# Questions for the Astrophysically-minded...



## Sephiroth (Mar 6, 2008)

I wonder if any of the Chrons' physics heads could help me out?


*1)  Calculating the zero-age main sequence luminosity of stars:*

I'm having trouble with this.  I've looked everywhere online for a formula that I can understand, but either I can't figure out what is being said (I'm not fluent in maths, although I struggle on...), or the formulae are listed but it is assumed that the reader already knows what the units/constants are.  So I'm confused.  The couple of ways I thought I had found are not yielding sensible answers, and I'm a bit stuck...

The importance of the ZAMS luminosity, of course, is that it determines the position of the 'habitable zone' around the star at the outset.  

The increase in luminosity over a stellar lifetime corresponds to the mass lost through hydrogen fusion.  I'm well aware that rates of mass loss for big stars (above 8 solar masses) are prodigious.  This leads me to believe that I cannot safely make the assumption that my star will lose the same amount of mass as our Sun, proportionately, over its lifetime.  

I am dealing with an F8V star with 1.1654141576188 solar masses.  

I know that it must lose more mass due to its greater luminosity, but I'm not sure if its shorter lifespan means that the proportion lost will end up being roughly the same, or significantly different from the 0.01% the Sun will lose over its 10-billion-year (main sequence) lifespan.  

I need a general formula that can calculate ZAMS luminosity for any MS star of a given size, mass, eff. temp., etc.  

Can anyone help?


*2)  Stellar magnetic field cycle and magnetic field strengths:*

This is very much of secondary importance for the time being, but does anyone know if there is a correlation between other physical properties of a star and the above-mentioned attributes?

Despite all the information I have about my star, I'm not sure whether or not I can use any of it to derive the stellar magnetic cycle. 

This is another area in which our Sun can serve only as the roughest of guides.  

I imagine that rotational velocity and rotation periods (equatorial to polar), which I have determined, might help.  I wonder if something like the effective temperature or bolometric luminosity can be used in conjunction to work this out?  

Probably I'm missing the point entirely.  This is all to do with electromagnetism.  Am I barking up the wrong tree?


Any advice would be greatly appreciated.


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## Xwing Mom (Mar 6, 2008)

Seph, I'm not a physic head, but I know where to find a good one...he specializes in astronomy and astrophysics.  you can reach him at todd-sullest.deviantart.com

Happy hunting!


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## MG1962 (Mar 7, 2008)

I hope this helps a little

Upsilon Andromedae - Wikipedia, the free encyclopedia

It specifically mentions the life zone.

I am interested why the star you are using has such tight parametres in terms of the story - I used to write a lot of astronomy based science fiction, and cant recall ever needing such precision as a matter of course for the story


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## Sephiroth (Mar 8, 2008)

*Xwing Mom:*

Cheers for your help!  I will certainly look him up and ask whether or not he might be willing and able to assist with this.  Much appreciated!  



* MG1962:*

Yes, thank you!  The fact that Upsilon Andromedae A has the same spectral class makes it very useful to me.  It's just under 10% more massive than my star (which makes me wonder a little about them being in the same spectral class, but then these are ranges, not absolutes...either that or I've made a mistake in my calculations, which is probable...).  It's 3.3 billion years old, too, which is reassuring, since that's very close to (but slightly younger than) the age of mine (being roughly middle-aged, like our Sun).  

That said, it seems to be very luminous for its size and it rotates more slowly than I would have expected.  Well, another site I'm looking at now does describe it as particularly bright for an F8V, which is reassuring.  I suppose it could have lost a lot of angular momentum due to the action of its planetary system, which seems to be very dynamic and contains two planets that makes Jupiter look small, both of which are much closer to the star than Jupiter.  

And I can't figure how they have its metallicity listed as 100%...I'm not sure what they mean by that, since the vast bulk of any star is hydrogen & helium, and I haven't seen a higher rating than 0.1 (or 10%) before.  I think this must be a mistake on Wiki, since another site lists its metallicity as 'roughly the same as that of the Sun'.  

Regarding luminosity, I suppose there is a great deal we still don't know about the detailed workings of individual stars, and I can't expect the simple, general formulae I'm using to come up with the right answers in the real world every time.  Another site lists the star as having 1.34 solar masses, which would make more sense with respect to its power output.  

