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Sun <==> Alpha Centauri gravity interactions
Some texts (notably one or two books I vaguely recall of Patrick
Moore's) often show projections of *brightest* and *closest* stars in the several thousand years before and after the present time. I distinctly recall that Barnard's star is one that will become the closest star to the Sun in so many thousands of years from now at around 3.5(?) odd light years and then it will start receding away from us again after that time. Firstly, how does one go about making such future positional guesses and secondly, how long has Alpha Centauri been in close proximity to the Sun? Is there any projections as to how long Alpha Centauri will stay this close? Could it be that Alpha Centauri (A+B+C) and the Sun are gravitationally *locked* together and share a common proper motion around the galaxy? I know we observe distinct radial velocity and proper motions that Alpha Centauri has relative to the Sun, but they are based on short term measurements in the current era... and I don't expect you can simply *extrapolate* forwards/backwards in time simply on the basis of their present values... or can you? According to "Solstation" (my number one favourite site for local star system research!):- http://www.solstation.com/stars/alp-cent3.htm the next nearest large star system to have any *significant* interaction with both the Sun and Alpha Centauri(A+B+C) is Sirius (A+B) - which is quite far removed at 8.6 LY away from the Sun and 9.5 LY away from from Alpa Cen. This means the Sun and Alpha Centauri system are relatively isolated in space, where it is conceivable that bodies orbiting far out around each system are gravitationally perturbed in the manner in which I illustrate he- http://uk.geocities.com/aa_spaceagen....html#midrange Its important I think to study our nearest triple star system in greater depth (if only it rose above my horizon... but then I can't do a lot with my tiny 8-inch Newtonian!). How much Hubble or other space/ground-based telescope time is devoted to Alpha Centauri, compared to all other stellar astronomy, I wonder... cheers Abdul Ahad |
#2
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Abdul Ahad wrote:
Firstly, how does one go about making such future positional guesses and secondly, how long has Alpha Centauri been in close proximity to the Sun? Is there any projections as to how long Alpha Centauri will stay this close? The gravitational interactions between stars are pretty small. Unless one star is predicted to pass very close indeed--say, less than a tenth of a light-year or so--the impact on their orbit around the galactic center is minimal. Testament to this is the relative stability of the galactic disc. You can map the radial and proper velocities of the star in three dimensions and get reasonably accurate results into the fairly distant future (say, about a million years) for the nearest stars. Could it be that Alpha Centauri (A+B+C) and the Sun are gravitationally *locked* together and share a common proper motion around the galaxy? No. I forget the exact figures, but the relative velocity of the alpha Centauri system is considerably more than permitted for a locked system. I know we observe distinct radial velocity and proper motions that Alpha Centauri has relative to the Sun, but they are based on short term measurements in the current era... and I don't expect you can simply *extrapolate* forwards/backwards in time simply on the basis of their present values... or can you? You sure can, because the gravitational interactions between individual stars separated by light-years are quite weak. the next nearest large star system to have any *significant* interaction with both the Sun and Alpha Centauri(A+B+C) is Sirius (A+B) - which is quite far removed at 8.6 LY away from the Sun and 9.5 LY away from from Alpa Cen. This means the Sun and Alpha Centauri system are relatively isolated in space... How do you figure? The Sun and alpha Centauri are 4.3 light-years apart, or half the distance from here to Sirius. Since Sirius has a bit more than twice the mass of the Sun, and is more massive than the entire alpha Centauri system, I don't think you can consider the Sun and alpha Centauri much more isolated from Sirius than they are from each other. The essential paucity of gravitational interaction is why the "stellar rape" hypothesis of the solar system's formation had such a short life in the middle of the 20th century. It's just too darned unlikely for stars to pass that close to one another. To give you an idea of how far apart the stars are from one another, consider that the Sun and alpha Centauri A are about the same size (1.4 million km across) and are separated by about 42 *trillion* km. That is, the distance between them is about 30 million times their diameters. To put it another way, if the Sun is a golf ball in San Francisco, then alpha Centauri is another one (or two golf balls and a marble) in Los Angeles, and Sirius is a slightly larger racquetball, accompanied by an incredibly dense BB pellet, in Boise. Picture that for a moment, and ask yourself how likely it is that they will interact with one another. And remember, they move in three dimensions, not two. Its important I think... Steady, Brian, steady... Its important I think to study our nearest triple star system in greater depth (if only it rose above my horizon... but then I can't do a lot with my tiny 8-inch Newtonian!). How much Hubble or other space/ground-based telescope time is devoted to Alpha Centauri, compared to all other stellar astronomy, I wonder... What you require is accurate astrometry, and this has all been done previously--most recently by the Hipparcos mission. Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt |
#4
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All right this is an something that is bothering me...Every period, new
Epochs are published for star charts. What is it 50 years? I have a chart that was from 2000.0. If I remember correctly the obsolete one was 1950. So, I am assuming that this is a trend. Every 50 years, a new epoch. 1.) Is there some object within this galaxy that we can use as a relative point that moves minimally enough to use as a "relative object" to which we can attach our reference frame? Or is motion just measured relative to neighbors? Or is everything just placed by position on the celestial sphere on these newer editions(epochs)? 2.) Are measurements made in the past (far by our standard, short in other frameworks) good enough to include with our more precise instruments to form observations about motion? BP Space motion (plus galactic motion) is used to determine how close stars will get to each other. Of course you can extrapolate from present values. Astronomers do it all the time. Very close stars can only take thousands of years to reach closest approach. Most stars are much farther away and take much longer (if it occurs at all). There is plenty of proper motion data going back 50-100 years which is enough time to pin down the space motion of the nearer objects. |
