# The Sun's twin star?

1. Apr 29, 2005

### hexhunter

The Sun's twin star???

a book i'm reading now, 'the history of nearly everything' by bill bryson says that some cosmologists, or whatever they are called, reckon that as most solar systems have dual stars, jupiter might be a star which is not ionic or on fire, or whatever...

is this a common thought, or is it rare. what do you think????

2. Apr 29, 2005

### SpaceTiger

Staff Emeritus
Basically, the question here is whether Jupiter formed from aggregation of material in the protoplanetary disk or gravitational collapse (like the sun). There are still some mainstream theories advocating the gravitational collapse of Jupiter, but aggregation is definitely favored at the moment.

3. Apr 29, 2005

### chroot

Staff Emeritus
Jupiter has the same chemical composition as the Sun; it is thus, in a way of speaking, a miniature companion "star." On the other hand, it has only about a thousandth of the mass it would need to actually ignite nuclear fusion and become an actual star.

- Warren

4. Apr 29, 2005

### hexhunter

but in dual-star systems one of the stars has a greater mass, right, this orbits the larger one, and am i wrong in saying that the stars swap mass, as to change which star orbits and which star has the most mass???

also, would it be possible for a liquid body out at space? we have gas giants, rocky planets and stars, so why no liquid ones, understandablyif it is too hot we'd get a gassey planet and if too cold we would get an icy planet, is it just never the right conditions and temperature???

Last edited: Apr 29, 2005
5. Apr 29, 2005

### chroot

Staff Emeritus
Binary systems with any combination of masses are possible. Stars rarely exchange matter unless they are extremely close together, and one is of much larger radius (not necessarily mass) than the other.

Both stars in a binary system orbit the common center-of-mass. Neither star orbits the other.
A liquid surrounded by vacuum would have to have an enormous surface tension to remain liquid. Liquids generally vaporize under such conditions.

It is possible that Jupiter contains a core of liquid hydrogen, surrounded by a thick envelope of gas. It is not possible for a sphere of liquid, unattended by such an envelope, to remain liquid.

- Warren

6. Apr 29, 2005

### hexhunter

thanks about that, i seem to have shifted my interests from sub-atomic particles to space, anyway, few more questions:

1) what is in the centre of the Milky Way that our Sun orbits???

2) are there any good sites or video's on the net which can help explain the topic to me in general???

thanks

7. Apr 29, 2005

### chroot

Staff Emeritus
There doesn't have to be a specific object at the center. All objects orbit the common center-of-mass. It happens that the center-of-mass of the galaxy is there, even if the space there were actually empty. The space is not empty, however; it appears there is actually a supermassive black hole.

- Warren

8. Apr 29, 2005

### tony873004

I think its more like a hundreth the mass. Jupiter is about 1/1000 the mass of the Sun, but a star only needs about 8% of the mass of the sun to burn H.

I think there does. This is just my own thinking, not something I read somewhere, but playing around using n-body, I could never get a system to hold together unless one object was much more massive than the rest of the objects. I don't mean combined. Just that the most massive objects needs to be something like 10x as massive as the 2nd most massive object.

I imagine that this "one object" could be a binary object, which would leave room in the middle. But if another object ever gravitationally perturbed the binary and split it apart, the whole system would evaporate away.

Maybe there's a perfect spacing that permits a group of objects to orbit their common center of mass, without a massive primary member, but I haven't found it yet. If my thinking is correct, Globular Clusters probably have a massive black hole in them. But I've never read that anywhere. It just doesn't make sense to me how something like Globular Clusters could have the ages they have without a massive primary member holding them together. I get the impression that the stars are just randomly wandering about the cluster in their orbits. But maybe there is some order to it?

9. Apr 29, 2005

### JamesU

Isn't there a supermassive black hole?

10. Apr 29, 2005

### chroot

Staff Emeritus
Thank you for the correction; I confused my ratios. :D
With stability issues considered, you're probably right -- there must be something in the center to make the system stable. I only belabored the point because it seemed that hexhunter was falling prey to the fallacy that "small objects orbit big objects," when in fact both objects actually orbit the center-of-mass.

