Binary star

  • Thread starter wolram
  • Start date
  • #1
wolram
Gold Member
4,267
557

Main Question or Discussion Point

how can (contact binary stars) exist? why is it that one does not destroy the other? where is there center of gravity?
 

Answers and Replies

  • #2
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
Originally posted by wolram
how can (contact binary stars) exist? why is it that one does not destroy the other? where is there center of gravity?
Labguy would be a good person to give a detailed reply, or any of several others Phobos etc

I will just skim the foam off this one---nice question.

Indeed close binary pairs do compromise each other's integrity.
Why shouldnt they.

Between the earth and moon there is a zero-gravity point
and likewise between two stars.
And one star can expand in a red giant stage to where its outer envelope of ionized gas touches this zero gravity point

and the expanded star can begin to flow material over to the more compact star.

Its great! Type Ia supernovas theoretically can occur this way---at the small compact partner which is receiving matter from its giant partner.

And ordinary novas too----Labguy has discussed this at least once so let him confirm or correct.

Stars even merge! A certain kind of merger is believed responsible for at least some of the observed gamma bursts.

When a red giant is feeding matter to a white dwarf partner the giant is distorted by the common gravitational field. It is no longer spherical.

At some point it may be better not to think of a binary system as two separate stars but as a collective distribution of mass that has two maxima. The center of mass is between the two and is found the same way with any other distribution of mass----roughly speaking a kind of average.

Maybe others will add detail. Always nice questions, wolram---combination of simple and evocative---always seem to lead somewhere. Good at it.
 
  • #3
Labguy
Science Advisor
731
7
Originally posted by wolram
how can (contact binary stars) exist? why is it that one does not destroy the other? where is there center of gravity?

Marcus covered most of the bases, but on some sites, and books, you will get more detail about when binary stars can and can't exchange material. As with a Type Ia supernova, Marcus explained that material is "transferred" from a (usually) red giant to a white dwarf. When that dwarf passes its Chandrasekhar mass (we don't use the word "limit" much anymore.. ), it can become a Type Ia or Type Ib supernova, depending on chemical composition. Most white dwarfs would ignite the outer layers by fusion and become a "regular" nova, not a supernova.

BUT, that is not what you asked. You asked about contact binary stars, which takes in a few more factors. For any mass transfer, one or both stars in a binary system must expand its outer layers (atmosphere) beyond its Roche Lobe limit. There are millions of binary stars that cannot do this due to orbital distance and mass. The Roche Lobe of a star is dependant on the mass and orbital distance of both stars. It is the limit (distance) from either star, where the star's atmosphere has extended/swolen to where the gravitational pull of the companion has as much effect on the gasses as does the expanding star. Basically, it is just the place where the "gravity-point" effecting the gasses (either star) is equal. I am sure that you have seen some of the "artist's conceptions" showing a large, egg shaped star spilling matter in a stream to the companion. This will happen only in close binary systems. Also, if the orbit of either star is too elliptical (eccentric), this transfer effect will not happen at all, or only happen sporadically when the stars are at the closest point in orbit. This last description is what sometimes happens in a nova that repeats in regular intervals. Quite a few stars are known to go nova at repeated and equal time intervals.

A mass transfer as Marcus described for a Type Ia (or b) supernova only involves mass transfer in one direction; from the giant to the white dwarf. The red giant's mass/size has reached the point where the gravity of the dwarf pulls matter through the Roche Lobe point onto the dwarf. All stars have a "Roche Limit", but transfer of matter through the Roche Lobe will only happen in binary star systems. Also, this "point of transfer" can only happen at the large star's L1, or inner Lagrangian point. This method of mass transfer occurs in many binary systems, but in common-usage terms, this still does not fit the description of contact binaries. In "contact binaries", both stars must have "atmospheres", which rules out any type of stellar remnant, or core, such as a white dwarf. Contact binaries are usually described as a situation where (a) the orbits are very close, therefore short period orbits and (b) where both stars have extended atmospheres beyond their Roche Lobes (limits), and material from both stars is intermixed. The center of gravity of such a system would be a common center of mass and would be no different than one calculated for any two orbiting bodies, This process will continue and increase until the stars eventually merge into one, larger star, very mixed up and unstable if the combined mass is high. In this case, sometimes the new, larger star will complete the evolution as a single star and may have enough mass to become a Type II supernova, not Type I.

