to become a significant factor until you hit ~1.3 solar masses (early F stars). Instead, it's because as the star burns hydrogen in it's core, the concentration decreases, and the fusion rate slows down. This causes the core to contract and heat up, raising the fusion rate again, stopping the collapse, and making the star brighter.

While it is true that the CNO cycle has a much higher temperature dependence than the p-p chain (T^16 versus T^4), it ends up not becoming a significant factor in stars where it wasn't to begin with. It also certainly does not fuel the transition to the RG, Horizontal, and AG branches on an HR diagram; gravitational collapse is a sufficient energy source for that, especially given that the transition only happens after the original core hydrogen is exhausted. None of this has any real bearing on your story, Autokrat; suffice it to say that a late F dwarf will still give its planets more than enough time to develop complex, sentient life.

[Bakustra]

Another note is that the sky would be darker thanks to a higher prevalence of violet light to scatter. I'm not sure what the star would look like from the planet in terms of color, especially since the reason why we perceive the sun as yellow is not quite understood, but it would probably be white or maybe bluish-white.

No, the sky would be brighter, assuming the atmospheric makeup is roughly identical to Earth's and the sentient life has eyes roughly equivalent to humans, because the blackbody peak of the star will move towards the blue, as Xeriar mentioned. This means there is more blue and violet to scatter, making the sky a brighter, deeper blue. It wouldn't be very much different from our Sun, though, given he picked such a late F star, which is only be about three or four tenths away from the Sun, a G2.

As for the reason the Sun looks yellow, that's pretty simple: blue light is preferentially scattered in our atmosphere, so that when the Sun is lower in the sky, it looks yellower. When it's high in the sky, though, it's not yellow at all; it's white, and also appears white from high altitudes, where there is much less air in the way to scatter away the blue light.

I was referring to the tone/color of the sky, not necessarily the intensity of the light. The hue would be darker, although not all that much, as you pointed out. I also am not sure that that is a total explanation, since clouds don't look yellow at similar levels of the sky, but then again, the sun always looks white to me (except on the horizon, of course).

[Ariphaos]

Gah, ignore me about the CNO bit. I think I was thinking about branches of the PP chain. It is irrelevant to the final point though - stars get brighter as they evolve.

I was referring to the tone/color of the sky, not necessarily the intensity of the light. The hue would be darker, although not all that much, as you pointed out. I also am not sure that that is a total explanation, since clouds don't look yellow at similar levels of the sky, but then again, the sun always looks white to me (except on the horizon, of course).

The tone is a function of how much atmosphere the light passes through. In addition:
http://www.everythingweather.com/atmosp ... tion.shtml
The light that finally reaches the surface of the planet is going to be moderated by the absorption spectrum, which will moderate the violet light striking the planet to some degree (less or more depending on the exact composition - nitrogen permits more violet light than oxygen does, etc.)

[General Zod]

This of course opens up a entire host of questions that needed to be answered; however, I'm scientifically deficient (a puny Sophmore working towards an Undergrad in Philosophy) so I have been unable to answer the following questions.

I would seriously suggest not focusing so much on the science fluff then, anyone with a degree of expertise would be able to pick it apart and point out where you're wrong. Way too many fantasy writers fall into the trap of trying to have a "super unique setting that's never been done" and ultimately fail at simple things like believable character interaction and motives that aren't incredibly juvenile.

[starslayer]

I was referring to the tone/color of the sky, not necessarily the intensity of the light. The hue would be darker, although not all that much, as you pointed out. I also am not sure that that is a total explanation, since clouds don't look yellow at similar levels of the sky, but then again, the sun always looks white to me (except on the horizon, of course).

When the Sun is high in the sky, clouds all over will appear white, because the air between the clouds and us is much too thick to support significant Rayleigh scattering (rather, you get Mie scattering). At sunset, when the Sun is very low, clouds obviously become violet, yellow, orange, or red depending on where they are in the sky (the higher they are, the more violet they are).

And yeah, the Sun always looks white to me too, because it is. I really don't know quite where the stereotypical noticeably yellow Sun comes from.

[Bakustra]

I was referring to the tone/color of the sky, not necessarily the intensity of the light. The hue would be darker, although not all that much, as you pointed out. I also am not sure that that is a total explanation, since clouds don't look yellow at similar levels of the sky, but then again, the sun always looks white to me (except on the horizon, of course).

When the Sun is high in the sky, clouds all over will appear white, because the air between the clouds and us is much too thick to support significant Rayleigh scattering (rather, you get Mie scattering). At sunset, when the Sun is very low, clouds obviously become violet, yellow, orange, or red depending on where they are in the sky (the higher they are, the more violet they are).

And yeah, the Sun always looks white to me too, because it is. I really don't know quite where the stereotypical noticeably yellow Sun comes from.

Phil Plait suggested in Bad Astronomy that it may come from people looking at the Sun with their eyes squinted or half-shut, since it's pretty hard to look at directly.

[Ariphaos]

And yeah, the Sun always looks white to me too, because it is. I really don't know quite where the stereotypical noticeably yellow Sun comes from.

It's less painful to look at when yellowed during the evening/morning.