# Question about the Alpha Centauri System

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#### Space_Goose

##### Guest
I was just wondering, if we were to find a planet orbiting either Alpha Centauri A or B in the habitable zones. How bright would the other star be in the sky of the planet. I found the below picture on Wikipedia depicting a planet orbiting Alpha Centauri A.

http://en.wikipedia.org/wiki/Filelanet-alphacen1.png

Is this picture accurate? I have read that if a planet orbited Alpha Centauri A, Alpha Centauri B would be brighter in the sky than a full moon on earth.

Also, what if the planet orbited Alpha Centauri B instead? I assume in this scenerio, Alpha Centauri A would appear even brighter in the sky than in reverse.

I guess the simple way of asking this question is, would it be possible for a planet orbiting either Alpha Centauri A or B two have the appearance of having two suns?

Also, how bright would Proxima Centauri be from the same planet?

I am sorry if this question is not clear but it was hard for me to figure out how to ask it.

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#### GraemeH

##### Guest
Hi Space_Goose,

Hmmmm, I think there is a little bit of artistic license going on there. Have a look at the descriptive view here;

[url=http://en.wikipedia.org/wiki/Alpha_Centauri#View_from_a_hypothetical_planet]http://en.wikipedia.org/wiki/Alpha_Cent ... cal_planet[/url]

........ For example, an Earth-like planet about 1.25 Astronomical unit (AU) from α Cen A (with an orbital period of about one year three months or 1.3(4) a) would get Sun-like illumination from its primary. α Cen B would appear 5.7 to 8.6 magnitudes dimmer than the Sun at visual magnitudes −21.0 to −18.2, respectively, or 190 to 2700 times dimmer than α Cen A, but still 170 to 2300 times brighter than the full moon. Conversely, some similar Earth-like planet at 0.71 A.U. from α Cen B would receive significant illumination from α Cen A, which would shine 4.65 to 7.3 magnitudes dimmer than the Sun at visual magnitudes of −22.1 to −19.4, respectively. Similarly, α Cen B would be 70 to 840 times dimmer or some 520 to 6300 times brighter than the full moon.....

Proxima Centauri is a very dim red dwarf and any unaided observer on a terrestrial planet orbiting, either Cen A or B, would not be able to make out the Promixa disk by any means.

With a visual luminosity that has reportedly varied between 0.000053 and 0.00012 of Sol's (based on a distance of 4.22 light-years)the star is as much as 19,000 times fainter than the Sun, and so if it was placed at the location of our Sun from Earth, the disk of the star would barely be visible

From http://www.solstation.com/stars/alp-cent3.htm

From Alpha Centauri ........ The closest star would be the low luminosity red dwarf Proxima Centauri at 0.25 ly in distance, shining as an inconspicuous 4.5 magnitude star. Its slow and gradual movement against the background stars would be readily apparent over several decades.

Taken from http://en.wikipedia.org/wiki/Alpha_Centauri

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#### Space_Goose

##### Guest
Thanks GraemeH,

I think the answer to my question is indeed in there somewhere. I have read both of the websites you linked to the problem I am having is, I am not an astronomer. I am simply a person who is interested in the Subject and who is doing research. I have seen magnitude comparisons between our own sun and other stars and I have a grasp of the Magnitude system. But to me the Magnitude system is like Kilometers. I of course know that a Kilometer is 1000 meters. I also know that a Kilometer is shorter than a mile. But even though I know both of those things, if you were to tell me "The next town is 50 Kilometers". It would be hard for me to estimate about how far 50 kilometers was. All I would really know is that it would be shorter than 50 miles but in my mind, I would probably envision 50 kilometers being a lot further than it actually is.

That is the problem I am having with all the magnitude stuff.

But, it sounds like from the information you listed above, that the best scenerio for a planet in the Alpha Centauri System to have the apperence of having two suns, would be for it to orbit Alpha Centauri B and not A. Would you agree with that statement? It also appears that in this Scenerio, that Alpha Centauri A would create enough light on the world that it may have periods when it didn't get fully dark, but dim? I am just trying to create a picture in my head.

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#### neilsox

##### Guest
Hi spacegoose; I wondered also. I think you are correct; the two sun effect would be stronger orbiting Centauri B. Perhaps a three sun effect orbiting Centauri C = Proxima. A near circular orbit 5 million miles from Proxima, (likely tide locked) would have a huge red sun filling much of the sky, but barely enough heat. A and B would often be be behind Proxima. At night A and B would be brighter than Venus, but not much bigger apparent disk. Humans would see colors better by A light than by the red of proxima or orange-yellow of of B.
I don't think GraymeH meant to imply that C is closer than either A or B as viewed near A or B. Neil

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#### Space_Goose

##### Guest
Hi neilsox,

I had not considered the affects of the slightly more orangish sun. From what I have read though, Alpha Centauri B is only slightly more orange than our sun so would the affects be drastic, would a human on a planet orbiting Alpha Centauri B even notice that colors were more difficult to see?

I don't think GraymeH meant to imply that C is closer than either A or B as viewed near A or B. Neil
No No, I didn't think GraymeH implied that at all. I was just trying to say that I didn't understand the Magnitude description enough for the numbers to mean anything to me. I am trying to paint a picture of a certain world but as I said, I did not consider the fact that Alpha Centauri B is slightly more Orange than our sun. I guess I didn't think it would make any difference.

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#### neilsox

##### Guest
Yes some difference, but not as bad as typical blue, orange or yellow street lighting, or stage lighting, does to human color perception. Our brains tend to compensate surprisingly well. Neil

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#### Pendragon1965

##### Guest
On the subject of the Alpha Centauri system, I have a question as well. Why is it that of all the hundreds of exoplanets and systems we've checked, most of which are hundreds of lightyears distant, we haven't looked at the Centauri system? It IS the closest to us, and as we have seen in many cases so far, the old thought that a habitable planet wouldn't exist in such a dynamic system is probably false. Why hasn't there been an effort to search our nearest neighbor for worlds, since it would be the first system we would likely be able to visit as our technology advances?

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#### MeteorWayne

##### Guest
Plenty of efforts have been made. Your assumption we've never looked is incorrect. It's just that so far, there are no indications of any planetary masses.

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#### bdewoody

##### Guest
Would the fact that there are multiple stars in that system make it hard to detect the wobble induced by planetary bodies?

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#### MeteorWayne

##### Guest
It would make it harder, but not that much. If there were planets in the system, we should have sufficient resolution by now to find anything of substantial mass. Remember, it is the closest star system, so has really been looked at from the beginning.

from Wiki:

"All the observational studies have so far failed to find any evidence for brown dwarfs or gas giant planets.[68][69]

However, computer simulations show that a planet might have been able to form within a distance of 1.1 AU (160 Gm) of Alpha Centauri B and the orbit of that planet may remain stable for at least 250 million years.[70] Bodies around A would be able to orbit at slightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants around A and B make the likelihood of terrestrial planets greater than otherwise. Currently, technologies do not allow for terrestrial planets like Earth to be detected around Alpha Centauri (unless they are a few Earth masses and orbiting very close to the star, as in the Gliese 581 and HD 40307 systems, for example), but this is expected to change in the near future."

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#### bdewoody

##### Guest
I read the Wiki clip and that kind of answered my next question. Since there are multiple stars the likelyhood of there being gas giants should be reduced also increasing the chances of there being small rocky planets whose effect on the stars would be so minor as to be undectable by current technology.

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