# Why do binary systems spiral INTO each other?

alex
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Topic 190055

I can see how black holes would spiral into each other...
Ie.. you're orbiting an object which is sucking in matter, so it's getting heavier... which means your orbit gets reduced.

Why would other objects spiral into each other?
Things like stars are getting lighter over time, right? and the solar wind is pushing dust away, so they can't accumulate matter.

Chipper Q
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### Why do binary systems spiral INTO each other?

Quote:
Why would other objects spiral into each other?

Hi, alex – according to theory, mass in motion produces gravitational wave radiation, which requires energy that can only come from the binary system itself. So the compact masses spiral in towards each other, which in turn increases the orbital speed and further increases the amount of gravitational radiation emitted, and hence increases the inspiral rate even more. The gravitational waveform that is produced is referred to as a “chirp”, at a frequency in the band where the LIGO detectors are sensitive to it.

barkster
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### (correct me if I'm wrong,

(correct me if I'm wrong, Chipper, anyone)...

Alex,

You are correct that changes in mass (for whatever reason) in one or both bodies of a binary system will have an effect on orbital paths.

However, their are many possible changes to, and forces acting on, a body... and they will have an cumulative effect on the orbit(s).

But, if all other factors are left unchanged (ideal), relativity would demonstrate that two bodies orbiting each other emit gravity waves corresponding to their period of orbit. Gravity waves are energy, and so the energy of the binary system is being "lost" (radiated out) simply due to the fact that they are two masses orbiting each other. As the binary system loses energy from radiation, the orbits decay and the orbit period decreases. i.e. they spiral in.

Not a perfect analogy, but.... Think of an ice skater that pulls her arms in to spin faster... there is still friction between the skates and the ice, so no matter how hard she tries, she will lose energy and begin to slow down. So she pulls in even tighter to keep up the spin... and this cycle (lose energy, pull tight, spin fast) keeps going until she finally disappears.

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

MarkF
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### ChipperQ: Just one thing, it

ChipperQ:
Just one thing, it is the acceleration of a mass that generates gravity waves.

Chipper Q
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### Yes, barkster, the ice skater

Yes, barkster, the ice skater is probably the classic illustration of the way angular momentum is conserved.

It might help to simplify things by considering only compact masses, i.e., combinations of neutron stars and black holes (NS/NS, NS/BH, and BH/BH), and so other kinds of electromagnetic radiation and other factors will be far less significant in the calculations, if not negligible..?

Thanks, Mark, for pointing out that the motion of the mass is one of acceleration – mass moving in a straight line with uniform velocity won't generate GWs. The masses in a binary system are continuously accelerating in the direction of the system's center of mass.

alex
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### um.. ok. Thanks.

um.. ok.

Thanks.

alex
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### So.. gravity waves are a

So.. gravity waves are a result of relativitistic motion?... ie.. measurable/noticable if the objects are moving/accellerating quickly as in a significant fraction of the speed of light, as in a binary Neutron Star system.

Chipper Q
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### RE: So.. gravity waves are

Quote:
So.. gravity waves are a result of relativitistic motion?... ie.. measurable/noticable if the objects are moving/accellerating quickly as in a significant fraction of the speed of light, as in a binary Neutron Star system.

Hi, alex - GWs are the result of mass that has accelerated motion (e.g., a circular, or elliptical orbit). The mass doesn't necessarily have to be moving at relativistic velocity. There's a pretty good overview on the nature of GWs and how they may be detected here. Click through the 'posters' there, and when you get to posters 5 & 6 you'll see a comparison between electromagnetic waves and GWs – the EM waves are generated by an electrical charge that's being accelerated (back and forth), and GWs can be generated similarly, but it's simply a mass being accelerated in that case, and the mass merely has to accelerate (in some way) to create GWs. Note that it's the change in motion of a mass – the acceleration, and not the velocity; it doesn't matter how fast the velocity is, if the velocity isn't changing (accelerating).

meckano
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### RE: I can see how black

Quote:

I can see how black holes would spiral into each other...
Ie.. you're orbiting an object which is sucking in matter, so it's getting heavier... which means your orbit gets reduced.

Why would other objects spiral into each other?
Things like stars are getting lighter over time, right? and the solar wind is pushing dust away, so they can't accumulate matter.

Black holes don't really suck in matter like a vacuum cleaner does. The 2 masses of matter are attracted toward each other due to the presence of matter. (nobody can really point a finger at gravity yet from what I have read, it is just accepted that there is an attraction based on the ol' falling apple.) Both gain matter, both orbits are reduced.

ADDIT: They also gain momentum.

Other objects also have mass and are attracted toward each other. Stars would get lighter over time all alone just because they are giving off light, which is energy, which is matter in another state, known as solar wind. (there is more than light in solar wind.) The ratio of lost weight to total weight is minimal so the 2 stars are still attracted toward each other.

I'd love to see 2 stars 'at' each other though.

ADDIT 2:
A very circular orbit in binary systems would have to be an oddity. The standard should be very eliptical, right? I am curious to know if our solar system is part of an early-stage binary system.

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Chipper Q
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### RE: I am curious to know if

Quote:
I am curious to know if our solar system is part of an early-stage binary system.

Hi, meckano – back in '94 when comet Shoemaker-Levy 9 was about to collide with Jupiter, I wondered if that would be enough to spark two sunrises in one day...

There has to be about 75 times the mass of Jupiter for a core temperature to heat up enough to generate sunshine. Is there enough leftover mass, in the whole solar system, for another star to form?

I was also wondering about the opposite possibility - if this system used to be a binary system – since the Earth has lots of iron, especially in its core, would that imply a nova remnant fragment?

meckano
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### RE: Hi, meckano – back in

Message 18637 in response to message 18636

Quote:

Hi, meckano – back in '94 when comet Shoemaker-Levy 9 was about to collide with Jupiter, I wondered if that would be enough to spark two sunrises in one day...

There has to be about 75 times the mass of Jupiter for a core temperature to heat up enough to generate sunshine. Is there enough leftover mass, in the whole solar system, for another star to form?

I was also wondering about the opposite possibility - if this system used to be a binary system – since the Earth has lots of iron, especially in its core, would that imply a nova remnant fragment?

I really don't know. About the nova remnant, I think it blasts out too fast, but I really don't know.

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