Hi,

In normal binary stars there is one star in a smaller orbit than the parent one about a common center of mass. Both stars move around their orbits in the same direction.

Is it possible for a binary system to exist there one of the stars is travelling in the opposite direction along its orbit, or would this be too unstable and end up with both stars flying off in oposite directions f it were to happen?

Cheers,
~Al

Hmmm .... no idea. It is an interesting question though.

Based on nothing other than gut feeling I think the orbits would be very unstable and that the system would probably fly appart or perhaps that stars might even collide (that would be one hell of a show!)

Bart.

If I understand your question, then you normally can't have a binary system unless both stars rotate in the same direction. When I say normally, I mean where both stars have approx equal mass and the centre of gravity is located somewhere out in space rather than inside near the core of one of the stars.
A situation where what you propose could exist is where you have a massive difference in mass between the two stars. Say you have a really really big star with huge mass. Then, you have another small star which is moving in space, which may for example have been flung through space due to a nearby supernova. As this small star travels through space, it may be captured by the gravity of the really big star. Their common centre of gravity could be very close to the core of the large star. In this situation, the large star would not have it's movement/rotational properties adjusted much by the small star. But the small star is now in orbit around the large star thus forming a binary situation which you propose.
However, my guess is that this would not be the most common situation.
Any of this make sense?
More importantly, does this answer your question Al ?

Michael

I have had a bit more of tha thnink about it and I would say that the situation you describe could not really happen except if a star is captured becasue both stars will be formed from hte same gas cloud and so be rotating in the same direction. Just like all the planets around our sun go the same way.

Hi,

Thanks for the replies folks. I'd imagine a lone star capturing another would be a fairly rare event! Though its a possible scenario.

both stars will be formed from the same gas cloud and so be rotating in the same direction. Just like all the planets around our sun go the same way.

Probably, but say the stars formed in an open cluster of many stars. Then due to gravitational tugging some could end up going the wrong way. In the extreme example of a globular cluster the stars go all over the place.

I got thinking about this by the way from wondering about what would happen in the case of a large star accreting matter to the accretion disk of say a neutron star. When I tried imagining what would happen if the large star went around the other way, I couldnt see how the accretion disk would form at all, and so started wondering if such binaries could exist.

Thanks,
~Al

If the star was small enough wouldn't it become tidally loked as io around jupiter. similar idea or am i totally off the mark :idea:

Hi,
Is it possible for a binary system to exist there one of the stars is travelling in the opposite direction along its orbit, or would this be too unstable and end up with both stars flying off in oposite directions f it were to happen?Cheers,~Al

Al,

Interesting thoughts but in reality, I cannot see how this scenario could happen. Binary stars are subject to Kepler’s Laws and, as you say, have elliptical orbits around their common centre of mass.

In other words, the two stars will always be at opposite ends of a straight line through the centre of mass. This would not be possible if the stars were orbiting in opposite directions.

Gordon

Al

I have to agree with Gordon. In fact, if you consider two stars and a centre of mass, then the centre of mass is only a construct we use to provide a handy frame of reference. So ... if you get rid of the CM, you have one star orbiting another, and the direction is immaterial.

Now if you asked about a system of multiple stars, that would be more interesting!

Owen

I think I understand what you're saying Gordon. Just so as i understand this concept of centre of gravity completely, if you have two stars orbiting each other, the centre of gravity is proportional to their respective mass and distance between them; is that correct?

Now, if you move the stars apart so as to increase their seperation by a factor of two, does the centre of gravity move and by how much? (assuming say, that one star is twice the mass of the other)

Do two objects have to be in orbit about each other for there to be a centre of gravity? Is there a centre of gravity between our Sun and say Proxima Centauri?

Thanks,

Michael