Binary Stars
- Thread starter Bones2
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There's no limit. Some binary pairs are so close that they're literally touching, their atmospheres merging together, while others are as many as several thousand AUs apart.
If there's a habitable planet around just one of the stars, though, then the companion would have to be at least 20 times farther away than the planet is, or its gravity would prevent the planet from having a stable orbit. In fact, you might need the stars to be substantially further apart; some research has suggested that the binary interaction would prevent planets from forming in the first place, or would leave them barren of volatiles (water and atmosphere). If the planet is around the A star, say, then the B star at that distance would just be a bright pinpoint, while the apparent size of the A star would depend on its spectral type and the orbital distance of its habitable zone. For instance, a cool red dwarf would have a much closer habitable zone and thus would appear larger in the sky than Sol, despite being actually smaller.
If there's a habitable planet around just one of the stars, though, then the companion would have to be at least 20 times farther away than the planet is, or its gravity would prevent the planet from having a stable orbit. In fact, you might need the stars to be substantially further apart; some research has suggested that the binary interaction would prevent planets from forming in the first place, or would leave them barren of volatiles (water and atmosphere). If the planet is around the A star, say, then the B star at that distance would just be a bright pinpoint, while the apparent size of the A star would depend on its spectral type and the orbital distance of its habitable zone. For instance, a cool red dwarf would have a much closer habitable zone and thus would appear larger in the sky than Sol, despite being actually smaller.
It's funny, Groans, but a short time ago, I was writing a short story and wanted it to take place on a planet orbiting a binary star. However, I researched the idea and found what Christopher was saying -- it would be difficult for an Earth-size terrestrial planet that could support life to form around a binary where both components were too close together, as its orbit most likely wouldn't be elliptical. So I abandoned the idea, setting it around a G-type star like the Sun that's (a) within 100 light years of Earth, and (b) considered a candidate for an Earth-type planet to form in its habitable zone. More info on such candidates here. -- RR
what about two close star5s and the planet moved around them both?
^That's possible, if the separation between the stars is less than 1/5 the orbital radius of the planet. It could possibly look something like Tatooine's suns, though it could be more interesting if it were a contact binary, with the stars close enough to affect each other's shape.
Where does that 1/5 figure come from? And are we talking about minimum separation, maximum, median?
Don't mean to put you on the hotspot here but I don't have figures for a planet orbiting two close-in stars and I'm curious.
Also, could two stars orbiting so close maintain their separation for a sufficient time for complex life to evolve (~3.5 billion years)?
Same principle as the 20x figure I mentioned for a wide binary -- it's so the orbit of a planet in the habitable zone won't be disrupted by their gravity. I don't remember the specific paper I got the figures from, though.
And I'd assume it's the maximum.
If they're a contact binary, maybe not. A close detached binary probably could.
Yeah, it'd be to have a habitable earth-like planet around one. So, keeping this 20x measurement, that would mean the nearest it could be is...around 2.5 light hours?Christopher said:
That's cool. I'd be looking for further away if anything. Maybe around 2 or 3 light months. Can anyone give me a fair idea of how big a star of that distance (say, for the sake of easiness, the same size of Sol) would appear from a planet orbiting the other star?
Well, that depends. If you're talking about a star whose continuously habitable zone is at a similar radius to that of Sol, and a planet at 1 AU like Earth, then the minimum distance would be 20 AU, which is 20 x 499 light seconds, which is 2.77 light hours, so yeah. But different types of stars have different-sized habitable zones. For instance, Alpha Centauri A's habitable zone centers at about 1.25 AU while Alpha Centauri B's habitable zone centers at about 0.68 AU, since B is a smaller, cooler star.
And as I said, that's the nearest for a planet to maintain a stable orbit, but that's discounting the formation of planets in the first place. It's possible that a star that close might prevent a planet from ever forming in the habitable zone. So you'd be safer going with a wider binary.
That might be excessive. In Googling for "widest binary," the largest figure I could find was something around 6000 AU, which is a little over one light month. Most binaries are much closer, in the hundreds of AU.
It'd be a point source at much, much less than that distance. The question isn't "how big" but "how bright." A Sunlike star at that distance would be the brightest star in the sky other than the planet's primary, but it would just be a point. If it were at 6000 AU, then by the inverse square law it would be 1/36000000 of Sol's brightness as seen from Earth, which translates to an apparent magnitude of about -7.84, which is about 350 times brighter than Sirius, 20 times brighter than Venus at its brightest, and 1/80 as bright as the full Moon (all as seen from Earth). It would be bright enough to see in the daytime sky, but it would just be a point and would generate no significant heat.
If Jupiter was traded for the smallest mass red dwarf possible it would still only appear a little brighter than the full moon.
Jupiter is kinda like a failed star. It's moons resemble a planetary system. I would surmise that if the we had a red dwarf between the orbits of Saturn and Jupiter, but no saturn or Jupiter, there may could the possibility of life on one of it's "moons" or planets. provided the satellite has enough water and is at the right distance. Of course Europa might have life too.
If you duplicated the Sun and placed it where Jupiter was, the amount of extra heat and light would be negligible as Jupiter is a lot further from us as the sun is.
Yeah, I've looked more into this, and I might change it to much nearer, maybe 1000 AUs, and tone down the ship speeds. Then I'll fiddle the star's size accordingly.