#5
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AA Institute wrote:
Could it be that Alpha Centauri (A+B+C) and the Sun are gravitationally *locked* together and share a common proper motion around the galaxy? Brian's already answered this, but you don't have to take his word for it. Your own page has the formulas you need to estimate the strength of the gravitational interaction. The gravitational force f between the sun and Alpha Centauri (assuming I haven't goofed this up) is roughly f = G M m / r^2 G = 6.7 * 10^-11 N m^2 / kg^2 Newton's Gravitational Constant M = 2 * 10^30 kg mass of sun m = 4 * 10^30 kg mass of Alpha Centauri r = 4 * 10^16 m distance f = 3.4 * 10^17 N That looks like a lot, but it takes a lot of force to move a star. The acceleration of the sun due to f is a = f / M = 1.7 * 10^-13 m / s^2 = 0.00000000000017 m / s^2 Pretty small. If the sun were a car powered by the gravitational attraction of Alpha Centauri, it would go from 0 to 60 (miles per hour) in about 5 million years. t = v / a v = 60 mph = 27 m / s a = 1.7 * 10^-13 m / s^2 t = 27 / (1.7 * 10^-13) s Finally, if you substitute the Earth for Alpha Centauri, m = 6 * 10^24 kg f = 5.4 * 10^23 N you find that the force between them is a million times stronger. - Ernie http://home.comcast.net/~erniew |
#6
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Wasn't it AA Institute who wrote:
Could it be that Alpha Centauri (A+B+C) and the Sun are gravitationally *locked* together and share a common proper motion around the galaxy? To be gravitationally locked, their relative velocity would need to be less than the escape velocity of one from the other. A quick calculation shows the relevant escape velocity to be about 81 metres/second at this distance. The radial component of the relative velocity is about 26400 metres per second, so they're not gravitationally locked. -- Mike Williams Gentleman of Leisure |
#7
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#8
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BP wrote:
All right this is an something that is bothering me...Every period, new Epochs are published for star charts. What is it 50 years? I have a chart that was from 2000.0. If I remember correctly the obsolete one was 1950. So, I am assuming that this is a trend. Every 50 years, a new epoch. 1.) Is there some object within this galaxy that we can use as a relative point that moves minimally enough to use as a "relative object" to which we can attach our reference frame? Or is motion just measured relative to neighbors? Or is everything just placed by position on the celestial sphere on these newer editions(epochs)? The main reason for the changing Epochs is to account for precession of the earth's axis. The axis shifts by about an arcminute every year (plucked from memory, could be wrong). Because of this the coordinates of objects on the celestial sphere changes with time, by a small amount each year. DaveL 2.) Are measurements made in the past (far by our standard, short in other frameworks) good enough to include with our more precise instruments to form observations about motion? BP Space motion (plus galactic motion) is used to determine how close stars will get to each other. Of course you can extrapolate from present values. Astronomers do it all the time. Very close stars can only take thousands of years to reach closest approach. Most stars are much farther away and take much longer (if it occurs at all). There is plenty of proper motion data going back 50-100 years which is enough time to pin down the space motion of the nearer objects. |
#9
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Right, okay that is the 26k year cycle. So, I guess really a stars movement
would not really be noticeable on the scale of a star chart. I am referring to the Sky Atlas (Tirion,Sinnott).. Bp The main reason for the changing Epochs is to account for precession of the earth's axis. The axis shifts by about an arcminute every year (plucked from memory, could be wrong). Because of this the coordinates of objects on the celestial sphere changes with time, by a small amount each year. DaveL |
#10
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Abdul Ahad wrote:
So I would say, from a perspective of gravitational interactivity, the Sirius system is relatively *far removed*... but you have every right to disagree of course! And I do. I still think you are entranced by this idea of the Sun and alpha Centauri as being partners in space, when they very clearly are not. Since the gravitational influence of Sirius on alpha Centauri is just about half that of the Sun's, I wouldn't call that at all far removed. Further removed, yes, but somebody has to be closest. That doesn't say anything about whether we're bound to each other. And, as it happens, we are not. So if Alpha Centauri is not gravitationally *connected* with the Sun, are you saying that its just another passing star system? Yes. We've said that a number of times, now. Is there a 3D model available to show the projected distances separating Sun from Alpha Centauri on a time-series basis like this:- Epoch: Distance: ================================================== ============ Now - 1 million years ? Now - 500,000 years ? Now - 100,000 years ? Now 4.3 LY Now + 1 million years ? Now + 500,000 years ? Now + 100,000 years ? You can construct one very simply from available three-dimensional velocity data. From the Doppler shift, one gets the velocity along the line connecting us and alpha Centauri; from the proper motions, one gets the velocity in the plane perpendicular to that line. Combine that, and you can derive the values in the table above. What about extrasolar planet detection efforts around Alpha Centauri, to your knowledge? To my knowledge, such efforts have been unsuccessful in detecting planets around alpha Centauri A, B, or C. That doesn't mean that there aren't any planets around any of those stars--only that if there are, they are too small (or, just conceivably, in an orbital plane that is too close to perpendicular to our line of sight) to have been detected yet. Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt |
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