This star makes a very interesting object of study, and I appreciate your bringing it to my attention.  Because it's so luminous, it seems like it's the biggest gas giant (out at 2.5 AU) that is in the habitable zone.  It's interesting to consider whether it might have sattelites capable of supporting life similar to that which emerged here on Earth.  Having said that, it looks like the eccentricity of the giant's orbit may make that implausible...



_ To answer your question:_

It's true that I don't need to know this stuff in such detail for the story.  I could make the story work without knowing any of these parameters, since the story itself is not reliant on mathematical details (as long as the physical situation is generally known).  The planet on which life evolves in my stellar system can be placed in the habitable zone and treated like an alternate Earth, with a similar energy budget, etc.  That way, I could get by without knowing any of the details.

And such details are for me alone, and would be kept to myself in the main, since the premise behind the story is so speculative that I consider it fantasy rather than proper SF anyway.  But that hasn't stopped me from creating an excruciatingly detailed planet with its own plate tectonics, weather systems, etc.  I've always taken the view that even if I never mention this stuff to the reader, it's important for me to know it.  It means that I can think about my world the same way I think about the Earth and the Solar System whenever I meditate upon them.  It makes the place seem as real as this one to me, and I figure that if I internalise enough of this stuff, it can't help but show through in the narrative even when I'm neglecting to mention it explicitly. 

And beyond my own writing projects, I would really like to know how to work these things out for myself.  Astronomy is one of my favourite subjects, and I'm constantly disappointed by my own limitations in the field, most of which are mathematical.  I really wish I'd paid more attention in school, because I was good at maths and physics up until the age of about 14-15, and then I just sort of lost interest in education altogether for a while.  This happened just when they were teaching the 'hard stuff' I would have needed had I wanted to pursue a career in astrophysics (which I loved the thought of, but I wasn't willing to work for it).  

So the more I can reclaim in my increasing old age, the better I will feel in relation to the wasted opportunities of the past.  


_ And regarding the story/setting..._

I would like to have other planets in the habitable zone.  This is where the calculations come in, since the HZ is a pretty tight space to squeeze more than one Earth-sized planet into, and its evolution over time (as the star increases in luminosity) then becomes very important in defining which planets could be habitable and for how long.  I would like to have the main planet somewhere in the middle of the HZ (further out than Earth, therefore, relatively speaking), with planets of a similar size (same order of magnitude) at the outer and inner fringes of the current 'middle-age' HZ. 

And luckily, manipulating the levels of atmospheric carbon dioxide will give me a bit of room to play around, there.  

But before I really start worrying about orbital mechanics (which is going to cause many a headache, I'm sure), I feel like I need to determine the rate of change in stellar luminosity so that I can map the outward movement of the HZ through time.


And where the stellar magnetic field is concerned, it's the implications for long- and short-term climatic cycles on my main planet that concerns me. If I'm ever going to be able to simulate an absolutely realistic climatic history for my planet, I need to know everything I can about my star. 


It probably seems crazy, but I would like this place I am creating really to exist, to the extent that, fantasy story elements aside, one could believe that it is one of those myriad points in our sky (or in a sky very similar to ours), and that I didn't just make it all up...





Thanks for your responses, guys.


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## MG1962 (Mar 9, 2008)

Yeah - a 100% metallicity would be a ball bearing lol - I think you are right - 10% sounds healthier. I understand the thirst for knowledge, just for the sake of knowing. A lot of info I gathered about my settings never made it into the story.

I would venture active tectonic plates would have a greater influence over the climate of the planet than the ongoing luminosity of the star. And fudging the COs content on the other planet will work a treat.

Having a moon orbiting a Super Jupiter has a bucket load of potential, and gets you out of the restriction of the habital zone. If you set up an Io like confluence you get lots of tidal stress creating heat and free water etc etc. Make the moon either big enough or dense enough to hold onto the atmosphere, and Bobs ya uncle lol

Anyway good luck with the project, please come back and give us updates


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## Sephiroth (Mar 9, 2008)

A ball bearing.  Heh, I like it.  


Fascinating idea about the Super Jupiter.  It makes perfect sense, and could be a really interesting setting for a story.  I had never thought about getting the required heat from a source other than the star.  I suppose the one question that might be raised about such a setting is its stability.  Our friend Io is not the most stable of chaps, as you know.  We would still need a situation with the potential for achieving the same kind of (punctuated) homeostatic stability as has allowed life to flourish here.  

But imagine the scenery!  I used to play Frontier, and I always liked the planets with gas giants in their skies; seeing that in real life would be breathtaking...


