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ScottEwen
2009-Jul-08, 02:47 AM
Hi everyone, I registered just to ask this question, but I need the help of people who know a thing or two about astronomy.

Okay, I run a "collaborative webcomic" which means there's 16 of us that all take turns doing a page and adding to the story. It involves a man who wakes up on another planet, and doesn't know how he got there, but he meets some local people and adventures ensue. I'd link it but I don't want to seem like I'm spamming.

Okay so in the first page of the comic, I showed that the planet our hero is on has two suns: a red giant and a smaller yellow sun. Then later on, I showed that there is a large moon (probably four times the relative size of our own) that covers both suns up in what the characters refer to as a "double solar eclipse." It is also mentioned that the double solar eclipse occurs once in a hundred lifetimes, and signals the arrival of the antagonist in the story, the Moon Tyrant. The Moon Tyrant is a talking staff who is held by a body that only says a few words, and has a floating black ball for a head.

Recent developments in the story reveal that the Moon Tyrant we see represents the "Eclipse Body," and is evil. The Moon Tyrant has several bodies, which are all good, and are based on the different phases of the moon. Since the Eclipse only happens once in a hundred lifetimes, the evil Eclipse Body only gets used once every few thousand years, when he poisons the mind of the Moon Tyrant and comes to the planet to wreak havoc.

Still with me? Okay now, since there are several bodies that are all based on phases of the moon, I wanted to figure out how this star system is laid out, and what the different phases of a moon in a double star system would look like. And also how it would be possible for a double solar eclipse to be such a rare event that it only happens once in a few thousand years. I thought of making the yellow sun orbit the red giant in a highly elliptical orbit that is perpendicular to the plane of the rest of the system, meaning they are only close to each other twice in the yellow sun's orbit. Does this make sense? Is something like what I'm describing even possible?

AndrewJ
2009-Jul-08, 03:47 AM
You better make sure your planet doesn't get much cloudcover. If there are two suns (one giant) there won't be much night-time. An unawareness of external reality might limit how technologically advanced your planet's inhabitants can get.

The Moon Tyrant sounds a bit like the Mekon from Dan Dare.

The Helliconia Trilogy by Brian Aldiss is an 80's science fiction classic about a civilization on a planet around a double star system. The cover art alone might give you some further ideas.

WayneFrancis
2009-Jul-08, 03:55 AM
Interesting, First thing to note is in many binary systems the 2nd sun would appear as not much more then a normal star. I'm not sure if the suns are actually close enough to each other to both appear large in the sky if there would actually be stable orbits for planets within the habitable zone. IE Tatooine would be a hard sell. The problem is any stable orbit of 2 stars within 10-20 AU of each other is bound to be very in hospitable for life.

Perhaps if the 2 stars where close to each other, with .1AU of each other then you might get some semi-stable orbit of a planet out at 2AU or greater, if they are G type stars. As for the phases on the moon, they would be pretty normal as the 2 suns would cast only slightly different shadows in the end.

I think there was a system of 3 stars where someone though there was a hot Jupiter in orbit around the main star but was later discounted.

ScottEwen
2009-Jul-08, 04:14 AM
You better make sure your planet doesn't get much cloudcover. If there are two suns (one giant) there won't be much night-time. An unawareness of external reality might limit how technologically advanced your planet's inhabitants can get.

Hm, I'm not sure what you mean. If the suns orbit each other (or the little yellow one orbits the giant red one), and the planet orbits them both, wouldn't it have more or less a normal day-night cycle?

Also, what do you mean by an unawareness of external reality?

AndrewJ
2009-Jul-08, 05:08 AM
Hm, I'm not sure what you mean. If the suns orbit each other (or the little yellow one orbits the giant red one), and the planet orbits them both, wouldn't it have more or less a normal day-night cycle?

From the rarity of the double eclipse I assumed your planet was orbiting one star as we orbit the Sun and the other over a much longer period (so "macro-seasons" would be centuries long). As your planet would be closer to one star than the other they wouldn't always be next to each other in the sky so nights would be reduced or there might even be perpetual twilight. However, Wayne pointed out that the further sun might appear just as a bright star (unless the further star was an absolutley humungous giant).


Also, what do you mean by an unawareness of external reality?

I think that our understanding of physics would have been hindered had Gallileo, Newton, Kepler et al not been able to compare celestial phenomena with those on Earth.

WayneFrancis
2009-Jul-08, 06:04 AM
Hm, I'm not sure what you mean. If the suns orbit each other (or the little yellow one orbits the giant red one), and the planet orbits them both, wouldn't it have more or less a normal day-night cycle?

Also, what do you mean by an unawareness of external reality?

If it has a normal day/night cycle then the phases of the moon would be normal too for the most part.

I don't know if there are any models for how planets could form within the habitable zone of a binary system where the stars are fairly close. Tidal forces from 2 stars orbiting each other would probably kick any close in planets out in to the frigged reaches of the system. If the 2 suns orbit far apart and the planet orbits one then most likely the 2nd sun wouldn't be that big. Remember out at Pluto the sun is only marginally bigger then any other star. At 5AU you'd only get 1/25 the amount of light from our sun.

