# The Skies of Alpha Centauri
**URL:** https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313
**Category:** Factual Questions
**Created:** [April 19, 2012, 7:55pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313 "2012-04-19T19:55:34Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Polycarp](https://boards.straightdope.com/u/Polycarp)
#### Post date: [April 19, 2012, 7:55pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/1 "2012-04-19T19:55:34Z")
Slightly hypothetical question with a factual answer here.
Contrary to the shorthand used for brevity in the thread title, what I’m presuming for our hypothetical question that is not demonstrable with current astronomical knowledge is the existence of an Earthlike world orbiting Alpha Centauri A at approximately 1 A.U. Feel free to adjust that approximation slightly to give a ‘shirtsleeves’ environment for this hypothetical planet. There may be other planets in the system, but nothing with a significant influence on our Earthlike world’s orbital dynamics.
Now, Alpha Centauri is a triple star. Alpha Cemtauri A is a G0 star marginally brighter than our own Sun (which is class G2). At approximately the distance from it that Uranus is from the Sun is Alpha Centauro B, an orange-red K star somewhat dimmer than ACentA or Sun, but still of some significance. At about a trillion miles (0.17 light year) from them is the dim M-class red dwarf Alpha Centauri C, called Proxima Centuri because at this point in its orbit it happens to be marginally closer to us than the other two. So these questions:
1. At its known mass and distance, does ACentB exert enough gravitational pull on our hypothetical Earthlike planet (HEP below) to pull it out of the almost-circular 1 AU orbit I presuppose around ACentA into a somewhat more elliptical one? If so, how much eccentricity is likely?
2. As a full-fledged star, albeit a relatively dim one, only about a billion miles distant, ACentB is going to be the second brightest object in HEP’s sky. How bright would it be? Would it show a disc at all, as opposed to point source? Would its insolation have any significant effect, other than light in the sky illuminating the night landscape, on HEP?
3. Would the casual naked eye observer detect Proxima at all? Or is it still too dim for naked eye vision at only a sixth of a light year?
4. Somebody (Asimov?) pointed out that our Sun would be a first magnitude star in HEP’s sky, the brightest star in Cassiopeia. Other than that an the fact that Centaurus is missing its brightest star, are there any other significant changes to the sky from the 4.3 LY shift from Earth to HEP?
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### Author:  [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)
#### Post date: [April 19, 2012, 8:26pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/2 "2012-04-19T20:26:33Z")
The Wikipedia page for [Alpha Centauri](http://en.wikipedia.org/wiki/File:Sky-from-alpha-centauri.jpg) has a sky map generated by Celestia that shows where the Sun would be. A little to the left of the W of Cassioipia and to the right of Capella. Sirius and Betelgeuse are positioned quite interestingly in AC’s sky.
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### Author:  [@chrisk](https://boards.straightdope.com/u/chrisk)
#### Post date: [April 19, 2012, 8:34pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/3 "2012-04-19T20:34:36Z")
> [@fiddlesticks](#):
>
> The Wikipedia page for [Alpha Centauri](http://en.wikipedia.org/wiki/File:Sky-from-alpha-centauri.jpg) has a sky map generated by Celestia that shows where the Sun would be. A little to the left of the W of Cassioipia and to the right of Capella. Sirius and Betelgeuse are positioned quite interestingly in AC’s sky.
That’s a fascinating addition to Orion, indeed! I presume that’s all because of Sirius’ movement relative to the motion of more distant stars - Betelgeuse is distant enough to not change based on the switch in observer points from here to Alpha.
I’m not sure that point 1 is easy to answer without a lot more speculation on how long the planetary system has been around and what other objects are present. I’d love to come up with answers to 2 and 3, but I don’t have time to work out the math just now. I’ll make sure to check in here later and see if there’s still work to be done.
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### Author:  [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)
#### Post date: [April 19, 2012, 8:50pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/4 "2012-04-19T20:50:11Z")
Undoubtedly Alpha Centaurians would have devised some sort of “twin” myth for those two stars (not realizing that their lining up like that was a coincidence with the redder of the two many times more distant).
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [April 19, 2012, 9:36pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/5 "2012-04-19T21:36:48Z")
Wiki also has a whole section on [the view from a hypothetical planet](http://en.wikipedia.org/wiki/Alpha_Centauri#View_from_a_hypothetical_planet).\
As for Proxima, [here’s a view from Earth](http://www.astrosurf.com/lorenzi/images/proxima.htm). It’s about 30 times closer to the rest of the alpha Cen system than it is to us, so it’d be about a thousand times brighter… But honestly, looking at that image, even making it a thousand times brighter, it’d still be practically nothing.
