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Dragonchild
2010-Jan-31, 09:07 PM
I am currently writing a fictional novel, and (among other things) one of my sticking points is the night sky. The work is essentially a fantasy novel, but I find an ounce of causality is worth both a pound of suspension of disbelief and pound of creativity. (What that means is that I intend to conduct similar amounts of research in other areas. I am limiting the topics discussed here to, well, astronomy.) In recent sci-fi movies, I think the night skies are entirely too Earth-biased or too unrealistic. In stories, they are often unmentioned (what a shame). I think if I can spend some time re-creating the circumstances on a different star system without stretching too many details, I can mentally paint a fascinating image of a night sky.

Basically, I am asking for help in creating a story that should NEVER appear in a Bad Astronomy blog post. If you are interested, please keep reading.

I imagine the story taking place on an Earth-like planet orbiting a K-type star of a binary system above the galactic plane. "Above the galactic plane" is problematic (globular cluster territory, Population II stars), but I figure somewhere along the 20 billion years it spends in the main sequence, a few crazy things can happen to the system that throws it out of a star-forming region without a catastrophic encounter that destroys my little planet. If the star somehow finds itself above the galactic center moving away from it, the effect would be that up to half the night sky would be filled with the spectacular sight of a galaxy up close (relatively speaking), while the other would be sparsely populated with stars.

The second feature of this world is that binary system, with the planet in an "S-type" orbit. A larger sister would be dangerous and a smaller one uninteresting, so I'm settling on a second K-type star, making the system somewhat like 61 Cygni. The sister star would need to be a safe distance away even at periapsis, of course. But here's where a strange thought struck me: Moons as large as Earth's are rare in the Solar System. (The world would have several smaller moons, if any, that would only transit the sun and minimally influence tides.) So, I calculated the distance a second K-type star would need to be to influence this planet's tides as much as the Moon does the Earth's. In rough calculations using semi-major axis as a rough reference, Newton's law of universal gravitation and equalizing the gravitation influence of the Moon on Earth with that of a star of 0.7 solar masses, I came up with. . . 11.2AU. In other words, a little beyond the distance to Saturn.

These all combine for what I consider a rather remarkable world. First,
with relatively little seasonal variation (the axial tilt is mild) but a binary companion on the horizon, the sister star -- as much as it drifts -- would be a better indicator of the passage of years than the seasons. (The year would be shorter compared to Earth's for a K-type star's habitable zone, but that would be irrelevant because years are divided into days, and the circadian rhythms would be based on the planet's own days. If the year is 10% shorter and the days 10% shorter, they may well have the same calendar.) Second, half the planet's civilizations could effectively use a GALACTIC CORE as its equivalent Polaris. I think that is cool. Finally, the semidiurnal tidal range would vary over a half-year cycle (not to mention a sub-cycle based on the binary system) instead of a two-week cycle. I'm still thinking over what that would do to the coastal flora & fauna.

Now, my questions:

1) I like the idea of a world that gazes on the face of a galaxy, but how plausible is the idea that the star system got kicked "upward" without any catastrophic close enounter with, say, a supergiant or black hole? If implausible, are there any plausible ways for a high-metallicity Population I star (w/ intact planetary system) to find itself above the galactic center?
2) If the galaxy was comparable in size & structure to the Milky Way and was at a distance that took up a majority of the night sky when viewed from the pole, what would the relative sizes and brightness be of the major features (arms, bar, center) compared to the Milky Way?
3) Is my result of 11.2AU correct? It's been a while since I used my college physics.
4) If correct, I'm worried that it's WAY too close, but I have no idea how to figure if such a close companion would harm the planet or its orbit. Orbital interaction is something that completely fries my brain. What would be any potential harmful effects, and if any, what would you consider a safe distance for a sister star of 0.7 solar masses?
5) If NOT too close, how bright would the star be? I'd imagine VERY bright. Not only could it be resolved as a disk by the naked eye, at that distance I daresay it would light up the night sky to twilight-like conditions for a large part of the year. That said, I could really use a more precise idea. If the sister star needs to be moved out (say, to 50 AU), I'd still like a clear idea of how bright it is. I'd calculate the apparent magnitude but there are too many factors (surface temperature, radius, distance, etc.) for me to be confident in my work.

Many thanks in advance.

Hornblower
2010-Feb-01, 01:13 AM
3) Is my result of 11.2AU correct? It's been a while since I used my college physics.Not even close. You are trying to get your secondary star to have twice the tide-raising capability of the primary. To do that, you would need to have it come closer than the primary. Out at 11 AU it would have less than 1/1000 of the tidal effect of the primary.

The tide-raising capability of a perturbing body is proportional to its mass and inversely proportional to the cube of its distance. It is easy to make mistakes when crunching the numbers, but when the two stars presumably have about the same mass, a simple check like this should catch it.

Dragonchild
2010-Feb-01, 01:55 AM
Ah, OK, I was way too simple -- I used the inverse square law of gravitation, completely ignoring the field gradient. If I want to create tides with a small moon, I guess the solution is to bring it in a lot closer? Not that tides are the be-all, end-all, but I just want to get the gravitational effects right.