Tog
2009-Jul-08, 07:20 AM
I'm not really going to be much help on the specifics, but i did to a short story that had a similar set up.

It would probably help a lot if you could give some specifics of the system. It sounds as if it's too late to change the basics, so we have two stars, a planet and a moon.

Which star does the planet orbit? Red, yellow, or both?
How far apart are the stars?
How far from the stars does the planet orbit?

AndrewJ
2009-Jul-08, 02:45 PM
Remember out at Pluto the sun is only marginally bigger then any other star. At 5AU you'd only get 1/25 the amount of light from our sun.

Suppose the second sun were a real mosnter like Aldebaran or Antares; you'd think it would be able to cast some significant shadows even at Pluto range. Having said that, sticking a second sun in the sky is a bit of science-fiction staple. Could the Moon Tyrant show up for some reason other than the double eclipse? If you put your planet within a globular there could constantly be a field of bright Sirius-like stars in the sky and a panoramic view of the mother galaxy half the time. Perhaps regular volcanic activity could produce bizarre will-o'-the-wisp effects. The more original the details the less the reader will be minded to find implausibility.

Rhaedas
2009-Jul-08, 03:38 PM
The only thing that comes to mind to allow both a close double star as well as a rare eclipse scenario would be if the orbit of the planet and of the suns were not aligned, so you'd only have an eclipse in a narrow field of the orbit, and then to have that match up with the moon as well would be further limiting (maybe too much).

A big problem is having a stable habitable zone with two close stars. If you have a major sun and a much smaller one, you won't have as dramatic an eclipse, but the varying radiation of two like size suns as they revolve around each other would make wide swings in what's received on the planet.

It's a question of how much you need the science to be realistic for the setting vs. making the reader accept what you describe.

Gandalf223
2009-Jul-08, 04:01 PM
Alien planet scenarios always require the reader to accept some kind of unlikely compromise with reality, so I don't have a problem with the varying radiation issue.

If you have the two stars orbiting one another in a different plane than the planet(s) or, similarly, have the moon in a different plane, then it's easy to imagine a scenario where many, many "years" would pass between the necessary alignment to create the double eclipse.

This could (theoretically) occur if the second, smaller star had been captured by the giant when it was passing through the system.

ScottEwen
2009-Jul-08, 04:42 PM
It's a question of how much you need the science to be realistic for the setting vs. making the reader accept what you describe.

Well, it's a sci-fi adventure comic, but it's not hard science fiction... what I mean is, I'd like to be able to figure out how the star system works, but it doesn't need to be 100% scientifically accurate at the expense of having cool stuff happen in the story.

My only problem is that I sort of wrote myself into a corner having one character say that the double solar eclipse only happens once in a hundred lifetimes. Now I have to figure out how that's possible, and the only thing I can come up with is if the stars are not close to each other most of the time.

ScottEwen
2009-Jul-08, 04:45 PM
If you have the two stars orbiting one another in a different plane than the planet(s) or, similarly, have the moon in a different plane, then it's easy to imagine a scenario where many, many "years" would pass between the necessary alignment to create the double eclipse.

Right, at first I came up with the idea of having the red giant at the center and the yellow star orbits it in an elliptical orbit that's perpendicular to the rest of the system. So for instance, the planet(s) orbit on the X plane, and the yellow star orbits on a Y plane. This would mean that the two stars, the planet, and the moon would only be aligned very rarely.

But I'm wondering if something like that would even be possible.

Tog
2009-Jul-08, 05:57 PM
Would it be possible for the moon and stars to be near the same plane but in a resonance of sorts, similar to the Earth-Venus one that allows for a transit every 100 years or so?

What I'm thinking is that the planet orbits star A, and Star B orbits both of those. The two stars will only be aligned every X number of years, and the moon will only be in position every Y number of cycles. I don't know how accurate it would be, but I think it's plausible, and resonance is a term that is both real and technobabbly enough to not likely need a better explanation.

I guess for this to work it has to come back to how bright the farther star is.

Jeff Root
2009-Jul-08, 06:19 PM
I think this whole thing about double eclipses only happening once in a
hundred lifetimes is a myth. Maybe started deliberately. Probably by those
weasley varmints on the Southern Continent that only live for a few months.

-- Jeff, in Minneapolis

George
2009-Jul-09, 02:38 AM
Okay so in the first page of the comic, I showed that the planet our hero is on has two suns: a red giant and a smaller yellow sun. I would recommend your hero be on a planet orbiting the yellow star (I just can't "yellow sun", sorry. ;) -- an inside joke); a red giant can be very problematic for long term civilization, or what's left of it in some cases.


Then later on, I showed that there is a large moon (probably four times the relative size of our own)... Ok, that is only half the distance for our own Moon.


