Binary Star Systems

[Space_Goose]

How do Binary Star systems work exactly? Being that they pretty common it seems likely that we will find a plenet orbiting one of them. What I don't understand is, would the planet orbit both stars like it was one big star or would it orbit just one of the stars? Also, have found any star systems that have more than two stars?

 

[MeteorWayne]

Planets can do it either way, depending on how far apart the stars are. There are tow stable position, either in a close orbit to one star, or far enough away that the planet would see the binary as a single mass.

Yes many systems are known with well more than two stars. I know of a few with 6 stars, but I suspect there are systems with more.

 

[SpaceTas]

The triple star systems and the multiple star systems follow a hierarchy. The 3rd star obits outside a close pair.
Multiple systems are a pair of pairs (4) or 1 star orbiting a pair of close double (5) or a third pair orbiting a pair of pairs. Working outward each pair is in tighter and tighter orbits.

 

[Saiph]

a system w/ more than 6 stars orbiting......a galaxy?

I haven't heard of anything with a tightly knit system w/ more than 6. It isn't impossible, just hard to find as such systems tend to eject members over time.

 

[MeteorWayne]

In checking around when this thread first came up, there is one possible septuple (7) system, but it not yet confirmed.

 

[3488]

Hi Wayne,

Coulld it be Mizar / Alcor system?

Article here is Mizar a seven star system?

Andrew Brown.

 

[MeteorWayne]

No that wasn't it; and BTW, they changed the article to sextuplet!

It is nu Scorpii, but as I said, it appears to be unconfirmed.

http://en.wikipedia.org/wiki/Nu_Scorpii

 

[SpaceTas]

A planet could orbit way outside a pair, or close in around a star. There is one other stable (just) orbit: a figure 8 shape round 2 stars (not sure; but I think the masses of the stars have to be about the same).

 

[neilsox]

Are Centauri A and B close enough to the same mass for the figure 8 orbit? It would make for interesting days, years and climate Daylight 70% of the time instead of less than 60%, average, the year around? I like it. I suppose habitable zone is low probability. Cooler is likely easier for colonists than hotter. Centauri C is likely far enough away not to make the figure 8 unstable? Neil

 

[Space_Goose]

According to Wikipedia, Alpha Centauri A and B are 11.2 AU apart, that’s about the distance from the Sun to Saturn. I am not sure how close the two stars would need to be in order to create a figure 8 orbit on a planet but the idea intrigues me. Using traditional planet finding methods, they have so far been unable to locate a planet of any kind in the Alpha Centauri system. Perhaps a planet in a figure 8 orbit could be the reason for that.

 

[neilsox]

I presume the closest approach would be to B with less mass = about 5 AU, so climate almost as cool as Saturn's moons, for a figure 8 orbit in the Centauri system. Dependent colonies would work, but intelligent life evolving there is very improbable. Neil

 

[eburacum45]

A general rule seems to be that planets could orbit one or other of a distant binary pair, at a distance of less than one third of the minimum distance between the stars. On the other hand close binaries can have a system of planets orbiting around both stars as if they were a single barycentre, but only if the orbit of the closest planet is wider than more than 3.5 x the distance between the stars.

Figure 8 orbits would be unstable so would be almost completely absent.

 

[Meeka]

New to this forum but very interested in binaries - in particular what /direction/ do the primary and secondary orbit? Both the same - i.e. clockwise [or anticlockwise] - or in opposite directions - i.e. one clockwise and the other anitclockwise?

If they do orbit in opposite directions what would dictate which partner orbited in which direction? Or is this just a random thing?

Finally, if both stars follow a slightly different eliptical orbit then how are their joint orbits calculated - i.e. do astronomers calculate the time it takes both stars to return to some designated starting position?

Apologies for the baby questions but I'm not an astronomer. Any help would be greatly appreciated!

 

[neilsox]

I think both stars typically orbit each other and rotate the same way as the planets orbit and our galaxy orbits. Exceptions are likely rare, but some tipping of the axis and orbital planes likely occurs. Mercury is tipped less than one degree with respect to it's orbit, but nearly everything else is tipped several degrees in our solar system. Venus rotates backwards, and Uranus has about 90 degrees axis tip with respect to the orbit of Uranus and Earth's orbital plane. Neil

 

[MeteorWayne]

I think both stars typically orbit each other and rotate the same way as the planets orbit and our galaxy orbits. Neil

Once again, sorry, but wrong. There is no preference for stars to orbit their barycenter or planets to orbit their star in the same direction as our system or the galaxy does.

 

[neilsox]

Eburacum typed, "close binaries can have a system of planets orbiting around both stars as if they were a single barycentre, but only if the orbit of the closest planet is wider than more than 3.5 x the distance between the stars."

