Proxima Centauri (don't they orbit?)

Factual Questions
1

I’m sure you’ve heard assertions like the following:

“Alpha Centauri is actually a triple star system. The third star, Alpha Centauri C, is actually the nearest individual star to us, and is also known as Proxima Centauri”.

That makes it sound astonishing permanent for an arrangement that must always be in flux if the three stars constituting the Alpha Centauri system are orbiting one another! Is the space between the three so large that it makes sense to say “is”, or will Alpha Centauri A or B move closer than C within a few hundred years or so? I don’t recall seeing a statement of orbital period or a date upon which C will swing farther away or one of the other two swing closer in. But surely there’s no way for one object in a triple system to always be the closest to a fourth object outside that system! (right?)

Disable Similes in this Post

2

I don’t know the details but I think it is possible for Proxima Centauri to have been the closest star to us throughout recorded history. The orbital period may be measured in tens of thousands of years and it may have spent all of recent history on our side of Alpha Centauri.

However, rather than guessing I’m sure there must be some astronomy type out there who has the Straight Dope. If not, I’ll see if I can find anything surfing the net.

If all you have is a hammer, everything looks like a nail.

3

I don’t know the details but I think it is possible for Proxima Centauri to have been the closest star to us throughout recorded history. The orbital period may be measured in tens of thousands of years and it may have spent all of recent history on our side of Alpha Centauri.

However, rather than guessing I’m sure there must be some astronomy type out there who has the Straight Dope. If not, I’ll see if I can find anything surfing the net.

If all you have is a hammer, everything looks like a nail.

4

From http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970717b.html :

–
TMR

5

I popped your question into a search engine and came up with [url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970717b.html] this … Looks like two ten year olds beat you to that question :slight_smile:

Anyway, I should add that the celestial mechanics of three body systems are almost a complete mystery to scientists (don’t even think about quadruples). The interactions are just a bit too complicated for us right now, plus we don’t have much observational data to work on either. If you read that article above, you’ll see that we don’t really know if Alpha Centauri C forms a stable triple star system with Alpha Centauri A and B; it could just as well be a rogue star passing in the vicinity after all.

For a long time, only two orbital arrangements of three body systems were generally accepted as being stable. The first is where the two most massive bodies form a binary system, and the third one orbits in the shape of an 8 as it circles around the other two. The second arrangement is basically the same as the first, except the smaller body goes into a circular orbit around the larger binary stars at a much large distance. There have been some promising new propositions in the last few years, but they’re all a little odd to say the least. I once read about one where 12-17 bodies would all follow a circular orbit like a merry-go-round, while 2-3 bodies would twirl up in the center. The computer animation looked pretty all right, but it just seemed a tad too neat to be true. I guess we’ll have to wait and see what surprises nature has in store for us :slight_smile:

6

Two minutes, damnit! Two minutes! And I messed up the darn UBB code again too :frowning:

7

According to “Measuring the Universe”, Kitty Ferguson, Walker & Co., 1999, pp. 142-143, the orbital period of Proxima Centauri around Alpha Centuri A and B is 500,000 years.

8

That’s true in a strictly mathematical way, but you can make practically stable systems. One example is where the distance between two objects is much smaller than the distance to the third one, like the Sun-Earth-Moon system. The (earth+moon)-sun system can be approximated as a two-body problem, as is the earth-moon system. The Alpha Centauri system is probably like this too. Another example is where one body is much less massive than the other two, like a satellite in the Lagrange point (a point where the gravity from the Earth and Moon balance).

As for the recent developments - there was an article on Nature a couple of years ago which claimed that a string of numerous masses travelling in any closed curve is a solution to the N-body problem. (G. Buck, 1998, Nature 395:51) Is that what you were thinking about?

9

Hrm… that sounds like the right article, ** scr4 **. Did you actually look that up? I’m too lazy sometimes :stuck_out_tongue:

Anyway, Alpha Centauri A and B orbit each other at a distance of 23 AU, while C is a comfortable 13,000 AU from the two of them. Maybe this is how large the separation has to be in order for the orbits to be stable, but then again, 13,000 AU is a long ways off and that’s why we aren’t really sure if it is a true three body system. Just a wild guess, but I don’t think we have enough data points to accurately calculate the orbit of Alpha Centauri C yet. That 500,000 year figure mentioned above is probably just our best estimate. Besides, I can’t tell if that figure assumes a stable orbit or not. You can still assign a period to Alpha Centauri C even if it’s in the process of being ejected from the system; it’s supposed to spiral out anyway…

I suppose the bottom line is this: we just don’t know :slight_smile: If Alpha Centauri C really forms a triple star system with A and B, then yes, some day it’ll be farther away from us than A and B, but it’s not going to happen in a few hundred years. If C doesn’t form a triple star system with A and B, then we’ll have to see which direction it’ll be ejected from the system, and maybe it will move farther away from us during its travels…

10

One thing I want to point out is, Alpha Centauri B is not Beta Centauri. It is Alpha Centauri B. Nobody has made that mistake on this thread, but I’ve heard people make that mistake before and it annoyed me. Well, it annoyed me that they didn’t listen to my patient explanation.