And I think you're right about the tectonics being a bigger influence, yes.  In fact, I guess we already have the proof, in a way - we're sitting on it!  In our case, what's interesting is that the increase in solar luminosity has coincided with the Earth becoming cooler than almost ever before (disregarding 'Snowball Earth' events) in the Cenozoic.  

So I guess for planets in the middle of the HZ, increasing luminosity (within reason) is a minor concern once you have an ecosystem.  

I wonder if it might still be very important to the planets on the HZ-fringe, though...but certainly the atmospheric CO2 has a much bigger influence.  



Thanks for showing an interest!  I'll certainly let you know how I get on...when and _if _I get on.  

(I'm dreading the part where I have to make sure none of the planets would eject each other from the system...)


Cheers.


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## MG1962 (Mar 9, 2008)

Io is not quiet as violent as you might think. The volcanic plumes are actually caused by the electrostatic interchange with Jupiters magnetic field. It is actually the same process as used by powder coating guns.

And no I am no super brain, I just worked for a company that made the guns, when they explained how the powder gun worked I said - Oh thats just like the volcanoes on Io

I got looks as though I had just denied the holocaust.

Yeah I agree, once you get an established bio-sphere, makes your life a lot easier. Your point about a cool Earth is interesting. I was only reading today that the Himalayain mountains have now began to retard Earths temperature track because of the increase in world rainfall is leeching CO2 out of the air.

Which means the whole gloabal warming fight has another element to deal with - but lets not go there lol - Much more fun building functioning solar systems


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## chopper (Mar 9, 2008)

definitely not a physics head, but one that used to create solar systems using Megatraveler RPG rules - they were quite complex and involved stellar mass, luminosity, and habitable zones. they might, or might not, be useful to you. i'd suggest scouring ebay or amazon for 2nd hand copies of the rulebooks (specifically the referee's rullebook) - or there's a site somewhere that'll bang em on a disc for you ( can't remember how much).

here's a starting place: MegaTraveller - Wayne's World of Books - Info & Sources

second point: if you have a super-Jupiter type giant in the habitable zone, surely you could have more than one satellite/moon with a breathable atmosphere orbiting that giant, thus solving at least one issue...

s


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## Sephiroth (Mar 11, 2008)

Io is a giant powder gun, then?  

I wonder if NASA would be up for a mission to paint Jupiter?  


Hmm.  I suppose the question is, how would this tidal/electrostatic interchange operate on a body of Earth magnitude (67 Ios), and one that has its own magnetic field, which would be embedded in that of its parent planet?  Io loses a lot of its atmosphere to space thanks to the Jovian EM-field, although it replaces this through its vulcanicity, and if it was more massive, it would no doubt hold on to a more substantial atmosphere. 

And a stronger magnetic field would protect from the radiation of the gas giant, which seems to be significant. 

I imagine there must be a lot of sulphur dioxide, etc, produced, however...

Volcanicity is a prerequisite for Earth-type life, but excessive volcanicity would surely preclude the possibility of complex life ever developing.  The further one moves the sattelite from the gas giant, the less the tidal heating becomes, though, and Europa, Ganymede and Callisto all experience a degree of tidal heating too (it's the reason for the sub-surface liquid water 'oceans' that are postulated to exist).  

So it's likely that an equilibrium point could be reached where the stresses produced just enough volcanism, but not too much.  


Having said that, the problem of tidal locking might be an insurmountable one. If the satellite has to be outside of the range for tidal lock, then it loses the benefit of tidal heating.  

But if the satellite is tidally locked to its planet, will it still receive sunlight across its entire surface, or will there be areas which are permanently lit and areas that are permanently in darkness?  Or at least, might the frequency of solar eclipses be so great that the planet is plunged into darkness frequently?

I haven't looked into this yet, so I don't know.  It's not as cut and dried as the case of planet tidally locked to a star, so possibly it will still receive solar radiation across its entire surface?


The other potential problem I thought of was formation.  At trans-HZ distances (and certainly when we get out to the equivalent distance of Jupiter), it seems that there is a physical tendency to favour the formation of lighter, less dense bodies, so having a planet with a close to Earth-type physical profile might not be a viable proposition. 

There's no reason why life might not evolve on a larger, less dense planet with close to Earth G, mind you, but it's an extra complication to consider...