With the secondary star irrelevant to tides, I might as well move it out to 50 AU or so. It'd still be a very bright star, but it wouldn't light up the sky or disrupt orbits from that distance, I presume.

Dragonchild
2010-Feb-02, 02:36 AM
Sorry for the thread bump, but I'm wondering if anyone has a take on questions 1 & 2. Also, I could use a sanity check on the 50AU distance for the secondary star. Thanks

Jens
2010-Feb-02, 02:49 AM
This is just a very individual reaction to your questions, which probably isn't what you really were asking, but anyway. I don't think that you really need to give a plausible explanation. Sometimes, as you say, a plausible explanation may help with believability, but sometimes it is just superfluous. I think that if it's a fantasy world, the people on the world would probably not know why their solar system was above the plane of the galaxy, so telling the readers could conversely take them out of the world that you're supposed to be leading them into, if you understand what I mean. Especially since it's fantasy. I mean, mostly in fantasy magic is accepted. Suppose that an author tried to give a plausible explanation using some kind of quantum mechanic solution - it would just be a hindrance to the suspension of belief. So in other words, I don't think you should worry too much about it. Even with the distance, I'm sure you could calculate it, but isn't it enough just to describe what it looks like?

I don't know how important it is for a novel, but I think a solar system situated somewhat outside of a big galaxy would be great for a movie.

Also, one other small comment, but you say it is a shame that the night sky isn't mentioned in much fiction. True, and to be honest, I can't think of any great novel that even discusses the night sky, though probably there are some where it is used in romantic scenes or perhaps as background for people talking at night. But again, I don't think it matters that much, because the characters and the story are what are important, so there's really no need to talk about the night sky unless it is important for the story. I think that in many cases, it isn't. Hence people don't discuss it. So it's not really a shame in my opinion.

Dragonchild
2010-Feb-02, 03:05 AM
This is just a very individual reaction to your questions, which probably isn't what you really were asking, but anyway. I don't think that you really need to give a plausible explanation. Sometimes, as you say, a plausible explanation may help with believability, but sometimes it is just superfluous. I think that if it's a fantasy world, the people on the world would probably not know why their solar system was above the plane of the galaxy, so telling the readers could conversely take them out of the world that you're supposed to be leading them into, if you understand what I mean.

I do, but some of this is for self-satisfaction. I like the idea of a world facing a galactic center, but I know that the galactic halo tends to be filled with Population II stars, and that bothers me. It's not like it needs to be explained in detail, but more like a treat for those who DO understand (like Phil). After all, there's enough fiction out there that gives no regard to causality. I don't need the distance, true, but I would like some perspective on how bright it might be. Unfortunately, I live in the city and never had a chance to see an unpolluted night sky, so I don't even have a good idea of how bright the Milky Way is; I've never seen it.


Also, one other small comment, but you say it is a shame that the night sky isn't mentioned in much fiction. True, and to be honest, I can't think of any great novel that even discusses the night sky, though probably there are some where it is used in romantic scenes or perhaps as background for people talking at night. But again, I don't think it matters that much, because the characters and the story are what are important, so there's really no need to talk about the night sky unless it is important for the story.

It is and it isn't. It doesn't drive the plot per se, but bear in mind historical records are filled with references to the night sky because at night, well, that was all there was to look at. Navigators, artists, scientists -- before the world industrialized, people all over the world were inspired by the night sky. No ancient civilization failed to create myths about why the sky looks the way it does (did).

That's one element I want to bring back with a vengeance. Sure, it's fantasy, and it's not like the story will revolve around astronomy. This is just one aspect of what I intend to make exhaustive research, and what better place to double-check my astronomy than an astronomy forum? But given the technology level will be pre-industrial, you can bet that everyone from navigators to bards will reference the night sky -- and I want it to be considerably more original than Luke Skywalker gazing at (gasp) TWO suns in the evening sky. The sister star (once I get an idea of where to put it) will be used to track the months, and (if it's not too wacky) explorers will navigate using the "Great Wheel". And I gotta admit, half the fun is in spending the time to make everything as plausible as possible. :)

Jens
2010-Feb-02, 03:39 AM
It is and it isn't. It doesn't drive the plot per se, but bear in mind historical records are filled with references to the night sky because at night, well, that was all there was to look at. Navigators, artists, scientists -- before the world industrialized, people all over the world were inspired by the night sky. No ancient civilization failed to create myths about why the sky looks the way it does (did).

That's one element I want to bring back with a vengeance.

If you put it that way, then definitely go for it. It is true that the night sky meant more to our premodern ancestors than it does to us, simply because they didn't have much lighting, so I agree it would be both logical and a way to create a different atmosphere from our modern life. Interestingly, just an hour ago I was googling about sleep (work related) and found out that in the premodern world, people often slept twice at night (in English, called first sleep and second sleep), and would spend the time in between talking or doing work or other things.

BTW, I don't know this for sure, but I suspect that if your solar system is out of the disk of the galaxy, and there is no big moon, then the night sky facing away from the galaxy would be really dark, and maybe you would see a lot of little fuzzy things like the way we see M31. Maybe, but I'm not sure. There may not be enough of them. Also, there could be satellite galaxies like the Magellanic cloud.