Of course, now that I have typed this all out, you will find out several other (special) cases where stellar remnants, like neutron stars, can also qualify as contact binaries. It is the "merger" (collision?) of neutron stars, and maybe occassional black holes, that are thought to be the source of the huge energy release of gamma ray bursts. Hope some of this helped, but there are always exceptions to the rules or generalizations, especially if I type them...

This site has a good summary of binary types and a cool computer simulation you can run, near the bottom of the page:

http://zebu.uoregon.edu/~js/ast222/lectures/lec05.html [Broken]
 
Last edited by a moderator:
  • #4
Dx
Originally posted by wolram
how can (contact binary stars) exist? why is it that one does not destroy the other? where is there center of gravity?
the simple fact they use 1's and 0's . Just joking I have no clue about this subject matter but I just read your explanations and find that interesting
Dx :wink:
 
Last edited by a moderator:
  • #5
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785


Originally posted by Labguy
Also, this "point of transfer" can only happen at the large star's L1, or inner Lagrangian point. This method of mass transfer occurs in many binary systems, but in common-usage terms, this still does not fit the description of contact binaries. In "contact binaries", both stars must have "atmospheres", which rules out any type of stellar remnant, or core, such as a white dwarf. Contact binaries are usually described as a situation where (a) the orbits are very close, therefore short period orbits and (b) where both stars have extended atmospheres beyond their Roche Lobes (limits), and material from both stars is intermixed.
I stand corrected. I was forgetting the definition given in a general astro course and talking only about a pair that was transferring mass.

A real "contact binary" I remember now, is what Labguy says:
both stars' atmospheres extend out to the lagrange point between them and touch there. It is not just that one star's atmosphere flows thru the lagrange point and spirals down onto the other, they have to both come out and mingle. Great subject, the natural history of stars---obviously a specialty of Labguy's.
 
  • #6
wolram
Gold Member
4,267
557
thanks for explanation LABGUY, MARCUS, its not clear to me what delays the merging process, i would have thought that once the two stars were captured in each others gravity wells the merging process would be very rapid, unless there orbital velocities are very high.
what effect does the merging have on the rotationan of the two suns?
at what point would there be a single magnetic field?
 
  • #7
Labguy
Science Advisor
731
7
Originally posted by wolram
thanks for explanation LABGUY, MARCUS, its not clear to me what delays the merging process, i would have thought that once the two stars were captured in each others gravity wells the merging process would be very rapid, unless there orbital velocities are very high.
what effect does the merging have on the rotationan of the two suns?
at what point would there be a single magnetic field?
You are right in both cases, the merging is rapid by, astronomical standards. And, the orbital speed (= angular momentum) is very high at this stage and keeps them from an immediate merge. Unless there happened to be an unusual case where the stars were "counter-rotating" on their rotational axis, the resulting "new" star would have a faster period of rotation than either of the original two stars. Your basic conservation of angular momentum.

If you have seen any of the magnetic field line representations of the field around any "normal" star, you know that the fields are usually (a) twisted by the fact that the equatorial portion of a star rotates faster than the polar regions and (b) the magnetic fields extend far beyond the photosphere to regions of 1.5 to 2.0 times the solar radius, at least. In our merging contact binary scenario, the "common" magnetic fields would form (merge) into one at, or even before, both stars reached their Roche Lobe limits and started the mixing of atmospheres. I don't know if even a Cray computer could simulate the complex pattern this field would have, but it would be a very odd and random shape depending on the mass and original fields of the two stars. Long after the final merger, I suppose it would settle into a "normal" pattern, but the increased mass of the new, larger star would accelerate its evolution rate and it might go BOOM before it ever settled into a happy, big star. Every case would be different.
 
  • #8
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
Originally posted by wolram
thanks for explanation LABGUY, MARCUS, its not clear to me what delays the merging process, i would have thought that once the two stars were captured in each others gravity wells the merging process would be very rapid, unless there orbital velocities are very high.
one can be mathematical about it and say how high
(and as Labguy says you are right about this, so
my comment is just a footnote)

consider two stars in circular orbit around each other, having total mass equal to that of
the sun plus the earth------like, each is about half the mass of the sun, since the earth mass is negligible in this context.

If the two are 100 times closer (0.93 million miles instead of 93 million miles as we are)
then relative to each other they have to be going 10 times faster

We are going 30 km/second so they would have to be going 300 km/second relative to each other. If they are about the same mass then as equal partners they would split that speed about equally.