Oh, and regarding the Himalaya - my understanding is the uplift of the Himalaya is considered to be a major factor (along with that of the Alps and the Andes) in the cooling of the Cenozoic Earth to the point that we got these Ice Ages in the first place.  So it really wouldn't surprise me to hear that they are having an effect today.  

I hadn't heard about the link between increasing rainfall and the leaching of atmospheric CO2, though.  Yet another complication!!  

Yeah, let's not go there........this is definitely more fun, if less relevant to our future.  



*Chopper*, thanks for the recommendation, it sounds like a useful tool, and I'll definitely look into it.   Cheers for the link! 


Regarding your second point, yes, it's something to consider.  That way they could be far enough out to avoid tidal lock and some of the other potential hazards associated with being close to a gas giant.  Also, we would be closer to the star, and therefore more likely to evolve heavier planets.  


The main potential problem would be the effect of increased solar heating on the gas giant itself, but perhaps it's only when we get closer in still that this becomes a problem.  At HZ distances, maybe the giant's magnetosphere would still do its job, and the solar wind would not strip the outer cloud layers away, creating a 'tail' that might be hazardous to the atmosphere of the sattelite and therefore to any life living there...

Oh, but the planets would be much closer together in either of these Super-Jovian scenarios, which has positive implications for space travel.


It's another cool idea.  This thread is giving me a lot to think about, and I'm enjoying the discussion. Your thoughts are most welcome, ta!


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## MG1962 (Mar 11, 2008)

Yes thinking about it - the idea of a tidaly heated moon is going to cause more issues than it solves, and the material composition is not something I'd thought of. As you suggest the amount of volcanic outgassing would be cronic. Might be a nice place to visit on the way to a real story lol.

I think the original HZ is the way to go - At least you have a frame of reference for any issues and problems, where the tidal moon you will be flying by the seat of your pants in a theoretical way

Well through the Cenozoic period the planet has definately been running on the cool side, and the relationship between rainfall and CO2 seemed well established. However when you go back to the Snowball period the prevailing theory suggested the interuption of global ocean currents by a central landmass stopped the gas interchange. **Shrugs** So I am not so sure now.

One thing that is interesting, all the Snowball theories agree that once the warming began, the ice retreated with amazing speed, in the order of a 1000 years, a mere geologic eyeblink.

That would be an amazing environment to set a story in


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## Sephiroth (Mar 11, 2008)

It certainly would.  Like the end of the Pleistocene gone wild!  Well, wild_er_, that was wild enough...

I suppose the sapient observers would have to come from elsewhere in such a situation, since it's highly unlikely they'd be around on the planet in the immediate wake of a Snowball event.  

What you say about the interruption of global ocean currents by a central landmass...I don't suppose you know of somewhere I could read more about that?  


The importance of the Snowball events and the timing of the latest ones in relation to the 'Cambrian Explosion' are fascinating with reference to the invention of an Earth-type evolutionary sequence on another world.  It's something else I really need to know more about...

At least dramatic events like this underline the fact that evolution is unlikely to be one long, gradual steady-state process, and so it should be perfectly plausible to imagine that on another world, sapient life might evolve in, say, 3.5 rather than 4.5 billion years, given a different set of conditions.  

This assumption is important to me given the shorter lifespan of my star.  


Regarding the position of the planets, I think I agree that the original HZ is the way to go, but it's a pity, because the Super-Jovian situation you described has created some fantastic mental images for me.  It's a very interesting scenario...

Definitely an idea with story potential.  Perhaps one of us should write a short story in that setting?  In a short story, we could get away with a lot more speculation?  And it wouldn't be necessary to build an entire elaborate structure on a possibly shaky foundation...


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## MG1962 (Mar 11, 2008)

SNOWBALL EARTH

Has pretty much everything you could need to know on the topic. I agree the Cambrian explosion and the last snowball show real causality, which kills my pet theory  

I have never been a fan of the steady state approach to evolution. There have been just too many extinction events that have lead to the rise of new fauna, the drama of which is far more to my taste. Given what has occured in the last 650 million years, I would be comfortable shortening a biosphere evolution down to as low as 2 billion years

The only thing that held life back for so long was oxygen, then enough of it to form an ozone layer. I have to tell you i get a kick out of telling people we are breathing excretement from ancient life lol. I rarely get follow up questions when I mention that lol


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## Sephiroth (Mar 11, 2008)

Yeah, I like telling those algae-haters out raking their garden ponds that they wouldn't be here without cyanobacteria.  

Thanks for the link!  What is your pet theory, incidentally?  I'm curious...