Correct me if I’m wrong, but each Greek letter is assigned to a visible constellation, no matter how many stars are it consists of. Roman letters are assigned to individual stars in that system. Numbers are assigned (theoretically) to planets.

Okay, that’s it for my little hijack. Carry on.

11

I just looked at the report I did on that article last year. Which is not to say I understood it - the math was easy enough to follow, but the approximations didn’t look very convincing. Still, if the Nature referee(s) thought they were valid, I guess that’s good enough for me… I’m just an experimentalist.

12

No no no. Greek letters are assigned to visible points in a constellation. Not to the whole dang constellation. A one-armed man hit “Submit Reply” before I could profread.

13

A minor point:

Proxima was almost certainly not formed at the same time and place as the A/B pair. Their metalllicity (percentage of elements heavier than helium, as inferred by spectroscopy) implies that they are somewhat older than the Sun, about 6 eons (1 eon = 1,000,000,000 years). Proxima, OTOH, is a flare star, a phase of instability that occurs in young class M (red) dwarfs, and is therefore though to be “only” about 4 eons old.

Whether the A/B pair has genuinely “captured” Proxima, whether it has done so, but so feebly that a passing star might wrench it from their grasp, or where Proxima is just cruising by in a trajectory that makes it appear to be gravitationally bound to the main pair, are questions that are disputed every few years, and for which there is no final answer at this time.

“I don’t just want you to feel envy. I want you to suffer, I want you to bleed, I want you to die a little bit each day. And I want you to thank me for it.” – What “Let’s just be friends” really means

14

No.

There are five Lagrangian points. All require that AA have a significantly greater mass than B, and that B have a significantly greater mass than c.

AA B c

Here, the extra gravity from B makes c orbit around AA faster than it would by itself, so that it keeps pace with B. This configuration is stable only as long as it is perfectly undisturbed.

AA c B

Here, the extra gravity from B makes c orbit around AA slower than it would by itself, so that it keeps pace with B. This configuration, too, will not repair itself.

c AA B

As long as c and B are in perfect opposition, this is stable.

These three positions are only hypothetical, in that they cannot be expected to occur naturally (though they can and do occur artificially). The following two positions, however, do occur in nature.
c

AA B

 d

c (in B’s orbit around AA, 60 degrees ahead) and d (in B’s orbit around AA, 60 degrees behind) are stable. If disturbed, they will tend to fall back into place. There are a great many asteroids occupying these positions with respect to the Sun and Jupiter (the so-called “Trojan” asteroids), and there is some evidence that there are dust clouds occupying these positions with respect to the Earth and the Moon.

I should add that Zor’s remark:

should be emended slightly. They are not a “mystery to scientists”; the equations have been well known since Newton’s time. However, as equations, they have never been generally solved for systems of more than two bodies. That doesn’t mean that they can’t be solved by arithmetic and brute force. Furthermore, Lagrange did manage to work out five real, but partial, solutions, and that is why the Lagrangian points bear his name.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

15

Close, but no cigar: “Visible only from latitudes south of about 25 degrees N, the star we call Alpha Centauri lies 4.35 light-years from the Sun.” More correctly, Alpha Centauri has a declination very near -60 degrees, so it is visible from latitudes at +30 or less.

Regarding the Alpha Centauri star system: The three stars orbit about a common center of gravity. The change in Proxima’s position relative to earth is negligible in comparison to the light years between earth and the three body system. Thus, it is still safe to say that Proxima is closest star to the earth (excluding the Sun.)

Heck, if you want to start splitting hairs, then Sirius B is technically closer to earth than Sirius A. However, it is accepted that Sirius is the second closest star to the earth (excluding the Sun).

If you can find Alpha Centauri in your skies, it is worth noting that Alpha and Beta Centauri have apparent close approximation, but these two stars should not be confused with the Alpha Centauri star system.

Happy Skywatching!

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