It is not soooo fast. 300 is only a thousandth of light.

As they spiral in closer (Labguy can probably say what time scale to imagine for this) they go faster according to that sqrt rule.
4 times closer (that is, 1/4 the distance) means twice the speed.
Rough approximation as always----neglecting corrections due to this and that.

It seems to me they might each be going roughly 1/1000 of the speed of light as they begin to merge, if one wants a very ballpark estimate. the combined object is going to be spinning like crazy. Labguy can refine this picture, or maybe he already has, since he knows the nitty-gritty about it.
 
  • #9
wolram
Gold Member
4,267
557
cheers LABGUY
 
  • #10
Labguy
Science Advisor
731
7
Originally posted by wolram
cheers LABGUY
I don't know if "Cheers" is a compliment or an invitation to expand on the subject....(?)

But, since I am sitting here, still recovering from Dr. Mengele's surgery, and with two, strong Percodans in my system, I will try to add a little and let it go from there. I lied on my PF "profile" about my birthdate, because I wanted to be young and handsome (again?).

one of the first things I discovered years ago, when I decided that stellar evolution was one of my main interests, is that even the basics of stellar classification are far, far more complex than the "Harvard" system of O-B-A-F-G-K-M (and the old R-N-S) standards, in addition to the 0-9 breakdowns of each, all based on spectral analysis. I think that today, with so many "exceptions-to-the-rule" stars identified, that anyone working in the field has to become a specialist in one, particular class and research only that one interest. That leaves far more specialists than even medical doctors would dare to dream of. Some guys might spend their entire career studying nothing but Carbon stars, while their buddy does nothing but T-Tauri studies. You get the point.

On the subject of Binaries, where this all started, I answered the basics (I hope) of contact binaries of the first order, that is "normal" mass transfer at the Roche Lobes. If I recall, I ended that long post with a statement that there would be many, specific exceptions to the rule. There are. Then, the subject of magnetic fields came up, and I made no mention of the "Polars", a class of binaries with unusual and polarized magnetic fields. I'm not going to lay all that out here, but do a search and there is a lot of info out there. Also, I didn't mention the class called "cataclysmic variables", so check that out too. Also, as far as the magnetic fields go, they can strongly interact before either star in a binary system has extended atmosphere to the Roche Lobe, and when one star does, the strength (at ~1 million Gauss plus) and the field-line pattern can often cause the outflowing gasses to NOT form an accretion disk around the smaller companion, but it can form as (a) a roughly spherical mass around the companion or (b) a disk that is oriented ~ 90 degrees from the rotational axis and allow infalling matter to descend only at the magnetic poles of the companion.

As far as orbital periods for close binaries, even the spectroscopic binary Algol has a period of 69 hours, and those two aren't even that close. I don't remember the distance (in AU) right now. Something around 25 to 40 hours is common for a one-way mass transfer binary with a giant dumping on (usually) a white dwarf. The most extreme examples would be a system where the binary stars are both remnants, like neutron stars, which I believe Marcus mentioned in his first summary. These are the suckers that merge rapidly and usually result in a catastrophic reaction (several ways) and are the source of the gamma-ray-bursts as Marcus explained. I do not remember any particular system's measured orbital period, but I do remember that the periods are measured in seconds, not minutes or hours! These are moving at "relativistic" velocities, and the precession per orbit results in very large numbers of degrees.

That is about all I can add. I'm kind of like your "family physician", where I know a little bit about a lot of stuff, but not a lot about any specialty. Too many different things out there.
 
  • #11
john shoemaker
please consider

Please consider my thread of 6 17 03 and tell me how to get attention of theory person. They pay you deserved attention thanks.

I havn't learned to weave threads yet but you asked for a "low brow" version of my question. I don't see any buttons for answering a post so I'll just edit my original.