I have never been a fan of any steady state theory, and yes, it's partially because I have a morbid fascination for the drama of cataclysmic events, that is true, but it's also because they seem to run counter to what we experience at any observable level in nature, from the birth and evolution of the universe to the workings of a living organism, where we see periods of relative 'stability' (or at least where a set of laws or assumptions hold for a time) followed by a rapid phase transition into some new state.  

The fascinating difference between the former and the latter is that the phase transitions of the universe have been toward a lower-energy state all the time, while those of bio-evolution have tended toward greater complexity and higher energy consuption, thus being (philosophically) anti-entropic. 


I was thinking along similar lines with regard to the minimum timescale for the evolution of a sapient species, so I'm glad you agree.  All I need is either a class of organism that can produce twice as much oxygen as our cyanobacteria, or a larger population of similar organisms existing in conditions more suited to the rapid production of O2.  Where the ozone layer is concerned, we have a higher UV-flux coming from the F8 star, which should speed that process along quite nicely...


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## MG1962 (Mar 11, 2008)

Well my pet theory, before the Cambrian explosion Edicaran life was dominant, as far as I am aware, there is no evidence of predation. Creatures basically lived in giant hippie colonies, absorbing light, watching soap opera's and reading poetry.

Then at some point, some life form realised it could take a bite out of something else and that took a lot less time and effort. Hence the creation of red of tooth and claw - and we have never looked back lol. In the Cambrian explosion, we see some really nasty predators on the prowl, and some amazing defensive mechanisms created by the prey, all in an amazing short time.

And please dont mention anti-entropic around creationists  They get over excited with information like that. There is however a bit of a shake up in the old complexity arguement as well. 

Micobiologists seem to finding some creatures after they evolve then move a state of less complexity. It is no biggie for evolution in general, but looks like it might shake out a few branches in the tree of life


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## Sephiroth (Mar 11, 2008)

Yes, I've oversimplified things as usual, I'm aware that not every branch of evolution follows the same pattern.  But I find the general trend intriguing, I must say.  And cripes, I try not to mention _anything _to creationists if I can get away with it! 

Well, it's not truly anti-entropic anyway, since we only borrow the energy for a while and then die and give it all back.  I suppose if we were immortal, it might be different... 


Your theory is interesting, and although it's far from my area of expertise, it sounds plausible on the face of it.  In what way do you consider that the Snowball Earth hypothesis kills it?  Could it not also be the case that the two things happened around the same time, possibly with changes in environment forcing adaptations (and these among them)?


I love the line about hippy colonies and soap operas, btw...


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## MG1962 (Mar 11, 2008)

Well with the Snowball - all life was under extreme pressure, so the need to compete for dwindling resources bought on the agressive prey predator relationship. Even after things improved and a world full of niches, rather than simply spreading out and getting back to normal, the competition continued.

In a sense it had to happen or we would not be here lol

I sometimes feel for creationist, many a lead to their beliefs by others who have their own agenda. When I was in the US in 2006 We had a Creationist on a dig with us for a day. Her mind was definately blown away at the thought of standing on a fossil coral reef 80 million years ago in the middle of Kansas. At lunch she was suprised to find that from a team of 10 - 7  believed in God

But I will shut up now cause I dont think this is the forum for such discussions


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## Sephiroth (Mar 11, 2008)

Ah, the old anthropic principle.  Well, the weak one is hard to argue against...


I've been in discussions with creationists many times before, and yeah, they tend not to end up anywhere good.  I've argued with people who seriously believed that the Earth was c. 6000 years old.  One actually claimed that 'the dinosaurs were put there to test our faith'.  There's no way to argue against that kind of reasoning, so I tend to stay away from those arguments.  


With regard to your theory: as you stated it, it was compatible with what you've just said, no?  Unless you were suggesting an alternate _cause _for the development of predation?


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## chrispenycate (Mar 11, 2008)

There is no way of proving the world was not constructed ready equipped with fossils and geology; mind you, there's no way of proving if wasn't created last thursday at three fifteen pm, with all our memories preprepared, global warming and the extinction of species that never actually required creating fully established.

But I don't think much of an omnipotent being who would cheat, develop a species with an inquiring nature and then plant masses of false evidence just to mislead them. Some earlier, more sadistic religions spring to mind (although the later ones can do a decent line in sadism, too, especially for a "loving" God)

Still, it's just an aesthetic judgement, not rigourous scientific proof.


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