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### Author:  [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)
#### Post date: [April 19, 2012, 10:50pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/6 "2012-04-19T22:50:06Z")
> [@John\_DiFool](#):
>
> Undoubtedly Alpha Centaurians would have devised some sort of “twin” myth for those two stars …
They are (or will be?) [dreamers, poets, clowns, and straight-up madmen](http://www.youtube.com/watch?v=ORdz72GSNeA) . . . so I’d expect a cool mythos. 😉
CMC fnord!
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### Author:  [@K364](https://boards.straightdope.com/u/K364)
#### Post date: [April 20, 2012, 1:07am UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/7 "2012-04-20T01:07:34Z")
On this page: [http://en.wikipedia.org/wiki/Proxima\_Centauri](http://en.wikipedia.org/wiki/Proxima_Centauri)
“Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star.”
So, it may not even be noticed by inhabitants of a A Centauri planet. Although, in the same article it mentions that Proxima is a “flare star”… so maybe they would notice that.
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### Author:  [@MikeS](https://boards.straightdope.com/u/MikeS)
#### Post date: [April 20, 2012, 1:46am UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/8 "2012-04-20T01:46:28Z")
Concerning question 1, [this seems relevant:](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122c.html)
> [@](#):
>
> An excellent example in the lines of the Tatooine example is the nearby solar-like stars Alpha Centauri A and B. They orbit each other at an average distance of 23 AU, however the eccentricities of each orbit bring them to as close to 11 AU and as far as 35 AU. Numerical simulations by Paul Weigert at University of Toronto have shown that each star has a “safe zone” about 3 AU in radius in which planets could safely survive for billions of years. Objects placed further out from each star than about 3 AU are dynamically ejected in a matter of millions of years or less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a “habitable zone” exists around BOTH stars within the 3 AU dynamic “safe zone.” Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr… plenty of time for life to develop if the planets have the right conditions…
Here’s the [paper referred to](http://arxiv.org/abs/astro-ph/9609106). It doesn’t look like they included information about the hypothetical planets’ orbits, but since the size of the “stable zone” is small compared to the average distance to the other star (less than 20%), it seems unlikely that they’d be noticeably different from ellipses with alpha or beta Centauri at the focus.
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [April 20, 2012, 10:04am UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/9 "2012-04-20T10:04:00Z")
SolStation’s [page on Alpha Cent](http://solstation.com/stars/alp-cent3.htm) will probably be of interest. They calculate the habitable zone to be between 1.17 and 2.33 AU for A while B’s zone is between 0.56 and 1.10 AU.
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### Author:  [@Polycarp](https://boards.straightdope.com/u/Polycarp)
#### Post date: [April 20, 2012, 12:40pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/10 "2012-04-20T12:40:47Z")
I’ve been reading the wonderful and informative responses here with great ionterest. My pologies and thanks to the people who were courteous enough not to say “Next time check Wikipedia first” 🙂 – if I had had any idea how much of my hypothetical setup would be answered substantively there, I would have done just that.
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### Author:  [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)
#### Post date: [April 20, 2012, 1:07pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/11 "2012-04-20T13:07:46Z")
Perhaps, but then _ **I** _ wouldn’t have seen the answer to your question!
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### Author:  [@Bricker](https://boards.straightdope.com/u/Bricker)
#### Post date: [April 20, 2012, 3:49pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/12 "2012-04-20T15:49:01Z")
Maybe still fantasy – but does anyone know if there’s a simulation web page that would show you a night sky and day sky from a known world, or – even better – allow you to build your own solar system and then see how it would look from a planet surface?
I have always wondered how Saturn looks from Titan’s surface.
And let me just add a “Thanks, Proxmire!!” Because when I was a kid I was firmly convinced that when got to be the age I am now, I’d KNOW.
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### Author:  [@Lasciel](https://boards.straightdope.com/u/Lasciel)
#### Post date: [April 20, 2012, 3:57pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/13 "2012-04-20T15:57:46Z")
> [@Bricker](#):
>
> Maybe still fantasy – but does anyone know if there’s a simulation web page that would show you a night sky and day sky from a known world, or – even better – allow you to build your own solar system and then see how it would look from a planet surface?
>
> I have always wondered how Saturn looks from Titan’s surface.
>
> And let me just add a “Thanks, Proxmire!!” Because when I was a kid I was firmly convinced that when got to be the age I am now, I’d KNOW.
I know that Celestia will do the former, and I believe it either has, or is working on, developing sky maps of the extrasolars we’ve found so far.
I don’t know if it will do the latter, but if it did, that would be really fun to play with.
(I don’t have a copy myself - I play with it very rarely on a friend’s computer.)
[http://www.shatters.net/celestia/](http://www.shatters.net/celestia/)
I think StarStrider does the same, but I haven’t used it myself, so I don’t know for sure.