Will do. It is not popular now but imagine that there is a probability that particles appear(begin existing)AND there is a probability that particles disappear(cease to exist). Now imagine that the probability of appearance of positively and negatively massed particles is the same. This applies to all particles with observable characteristics; note this is the wish and working assumption of the folks working to justify bigger particle accelerators.
Negative and positive matter will attract like and repel the other leading to a clumping like oil and water shaken. This theory explains the experimental evidence that there "appears" to be some strange matter "dominating" space--that makes the arms of galaxies spin as fast as the near-center stars. Your original question finally drew an answer invoking the concept of angular momentum which is the heart of the surprising galaxy arms.
The lucid wording of your question drew answers from persons familiar with the undergraduate level formula mv^2/r=mM/r^2 which determines the velocity of a planet(little m) in relation to its radius from big M. This relation comes from equating the centripetal force necessary to maintain a planet in a circular orbit WITH the gravitational attraction between them.
The "math" I presented simply points out that these assumptions preclude "black holes" since General Relativity will have the clocks near a concentration of negative mass run faster(in our frame of reference living here in a clump of positive matter) thus increasong the numbers of positive particles apppearing.
Understanding the math is not nearly as important as understanding the assumptions and "feeling" a bit of their consequences. I included the math for a collapsing neutron star as a ceremony of respect for Maria Math, without which some priests of physics will not consider anything.
Since persons adept at the relevant math are attracted to your wordings I thought perhaps if you found some interest in the theory you could word your interest in a question that would attract their attention. Thank you for your attention--John Shoemaker
 
Last edited by a moderator:
  • #12
wolram
Gold Member
4,267
557
im sure your post deserves attention, but i am not qualified to
do so.
may be you can reformulate your theory in a more discriptive way
that a "low brow", like me can understand.
best wishes.
 
  • #13
wolram
Gold Member
4,267
557
and LABGUY i see very litle gratitude expresed for your time and
experiance in this forum but i for one thank you.
im not going to inflate members of this forums egos to
much, so i will say thankyou all, and leave it at that.
 
  • #14
john shoemaker
please note

I had difficulty understanding how to register to post. Please look again at my earlier post that I edited 8 25 which is an attempt to meet your request. Thanks, John
 
  • #15
43
0
Guys the binary stars seem to couple like atoms?
 
  • #16
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
Originally posted by wolram
and LABGUY i see very litle gratitude expresed for your time and
experiance in this forum but i for one thank you.
im not going to inflate members of this forums egos to
much, so i will say thankyou all, and leave it at that.
I just got around to reading this thread, which I had not visited for quite a while and saw this commendation of Labguy.
To which I concurr.
Labguy thanks. You make a point of careful thorough answers
and not just saying the obvious.
Apparently by asking questions we are keeping Labguy out of
trouble, if we didnt he would probably eventually wind up out
on the streets pushing dope and holding up gas stations
and taking lunch money from the other kids.
But by asking good challenging questions about astronomy we can actually keep him on the path of righteousness. So think of some good ones everybody.
 
  • #17
Misty
Another binary question...

Ooookay... Let me start by saying I'm no physicist, just trying to conjure a setting for a story... Was searching the web up and down, looking for an answer for one question, mainly... *inhales deeply* ...can a close binary pair host a habitable planet? I guess, from what I've read in this thread till now, that these contact binaries will be at an early stage of their evolution, so they won't get to 'touch and share' yet... uh... shall try to say it in physics... neither will expand beyond its Roche Lobe limit... yet. So,do you think it possible, twin stars (what lovely sunrises!) having their own planet? Will the twin stars have similar masses then? Could the twin stars be main sequence? Well, reading that you understand I'm fumbling in the dark when it comes to physics... Thank you for your time reading this - and even thank you more if you answer (not polite of me, I know ).
 
  • #18
wolram
Gold Member
4,267
557
it seems that binary stars can have habbitable planets
im not a romantic, but the views would be awsome.


http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122c.html

Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a "habitable zone" exists around BOTH stars within the 3 AU dynamic "safe zone." Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr... plenty of time for life to develop if the planets have the right conditions...
 
  • #19
Misty
Thanks for answer and link, worlram!

Sorry to draw on movies, but talking binaries, Luke Skywalker hopped into my mind, along with his home planet, Tatooine, orbiting twin suns -
Look here, from 'Bad Astronomy' site - a website managed by an astronomer called Phil Plait:
http://www.badastronomy.com/bad/movies/starwars2.html

And someone went to a great deal of trouble calculating Tatooine's suns' masses, separation, distance from the center of mass... look:
http://www.theforce.net/swtc/orbs.html#tatooine
 
Last edited by a moderator:
  • #20
wolram
Gold Member
4,267
557
hi MISTY.

it has been said that what is sci fi today will be
science fact tomorrow.
 

Related Threads on Binary star

  • Last Post
Replies
6
Views
772
  • Last Post
Replies
22
Views
4K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
4K
  • Last Post
Replies
2
Views
3K
Top