- Warren

11. Apr 29, 2005

### SpaceTiger

Staff Emeritus
None of these systems are stable for indefinite periods of time, including globular clusters, but the timescales for instability vary. Generally, we look at the "relaxation time" to determine how long it will take for a gravitationally bound system to change in a significant way. One parameterization of this is:

$$t_{relax}=0.34\frac{\sigma^3}{G^2m\rho ln\ \Lambda}$$

where $$\sigma$$ is the velocity dispersion, m is the mass of the individual stellar components, $$\rho$$ is the stellar mass density, and $$ln\ \Lambda$$ is the Coulomb logarithm (order unity).

Globular clusters have relaxation times ranging from 108 to 1010 years and have been modelled to undergo core collapse in a few hundred relaxation times. From this, can you figure out why we don't see globular clusters with relaxation times less than about 108 years?

For comparison, the relaxation time of the Milky Way is something like 1019 years!

12. Apr 29, 2005

### DaveC426913

Well, Pluto and Charon would have issue with that. Depending on who you ask, Charon is somewhere around 1/10th to 1/5th of its parent's mass.

Their common centre of rotation is somewhere in the intervening space between them.

13. Apr 30, 2005

### SpaceTiger

Staff Emeritus
I think he was implicitly assuming more than two bodies. Ideal two-body systems are completely stable, but things get troublesome when you have any more than that. Tony's statement probably isn't too far from true in these cases. Given that they're still exploring the stability of our solar system, I don't think anybody knows for sure.

14. Apr 30, 2005

### Chronos

Good observation, ST. I think the supermassive BH at the center of MW answers some of those questions. My guess is it is even more massive than accused. And it appears possible there may be more than one of them in that neighborhood.

15. Apr 30, 2005

### SpaceTiger

Staff Emeritus
Why do you say that? I thought the mass determination was pretty solid.

16. Apr 30, 2005

### turbo

What will a globular cluster look like when it has undergone core collapse? Do we currently see such animals (remnants from those clusters that might have collapsed on the 108 time scale?)

17. Apr 30, 2005

### hexhunter

so the centre of gravity is between the two bodies, closest to the denser of the two or more bodies...

so is this why the sun moves, does this mean that earth gets pulled around slightly by the moon, and does the earth actually orbit around the sun between it and the moon???

18. Apr 30, 2005

### SpaceTiger

Staff Emeritus
Here is a nice paper on the subject.

Really, things are a bit more complicated than I said. In addition to core collapse, you have a process known as evaporation occurring on similar timescales. This is basically analogous to evaporation from an atmosphere, in which the high-velocity tail of the Maxwell-Boltzmann distribution is able to escape from the system.

Also, there is mass segregation, a process that tends to separate the more massive stars from the less massive stars. Why does that happen? Well, the clusters are trying to establish equapartition of energy (same basic deal as in a gas), so the massive stars will tend to end up with smaller velocities and sink towards the core. This can be observed, but it is difficult in practice.

There are many papers on these processes in ads and astro-ph, so I suggest a search if you're interested.

19. Apr 30, 2005

### SpaceTiger

Staff Emeritus
It's closer to the more massive of the two.

The sun moves about the galactic center due to the combined gravitational field of many stars, dark matter, gas, etc. The sun also wobbles a little bit as a result of gravitational interactions with the planets in the solar system. This is the wobble that many extrasolar planet searches look for.

Three-body interactions are a bit more complicated, but the earth does wobble in its orbit as a result of interactions with the moon.

20. Apr 30, 2005

### turbo

Complicated indeed! A search of CiteBase turned up numerous papers including this one:

http://citebase.eprints.org/cgi-bin/citations?id=oai%3AarXiv%2Eorg%3Aastro%2Dph%2F0406227 [Broken]

Many of the papers I found said that the presence of a large binary at the core of a dense cluster could provide the dynamics to disperse stars and prevent further collapse. This paper says that a large enough black hole can do the same thing. Maybe the figure in the paper you cited (20% of observed clusters exhibit collapsed cores) is indicative of the small (1 in 5) number of clusters that have not come to some sort of dynamical equilibrium, in which the perturbations caused by the BH or binary at the core balance the tendency toward collapse?

Intuitively, a tendency toward core collapse will accelerate the formation of binaries and/or increase the mass of a central BH in the core, which will then provide the perturbative effects that disperse the core stars - kind of a feedback mechanism to keep the cluster stable against collapse.

Last edited by a moderator: May 2, 2017