[http://www.starstrider.com/](http://www.starstrider.com/)
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### Author:  [@Foggy](https://boards.straightdope.com/u/Foggy)
#### Post date: [April 20, 2012, 4:35pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/14 "2012-04-20T16:35:18Z")
A bit of a hijack, but I read somewhere sometime ago that some scientists\* were thinking that Proxima might not be part of the Alpha Centauri system, that it was just passing by.
Is there any new info regarding this?
\*Could I be more vague? Why, yes I can.😃
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### Author:  [@MikeS](https://boards.straightdope.com/u/MikeS)
#### Post date: [April 20, 2012, 4:46pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/15 "2012-04-20T16:46:43Z")
> [@Foggy](#):
>
> A bit of a hijack, but I read somewhere sometime ago that some scientists\* were thinking that Proxima might not be part of the Alpha Centauri system, that it was just passing by.
>
> Is there any new info regarding this?
As of 2006, [the data was still inconclusive](http://arxiv.org/pdf/astro-ph/0607401v1.pdf) (PDF). The uncertainty in the measurements of the three velocities of the stars translated into a 44% chance that Proxima was “bound” to the other two stars, and a 56% chance that it was “just passing through”. Better measurements of Proxima’s radial velocity (the speed it’s moving along our line of sight) are needed to nail down whether it’s gravitationally bound or not.
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### Author:  [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)
#### Post date: [April 20, 2012, 6:56pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/16 "2012-04-20T18:56:05Z")
> [@Bricker](#):
>
> Maybe still fantasy – but does anyone know if there’s a simulation web page that would show you a night sky and day sky from a known world, or – even better – allow you to build your own solar system and then see how it would look from a planet surface?
>
> I have always wondered how Saturn looks from Titan’s surface.
Check out the previous thread: [What would the night sky look like from another planet?](http://boards.straightdope.com/sdmb/showthread.php?t=646251) and the Stellarium program referred to in post #2.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [April 20, 2012, 8:29pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/17 "2012-04-20T20:29:46Z")
Excellent find, **MikeS**! That’s the sort of question that can really only be solved by numerical modeling, and I hadn’t been aware that anyone had actually done it.
> [@](#):
>
> Better measurements of Proxima’s radial velocity (the speed it’s moving along our line of sight) are needed to nail down whether it’s gravitationally bound or not.
Are you sure you didn’t mean transverse velocity, there? Radial velocity is easy to measure, via Doppler shift. With a star that close, transverse velocity can also be measured from proper motion, but I’d expect the error bars on the transverse components to be much larger.
And Proxima being a flare star probably wouldn’t be enough to make it noticeable to the naked eye (or indeed, to any instrument within an atmosphere) since most of the energy from flares is in the X-ray range.
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### Author:  [@MikeS](https://boards.straightdope.com/u/MikeS)
#### Post date: [April 20, 2012, 9:31pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/18 "2012-04-20T21:31:25Z")
> [@Chronos](#):
>
> Are you sure you didn’t mean transverse velocity, there? Radial velocity is easy to measure, via Doppler shift. With a star that close, transverse velocity can also be measured from proper motion, but I’d expect the error bars on the transverse components to be much larger.
Surprisingly, it’s the other way around for Proxima Centauri. The proper motion is known to within about one part in 1000, while the radial velocity is only known to within about one part in 100. (See table 1 of the linked paper.) The largest source of error in our knowledge of the transverse velocity appears to be in the parallax measurement, which is closer to 1 part in 300.
In any event, the argument in the paper is that the slop in the radial velocity measurements is the main reason that the total energy of the Proxima Centauri has an error bar that crosses zero (i.e., bound vs. unbound), and that nailing this speed down to a higher degree would be the easiest way to discover whether it was unbound or not.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [April 20, 2012, 9:35pm UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/19 "2012-04-20T21:35:54Z")
Huh, I did not know that.
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### Author:  [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)
#### Post date: [April 21, 2012, 1:35am UTC](https://boards.straightdope.com/t/the-skies-of-alpha-centauri/619313/20 "2012-04-21T01:35:48Z")
> [@Bricker](#):
>
> Maybe still fantasy – but does anyone know if there’s a simulation web page that would show you a night sky and day sky from a known world, or – even better – allow you to build your own solar system and then see how it would look from a planet surface?
>
> I have always wondered how Saturn looks from Titan’s surface.
>
> And let me just add a “Thanks, Proxmire!!” Because when I was a kid I was firmly convinced that when got to be the age I am now, I’d KNOW.
Go to [http://www.stellarium.org](http://www.stellarium.org).
Your wish will come true.
But you will have to do some work. Once the program is up and running you need to “go to” Titan. Remove the ground if necessary and scan around.
This will make sense once you install the (free) program and read the directions.
I put this program on every computer that I own or have access to.