What I'm after is having the star around 3 months' travel away, and the star quite prominent in the night sky, say around an 1/8 of the size that the moon appears. Also, maximum ship speed of around .4c at a push.
But then a sciency friend was telling me about how time dilation would muck things about. Does that ruin the Star Trek concept as well? Meh, I don't much care for relativity anyway.
What I'm after is having the star around 3 months' travel away, and the star quite prominent in the night sky, say around an 1/8 of the size that the moon appears. Also, maximum ship speed of around .4c at a push.
But then a sciency friend was telling me about how time dilation would muck things about. Does that ruin the Star Trek concept as well? Meh, I don't much care for relativity anyway.
There's no way. One-eighth the apparent diameter of the full Moon is 1/16 of a degree, or about 450 arcseconds. 1000 AUs is 149.6 billion kilometers. By the small angle formula, the star would have to have an actual size of (450)(1.496x10^11 km)/206265 = 326.4 million kilometers. That's 234 times the diameter of the Sun, making it a supergiant star. Such a star would have a life expectancy in the millions of years, so there's no way it could exist in a system old enough to have an inhabited planet.
If you want the star to be 3 months' travel away, it doesn't have to be that distant at all. The fastest space probes ever launched to date travel at only 0.02% of the speed of light. So a ship travelling at, say, 2% of lightspeed would be extremely fast for an intrasystem vessel. And 3 months' travel at a fiftieth of lightspeed is only about 320 AU, or 48 billion km. At that distance, though, the star would still need to be 100 million kilometers across, still over 70 times the size of the Sun.
Is there any particular reason why the star needs to have a visible disk? Even as a point source, it would still be the brightest star in the sky. If it's a sun-sized star at c. 300 AU, it would subtend about 6 arcseconds, about twice the angular diameter of Uranus as seen from Earth; that would be resolvable as a disk through a telescope.
Time dilation at 0.4c would be minimal -- we're talking 55 minutes to the hour.
I think the best place to find habitable planets would be around Yellow Dwarfs. Stars that would be hot enough for life, but small enough to live tens of billions of years. Also the habitable zone would be far enough out that planetary tidal locking would be at a minimum.
Can stable orbital habitable zones exist int he alpha centari system?
Can stable orbital habitable zones exist int he alpha centari system?
Actually that would be red dwarfs. Or maybe orange, depending on how you choose to classify K stars. Tidal locking doesn't really become an issue until you get down to K5 or so. And K stars are a lot more abundant than yellow G stars.
Yes, around both the A and B components, although it might be a bit borderline for the A star. As I said, some models suggest that the stars are too close to have allowed planets to form in the habitable zone, but I've just now found a paper that argues that planets which formed closer in would probably migrate outward, meaning that there could be one in the HZ. There's kind of a lively debate going on about it in the extrasolar-planet community. But there are also efforts underway to search for planets at Alpha Centauri, and we may get results within a couple of years, maybe even months.
Now, if not for Proxima Centauri, any planet around Alpha Cen A or B would be barren, since the proximity of the two stars would've seared all volatiles (including water) out of the protoplanetary disk. But Proxima's gravity could perturb the outer cometary cloud and send icy chunks inward to bombard planets within the system. Which would be a mixed blessing, since it would give the planets water but would also give them a relatively high bombardment rate.
I wonder about those K stars. Even those K0-K4 are going to subject a planet in the habitable zone to some pretty strong tidal forces which probably translates to very slow rotation rates.
There is also speculation that the diminished hard radiation of K-types might be inadequate to form an ozone layer which would result in an increase of such radiation at the surface.
There is also speculation that the diminished hard radiation of K-types might be inadequate to form an ozone layer which would result in an increase of such radiation at the surface.
We don't yet know for certain that it doesn't. There could be a brown dwarf out there somewhere that we haven't found yet.
It's like, there's a shopping list of things I ideally want, and hopefully they can all fit into place.
So...
1. I'd like the star to be blue, and noticeably blue in the night sky of a planet orbiting the other star. Fairly prominent. Maybe 1/8 of the moon's size is quite large, perhaps considerably less could still work, but as long as it's more noticeable than your average star, and blue in appearance if that's possible.
2. Around 2 months to get there travelling at .4c.
It looks to me like the obvious variable can be in the size of the star itself. But I'd also fiddle the speed of the craft/time to get there, if it'd help. I'm really no expert on these things.
So...
1. I'd like the star to be blue, and noticeably blue in the night sky of a planet orbiting the other star. Fairly prominent. Maybe 1/8 of the moon's size is quite large, perhaps considerably less could still work, but as long as it's more noticeable than your average star, and blue in appearance if that's possible.
2. Around 2 months to get there travelling at .4c.
It looks to me like the obvious variable can be in the size of the star itself. But I'd also fiddle the speed of the craft/time to get there, if it'd help. I'm really no expert on these things.
You'd probably be better fiddling the speed of the craft since the colour of the star depends on its size. If its blue then it probably needs to be significantly larger than our own sun.
Forget blue stars. They are type B or O and are very massive. Lifetime will be in millions of years. Any planets in a system with such a star might not even have time to cool (from initial bombardments) to a reasonable temperature before the star goes boom.
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