# In binary star systems, how far apart are the stars?

Reminiscing about Star Wars the other day, I wondered about Luke’s planet having two suns.

Maybe it’s just because in my neighborhood, the nearest star is 4ly distant, but I tend to think of the proper spacing for stars as being measured in light years, rather than, say, miles, or kilometers, or football fields.

But I know that binary stars are a thing, and they consist of two stars orbiting an intermediate point. On a scale of even one ly, this probably makes no sense, the inverse square property of gravitational force being what it is, so I ask the astronomy mavens of the SDMB:

How far apart is it realistic to call the two suns of Tatooine? And speaking of binary systems with planets, do the planets orbit both stars as a unit, or do they travel in a sort of figure-eight path?

t varies considerably. In the system Algol (beta Persei), for instance, the two main stars are only 0.062 astronomical units apart, or 1/16 the distance between the Earth and its Sun. (That’s closer than Mercury is to the sun)

There is a third companion 2.7 AU from this pair – that’s about where the Asteroid Belt is in our system.
By comparison, Alpha Centauri – also a triple star – has two major members whose distances vary between 11 and 36 AU (that’s sun-to-Saturn and Sun-to-Pluto, for comparison). I recall Isaac Asimov writing about how, to an inhabitant on a planet in this system, they wouldn’t see the stars as double.

Where would a planet orbit? Depends upon how far apart the stars are and a lot of other factors. It could orbit the pair as if they were one star – I could see a planet far fgrom the Algol main pair doing this. If one star is small enough relative to the oother, I could see a planet following in its wake at the Trojan point. There are lots of other possibilities, including pretty odd orbits.

It’s not clear if any of these odd situations would allow life to form and thrive in the system. I suspect Algol wouldn’t be a great plac e for humans to live, but possibly something could adapt to its conditions.

I’d be more inclined to suppose arise in.

You know, unless you’re talking about a panspermia situation.

Phil Plait has given some thought to this question in his review of The Phantom Menace (in his Bad Astronomy column).
(Scroll down.)

You can get even more extreme than Algol. W Ursae Majoris, for instance, is what’s called a contact binary: The stars are so close they’re actually touching.

For planetary orbits, there are only three stable configurations: You can have both stars really close together, with the planet far out and orbiting both of them as if they were one star (this is probably what Tatooine has), you can have the stars really far apart, with the planet orbiting one of them and basically ignoring the other, or (if the two stars have significantly different mass) you can have the two stars and the planet forming an equilateral triangle. More complicated orbits like figure-eights are possible, but they’re unstable, and are likely to lead to the planet being ejected before it’d have a chance to do anything interesting like develop life.

So what would that look like, anyway? 11AU ain’t very far as system distances go. Would a star the size of Sol show a disc at 11AU? What about at 36AU?

Definitely in both cases, though of course it’d be a smaller disk than the one we see. Of course, it’d be hard to make out except at sunrise/set, so the poets of such a world would probably describe it as a pinpoint.

Too lazy to find the thread right now, but I once asked (here, I mean) about proxima centauri: it’s simultaneously described as a third partner star of the alpha centauri system AND as the nearest star to us. And I asked “Waitaminnit, don’t the damn things orbit? So fast-forward a few years and won’t proxima cease to be the closest due to its orbit?” And people with more astronomical savvy than I’ve got said “well, yeah, eventually, but they’re sufficiently far apart that several hundred centuries will slip by before the orbit makes a difference in their position relative to us”

For a planet orbiting a contact binary, assuming the planet orbited in the same plane as the stars, the two suns would merge together in the sky about half the time to look like one star, so you’d only get one shadow.

Even for a close non-contact binary star there would be minimal shadow separation for a good fraction of the time - unless you were to glance at the suns (risking eye damage) or look at them at sunset you’d be hard pushed to notice much difference. Even at sunset the stars might be so close together on occasion that they would look like one object, perhaps with a rather strange shape.

Looking at the stills from the famous sunset (trivial to find on the net). The angular size of the suns is clearly intended to be much the same as ours - ie about half a degree. This puts the twin suns at about 1 degree of separation.

There is no chance at all that you would visually be able to perceive dual shadows cast by ordinary objects. Under contrived circumstances you would see the twins. Say a hole in a roof, it would cast two discs, or thin slits would cast double images. But ordinary shadows seen at ordinary scales? No chance.

For ballpark, your little fingertip at arm’s length is the same angular size as the Sun (or Moon).

At 11AU the Sun would be 1/11th the angular diameter we’re used to seeing. So it’d be about 1/11th that angular size. So about the size of a sesame seed at arms length, maybe a bit bigger.

At 36AU the Sun would be roughly 1/3rd that size. So kinda like a poppy seed at arm’s length.

Jupiter is 1/10th the absolute diameter of the Sun. And averages about 5 AU away from Earth. IOW, Jupiter seen from Earth is about 1/50th the angular size of the Sun seen from Earth.

So the Sun at 36AU would be roughly a third bigger than Jupiter looks to us. Which is a barely discernable disc. And much more discernibly a disc when seen from a vacuum than when looking through an atmosphere.

I read an APOD about that. They said the brightness of the Sun seen from Pluto would still be horrific; like looking at a welder’s arc. The total illumination would be dim, like twilight on Earth. But it’d be coming from this one painfully bright welder’s arc in the sky. Which would make very sharp and very black shadows.

Late edit: update the first sentence / paragraph as:
For ballpark, your little fingertip at arm’s length is [del]the same[/del] *double the * angular size as the Sun (or Moon). IOW the Sun or Moon appears half as wide as your little fingertip.

Kepler-16b is the first confirmed Tatooine-style circumbinary planet, orbiting both parts of a binary sun pair. The planet is a *bit *less hospitable than Tatooine, though.

Kepler has found 7 circumbinary planets so far
Kepler 16b
Kebler 34b
Kepler 47b,c,d
Kepler 413b
Kepler-451b
and there are others, not found by Kepler; the pulsar planet PSR B1620-26, the planet HD 202206c (where HD 202206b is a brown dwarf- does that count?) and possibly HW Virginis b (disputed).

There seems to be a ‘rule of three’ involved with binary planets; the closest a circumbinary planet can orbit the pair of stars is around three times their separation, while a planet can orbit a single star in a wide binary pair, so long as the other star is at least three times as far away as that planet is from the star it is orbiting.

Kepler16b is very close to the first case, whereas no confirmed planets fave been found that match the second case, although theoretical studies of the Alpha Centauri system have suggested that planets might be stable out to one third of the stellar separation there.

Or to see an illustration.