Alpha Centauri physics (crosspost)
Jedidiah Whitten
Brian Reynolds (brey...@firaxis.com) wrote:
: (I've crossposted this because I'm not sure which group an answer is
: most likely to come from. Apologies if you read more than one group; I
: won't do it again).
:
: My question relates to the Alpha Centauri star system.
:
: Given a sun-like primary star (magnitude 0.33) and a smaller secondary
: (magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
: aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
: completing an orbit about every 80 years.
You're using the apparent magnitudes here, which are irrelevant if you're
talking about the physical properties of the stars. This is just how bright
they appear from Earth. You may want to use absolute magnitudes instead.
: Now, say we want to imagine a world of roughly Earthlike size and
: temperatures.
:
: * Can such a planet maintain a stable orbit around the primary, and
: would that orbit differ significantly from Earth's in shape and/or
: radius?
Yes, a planet at 1 AU would probably have a stable orbit. It may need to be a
little more than 1 AU from Alpha Cent A since that star is brighter than our
Sun, and a little less than 1 AU from Alpha Cent B, which is dimmer, if you
want the planet to receive the same amount of radiation. Both stars could
support an Earthlike planet.
: * Is the secondary a significant factor in terms of solar heat for this
: world? About how much difference in heat?
At minimum separation, the other star in the system would add about 1% to the
total radiation received. It would be significant, but not enough to preclude
the possibility of life, I think.
: * On its closest approaches, how much illumination will the secondary
: provide to the dark side of the planet?
It would be about 1% as bright as the Sun, or magnitude -22 or so at closest
approach. Much brighter than the Moon (-12 when full). The Sun is about -27.
I'm just working these figures out in my head, so they are not very precise.
At farthest separation, the secondary star will be about .02% as bright as the
Sun, which would be magnitude -16 or so.
Another important thing to consider is that although stable orbits exist around
each of the 2 stars, they are probably not stable enough to have allowed
planets to form. The other star would interfere with planet formation in the
same way Jupiter stopped a planet forming between it and Jupiter. So all that
might exist around these stars is an asteroid belt.
--
Jedidiah Whitten (jswh...@ucdavis.edu)
+------------------------------------------+
| University of California, Davis |
| http://wwwcsif.cs.ucdavis.edu/~whitten |
| http://wwp.mirabilis.com/6569964 |
+------------------------------------------+
Steven B. Harris
In <34CC1605...@firaxis.com> Brian Reynolds
<brey...@firaxis.com> writes:
>
>(I've crossposted this because I'm not sure which group an answer is
>most likely to come from. Apologies if you read more than one group; I
>won't do it again).
>
>My question relates to the Alpha Centauri star system.
>
>Given a sun-like primary star (magnitude 0.33) and a smaller secondary
>(magnitude 1.7) orbiting it w/ perihelion 11 astronomical units and
>aphelion of 35 astronomical units (e.g. approx. Saturn to Pluto)
>completing an orbit about every 80 years.
>
>temperatures.
>
>* Can such a planet maintain a stable orbit around the primary, and
>would that orbit differ significantly from Earth's in shape and/or
>radius?
No, you'd have a pretty Earthlike orbit about Alpha Centauri A,
with a distance of 1 AU, since this is a sister star to our own at G2
class.
Most computer problems looking at double stars find that planetary
orbits are stable around one star if the radius is less than about 1/5
of the distance between stars. That's influenced somewhat by the mass
of the stars, but with the much smaller and farther away Alpha Centauri
B, you'd bet essentially no problems. I'm willing to bet anything.
>
>* Is the secondary a significant factor in terms of solar heat for
this
>world? About how much difference in heat?
Nah. The ratio of luminosities for Alpha Centauri A and B is
about 1:4, and even at close approach you have 11 AU squared = 1/121 *
4 = about 1/500th. How can increasing the Sun's power by 1/500 make
any significant difference?
>
>* On its closest approaches, how much illumination will the secondary
>provide to the dark side of the planet?
Well, 1/500th of it. I don't know how that compares with a full
moon. Probably close.
>
>Thank you very much for your help. I am neither a physicist nor an
>astronomer, but rather a computer game designer. :-)
>
>If you can, please send me an e-mail of your reply as well as replying
>to the group.
>
>Brian Reynolds
>Firaxis Games
Frank Crary
In article <6ak0h4$a...@dfw-ixnews7.ix.netcom.com>,
Steven B. Harris <sbha...@ix.netcom.com> wrote:
>>* Is the secondary a significant factor in terms of solar heat for
>this
>>world? About how much difference in heat?