If the stars are one AU apart, the diameter of the orbit can be 3.6 AU wide = radius 1.8 AU and circular. If the suns are the same mass and luminosity as our Sun, the the year is about twice as long as Earth's, but the closest approach (almost 1.3 AU) occurs once per binary year for A and once per binary year for B = hot season at 365 day intervals. The stars eclipse each other briefly near the beginning of each summer, assuming the plane of the orbit of the stars around each other is closely aligned with the orbital plane of the planet. At the beginning of winter the planet is 1.8 AU from the barycenter and thus about 2.2 AU from both stars. This will produce a rather large summer-winter temperature difference, but likely tolerable for humans over some portion of the planet, assuming Earth like atmosphere, oceans and air circulation. At the poles the suns will always appear (due to bending the image in the atmosphere) about 1/4 degree above the horizon assuming approximately zero axial tilt. Admittedly these are very tight specifications and the climate change over each binary year will be more severe as variations grow larger. Am I wrong that the cold weather distance to the barycenter will be less than 1.8 AU because the suns are not very close to a point source, thus making the orbit somewhat eliptical instead of circular = less severe winter-summer temperature variations? It will be daylight or twilight about 70% of the time, average, except at the poles, compared to about 60% for Earth average? Can the planet have a small moon orbiting in a circular orbit with a radius of about 50,000 kilometers? Can each star have a Mercury type panet orbiting at about 0.3 AU? I presume a Jupiter type planet closer than about 12 AU would make the Earth like planet unstable. Are my other conclusions correct? Neil

 

[Meeka]

Thanks for your input guys. Would be interested in knowing what the situation would be with a distant binary, e.g. like Alpha Centauri AB. Read recently that small, rocky planets could be in the habitable zone of Alpha Centauri B.

If there were a more or less earth-like planet around B then I'm assuming it would have a shorter 'year' as B is smaller than our sun? However from the viewpoint of this possible planet would there not also be cyclic epochs during which Alpha Centauri A was either much closer or much further away? When it was further away would the climate of the planet be appreciably cooler and vice versa would it be much warmer when Alpha Centauri A was at its closest? Would there be any other effects on the planet?

Finally, I want to make a simple gif animation of the orbits of a binary pair much like Alpha Centauri AB. From what I've read I'm guessing that Alpha centauri A would have a shorter orbit as it's more massive. Logically B would have a longer orbit. If both are travelling at the same speed then wouldn't B take longer to make one orbit? If yes then surely it would take a number [many?] of orbits for both to return to some arbitrary starting position?

Again apologies for the very non-technical nature of these assumptions and questions. Hope the meaning gets through.

 

[neilsox]

Since Centauri A has more mass, somewhat more distant planets are stable, but these are cooler than optimum. More distant means longer orbit and slightly longer year with faster orbital speed. A circular orbit with a radius of 1.2 AU would have a year of about 370 earth days. At closest approach = once per decade? perhaps B would make the planet 5 degrees c warmer for a few days.
A may have planets with circular orbits, less than one AU radius, but they are likely too hot.
Since B is cooler, about 0.8 AU is likely optimum radius.
A, at closest approach, would make the planet perhaps 10 degrees c warmer, for a few days. Please correct if I have some of this wrong. Neil

 

[Meeka]

Thanks Neil, this gives me a great base to build on. Let's say you were on a planet circling Alpha Centauri B. If you looked up at the sky during the lead up to the period of closest approach [of the two binaries] what would you see?

Any and all ideas gratefully accepted!

 

[neilsox]

If 11 AU is the closest approach rather than the present spacing, then A would look only slightly larger than Venus, but much brighter and with closer to pure white light than Sol, because A has a hotter photosphere temperature than Sol or B. Neil.

 

[Meeka]

Thanks Neil! I'm going to go away and try to model this now. If I get anything half way decent I'll post back. Cheers!

 

[csmyth3025]

I think both stars typically orbit each other and rotate the same way as the planets orbit and our galaxy orbits. Neil

Once again, sorry, but wrong. There is no preference for stars to orbit their barycenter or planets to orbit their star in the same direction as our system or the galaxy does.

Just to be clear, is the axis of rotation (and orbits) of multiple star systems and planetary systems in the Milky Way random with respect to the plane of the Milky way?

Chris

 

[MeteorWayne]

Yes as far as we know. Kepler may supply some interesting data on that point.

 

[SpaceTas]

So far no preferred orientation of star spin or planet orbit with respect to galactic plane.

MOST planets orbit in the same direction as the rotation of their host star. The exceptions (finding more all the time) are very interesting .... How do you get opposite rotation using the standard disk model for the formations stars+planets? That is a hot question now.