...that covers both suns up in what the characters refer to as a "double solar eclipse." A Solar eclipse involves a certain star by the name "Sol".


I thought of making the yellow sun orbit the red giant in a highly elliptical orbit that is perpendicular to the plane of the rest of the system, meaning they are only close to each other twice in the yellow sun's orbit. Does this make sense? It does to me, and you don't even have to make it all that perpendicular, but close would be helpful.

Using crude equations, using a red giant mass of 2 solar masses, and the yellow star with 0.7 solar masses, I think you could separate this binary system by about 1,650 AUs and get an orbital period of about 14,500 years, or a little over 7,250 years (100 lifetimes) in crossing the planetary orbital plane.

This red giant would still be about 28x brigther than a full moon, or -16 mag. But the apparent size of the red giant would be quite small. If it were no bigger than Aldeberan, it would be just under the size of a point light source (1 arc min). Let it swell up to the size our Sun will someday be, and it would appear about 1/10 the size of our Moon.

Will this work for you? If so, perhaps others will verify this if you'd like.

WayneFrancis
2009-Jul-09, 04:18 AM
Suppose the second sun were a real mosnter like Aldebaran or Antares; you'd think it would be able to cast some significant shadows even at Pluto range. ...

Sorry I was thinking he was dealing with G type stars.

The problem with really big stars is ...well they don't live very long. Orbit near one of these guys and you'll be lucky to get self replicating molecules going before the star goes supernova and disintegrates the planet.

I understand that it is a staple of sci-fi but so is traveling into a black hole or worm hole, faster then light travel without any change in relativity/causality except when they want to travel back in time and flying through a nebula like it is a thick fog.

Gandalf223
2009-Jul-09, 06:11 AM
But I'm wondering if something like that would even be possible.

Not to put too fine a point on it, but I think it stands up pretty well in light of the story line you gave us. Please re-read the first sentence in my previous post! :shhh:

chornedsnorkack
2009-Jul-09, 08:16 AM
Sorry I was thinking he was dealing with G type stars.

The problem with really big stars is ...well they don't live very long. Orbit near one of these guys and you'll be lucky to get self replicating molecules going before the star goes supernova and disintegrates the planet.


But not all of them go supernova!

A star as massive and shortlived as Sirius B did NOT go supernova - it shed its mass as a planetary nebula and turned into a white dwarf.

Now, assuming that the action goes on on a planet orbiting a longlived G type star, where life has evolved.

Assume that it is one of a double (or bigger) star system, that also contains a star that (at zero age main sequence) is just slightly more massive and shorter-lived than Sun.

During its main sequence, it is rather remote and dim, but still much brighter than all fixed stars.

But then it reaches the red giant stage...

I have seen estimates that the Sun should reach over 2000 times solar luminosity at the end of red giant branch for millions of years before helium flash, and then shortly exceed 5000 solar luminosities at the end of asymptotic giant branch.

Now, since the smaller star is nicely in the middle of its main sequence, the red giant should be at least 100 au away to prevent seriously messing up the climate.

100 au would imply orbital period of at least 1000 years. Longer if the orbit is eccentric.

The triple stars where both orbital planes are known commonly have large angles between orbits. So, it makes sense that with a star further than 100 au away even at periapse, the planetary orbits might actually be at a large angle. In which case, passing the nodes should be a short event, as well as very rare.

I believe that Alrai has a planet on a stable orbit while possessing a satellite star with orbital period as short as 68 years.

Now consider such a well-visible system as Keid!

The orbit of A to BC is unknown, which means that, within observational constrains, it should be possible to make it up as the story requires. In any case, the mass loss of B in the red giant/planetary nebula stage would be expected to alter the orbits.

The observed distance of A from BC is no less than 400 au. The long semiaxis is thus unlikely to be under 250 au or so, and can be longer depending on what the true present distance is and where A is along the orbit. The period is at least several thousands of years.

What do you think could Keid BC look like from A back when B was a red giant?

Keid A is not a G star (it is K1), but at 0,36 times solar luminosity, it should be bright enough to support a free rotating planet, with maybe 0,5 year orbital period.

neilzero
2009-Jul-10, 11:07 AM
When the planet is closest to both suns simultaneously occurs rarely and briefly in any likely arrangement. Also the apparent size of one or both suns, varies considerably over periods from hours to centuries. This produces wide temperature variations for the planet, unless one of the suns always provides less than about 20% of the heat. A possible exception is the axis of rotation (North pole) of the planet points toward the sun that produces the lest heat, once per year Then the southern hemisphere is not heated directly by the lower heat sun, but convection heating (wind) may bring lots of heat to dimmer lit southern hemisphere. The rest of the year still has wide temperature variations
If the north pole points toward the the sun (once per year) that provides the most heat, then the southern hemisphere may get so cold, that nearly all the planets water is trapped in a giant ice cap. Double suns cause lots of mischief, unless one always provides less than about 20% of the heat, or so it seems to me. Neil