> Nah. The ratio of luminosities for Alpha Centauri A and B is
>about 1:4, and even at close approach you have 11 AU squared = 1/121 *
>4 = about 1/500th. How can increasing the Sun's power by 1/500 make
>any significant difference?
You might be surprised. Computer models of the Earth's climate suggest
that a 0.3% change in solar output would cause climate change. This
tends to come up when man-made versus natural climate change is debated.
(Solar variability seems to be under 0.3%, but it's close enough and
the models are uncertain enough, that the matter gets a fair amount of
attention.) I'd say a 0.2% change in heating due to Alpha Cent B
could cause some interesting weather/climate patters. But it certainly
isn't enough to make such a planet uninhabitable.
Frank Crary
CU Boulder
radiospace
On 28 Jan 1998 01:42:38 GMT, fcr...@rintintin.Colorado.EDU (Frank
Crary) scribbled with the virtual quill:
If Alpha Centauri B has an interesting but not radical effect on this
fictional planet's weather and climate, and it has an 80-year orbit
around AC-A, does this mean that our little planet would have an
80-year cycle of climate change? (Warmer temperatures, more rainfall?
Bigger storms?).
What if the situation was reversed and the fictional planet is
orbiting Alpha Cent. B? What would be the luminence and climate
effect from Alpha Cent. A? (Clearly it would be greater {4
times??]since it is brighter, but what of the effect of the planet
orbiting about 1.7 AU closer/further away as it moved around AC-B, in
addition to moving closer/further away in tandem with AC-B's orbit
around AC-A?). If this isn't too confusing (sorry) it would be
interesting to speculate what the weather and climate might be like on
such a planet. (I'm guessing significant yearly variation due to the
1.7 AU difference and a possibly traumatic 80 year cycle?)
Patrick
Frank Crary
In article <34d47837...@news.earthlink.net>,
radiospace <radio...@earthlink.netSPAM> wrote:
>>You might be surprised. Computer models of the Earth's climate suggest
>>that a 0.3% change in solar output would cause climate change. This
>>tends to come up when man-made versus natural climate change is debated.
>>(Solar variability seems to be under 0.3%, but it's close enough and
>>the models are uncertain enough, that the matter gets a fair amount of
>>attention.) I'd say a 0.2% change in heating due to Alpha Cent B
>>could cause some interesting weather/climate patters. But it certainly
>>isn't enough to make such a planet uninhabitable.
>If Alpha Centauri B has an interesting but not radical effect on this
>fictional planet's weather and climate, and it has an 80-year orbit
>around AC-A, does this mean that our little planet would have an
>80-year cycle of climate change?
Possibly. It wouldn't really be a 80-year cycle in the usual sense.
The orbit of Alpha Cent B is elliptical, so it would be reasonably close
to Alpha Cent A for perhaps a few years out of the 80 year period.
That could give you a few years of very unusual weather every 80 years.
Overall, this might or might not happen, and someone on that planet
might or might not notice. First, when climate modelers say a 0.3%
change in insolation could be significant, they mean that they turned
up the solar flux in their computer models by 0.3%, let them run,
and noticed a significant difference in the results. The models
could be wrong (in either direction), and it might take more than
a few years for a 0.3% difference to produce significant results.
As far as I know, no one has tried pulsing the extra heating on and
off. Second, the difference might not be noticeable over other
events. Here at the University of Colorado, we have a fairly large
climate modeling group. In the six years I've been here, they have
_always_ been talking about some unusual event causing unusual
weather: Only a few of those six years have been ``typical''. There
was a strong El Nino pattern in the first year or two of the 1990s,
and another one last year. There was a large volcanic eruption
in (I think) 1992, which supposedly was responsible for atypical
weather for a couple of years. So Alpha Cent B might cause unusual
weather on our hypothetical planet, for a few years out of every 80.
But there would be lots of other things causing unusual weather,
and it isn't clear if the effects of Alpha Cent B would be more
or less significant.
>What if the situation was reversed and the fictional planet is
>orbiting Alpha Cent. B? What would be the luminence and climate
>effect from Alpha Cent. A? (Clearly it would be greater {4
>times??]since it is brighter, but what of the effect of the planet
>orbiting about 1.7 AU closer/further away as it moved around AC-B, in
>addition to moving closer/further away in tandem with AC-B's orbit
>around AC-A?).
I'm afraid I don't have the orbital elements and luminosities handy...
But from what you have said, the effect on a planet around Alpha Cent B
would be stronger than the luminosity ratio implied. The larger
semi-major axis of the planet would mean that the closest approach
distance to Alpha Cent A would be smaller (i.e. than the closest
distance between a planet with the same average insolation around Alpha
Cent A and Alpha Cent B.) So the variability in insolation from the
second star would be greater.
Frank Crary
CU Boulder