<p><strong>tl;dr</strong> Yes there have been theorized, and <strong><em>extremely</em></strong> possible binary systems that have not been observed. One such thing is a TZO, or a Thorne–Żytkow_object. This is a neutron star-red giant binary. </p>

<hr>

<p>There are many kinds of binary pairings that have not been discovered yet, if they exist. A hypothetical star is the quasi-star, which you could say it is a binary between a black hole and an extremely massive star. You can also consider a Wolf–Rayet star, which can occur from a binary system. Other binary systems would include theoretical stars, also known as exotic stars. </p>

<p>However, I do know of a binary system that is composed of real observed types of stars or stellar remnants. This system is composed of a red giant and a neutron star, with the neutron star spiraling in towards the red giant's core. It starts off with a binary system with a star and a neutron star. When the star becomes a red giant, its atmospheric friction on the neutron star(the red giant envelops the neutron star) makes their orbit deteriorate, and the neutron star and the red giant's core will spiral into each other. After the neutron star meets the core, if the mass of the core does not overcome neutron degeneracy pressure, then the core is replaced with a neutron star. If it does, then a supernova resulting in a black hole occurs. This binary system is particularly significant because this can shed light onto how fusion works, and how binary systems develop, because in this model the neutron star does not absorb a lot of the red giant's mass. The surface of the neutron star would also be so hot that fusion can occur, demonstrating a new way of fusion. </p>

<p>Astronomers can observe these stars by their emitted wavelengths because the fusion creates heavy elements that may find their way to the surface of the giant:</p>

<blockquote>
  <p>To look for a TZO, the team searched the spectrums for some of the elements predicted to be produced in high quantities in these objects: lithium, rubidium, strontium, vanadium, zirconium, and molybdenum. Not all of these are easy to image, so they focused on lithium, rubidium, and molybdenum. They compared the levels of these elements with elements that create spectral features nearby but aren't expected to be enhanced in TZOs: potassium, calcium, iron, and nickel.</p>
  
  <blockquote>
    <p><a href="https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/" rel="nofollow noreferrer">https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/</a></p>
  </blockquote>
</blockquote>

<p>I would say that no true examples of this kind of binary pairing has been found. However, there are three possible candidates that are observed, though recently, these stars are under serious questioning on their validity(that may not answer part of your question on "at least suspected :("), according to</p>

<blockquote>
  <p><a href="https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Thorne–Żytkow_object</a></p>
</blockquote>

<hr>

<p>There is another one mentioned by mistertribs(all credits to him), that a black hole and neutron star binary is not observed to exist. These type of systems are possibly the most interesting from a gravitational point of view. They should produce gravitational waves, which could be detectable by LIGO. LIGO has yet to observe such a system and confirm such, which seems a bit puzzling, although something like the blue supergiant in Cygnus X-3 could evolve into a neutron star in the future. This is a case where more data and observations are needed. </p>

<p>However, scientists were only able to constrain the merger rate enough to eliminate the most optimistic models, and that it would take several more observing runs with no results before a continued failure became problematic. </p>

CC BY-SA 4.0
2019-03-01T01:11:43.843Z
Excellent, this is just the kind of thing I was thinking of, thanks!
uhoh
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2019-03-01T19:21:48.393Z
@uhoh updated the answer
Max0815
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2019-03-03T00:40:14.097Z
@uhoh what do you think? Possible to mark an answer as correct?
Max0815
CC BY-SA 4.0
2019-03-03T00:48:06.463Z
Your answer is long and interesting, but you don't really propose a binary object at all. A TZO is "[...a red giant or supergiant contains a neutron star at its core.](https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object)". If you would like to call this a binary system, I think you'll have to make an argument for that. However, if you want to make it a lot clearer that you are naming a "neutron star (in the process of) spiraling in towards the red giant's core" as the object, and say that this has never been observed, then it may qualify as a type of binary object.
uhoh
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2019-03-03T00:49:26.240Z
@uhoh yeah that's how they form I'll add it.
Max0815
CC BY-SA 4.0
2019-03-03T00:52:24.760Z
I often add a **tl;dr** to the beginning of longer answers in lieu of an abstract, to make it easier for readers to know where the answer is going (e.g. https://space.stackexchange.com/search?q=user%3A12102+%22tl%3Bdr%22). btw some questions have more than one excellent answer and occasionally an OP will be so torn that they will accept neither. It should not happen most of the time, but it does happen. Is a TZO *clearly more correct* than a BHNS?
uhoh
CC BY-SA 4.0
2019-03-03T05:33:38.883Z
@uhoh... you make a great point!
Max0815
CC BY-SA 4.0
2019-03-05T23:37:56.133Z
@uhoh I added the tl;dr.  Well, lets first see, your first question, "Are there any kinds of binary pairings that haven't been discovered yet?" is answered in both posts. "Any that are particularly significant, or that might shed some light on binary stellar evolution or theories of capture processes" is answered in mine, but not his, and "so far no examples have been found or at least suspected?" is answered in his, but not mine. I see why you would say this is a stalemate! Cheers!
Max0815
CC BY-SA 4.0
2021-07-04T00:23:41.383Z
There has been some news about BH + NS pairs being detected (posthumously) by gravitational wave detection, maybe you can add a little "update note" at the beginning or end if you have a chance.
uhoh
CC BY-SA 4.0
2019-02-28T21:52:35.033Z
2019-03-05T23:36:17.983Z
2019-03-05T23:36:17.983Z
Max0815
<p><strong>Question:</strong> Are there any kinds of binary pairings that haven't been discovered yet? Any that are particularly significant, or that might shed some light on binary stellar evolution or theories of capture processes, that are sought, but so far no examples have been found or at least suspected?</p>

<p>By "pairing" I mean a binary object where each is <em>a type of star</em> or a <em>black hole</em>.</p>

<p>Stars can be anything from brown dwarfs to neutron stars. Black holes should be very roughly comparable to stars in size. (I'm not asking about a star in orbit around a supermassive black hole in the center of a galaxy)</p>

<p>For example, <a href="https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations" rel="nofollow noreferrer">recent observations of gravitational wave events</a> have suggested a pair of black holes merging, and a pair of neutron stars merging.</p>

CC BY-SA 4.0
2019-02-26T20:14:08.443Z
I doubt there are any. Other than if we discover the theoretical stars such as planck stars or quark stars.
Max0815
CC BY-SA 4.0
2019-02-27T23:46:18.530Z
there is a TZH binary system where a neutron star spirals into the core of a red giant. It demonstrates a new way of fusion...
Max0815
CC BY-SA 4.0
2019-02-28T00:06:16.243Z
Err, I means TZO. https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/
Max0815
CC BY-SA 4.0
2019-02-28T00:30:11.413Z
@Max0815 ah, [Thorne–Żytkow objects](https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object) I'd never heard of them, thank you very much! I noticed that the [Ars Technica article](https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/) has a big picture of one of my favorite objects, V838 Monocerotis for bling, but I'm not sure why. ([V838 Monocerotis “light-echo” images morphed into nice video, but why so few original images?](https://astronomy.stackexchange.com/q/14660/7982))
uhoh
CC BY-SA 4.0
2019-02-28T00:34:07.260Z
@Max0815 the first line of the Wikipedia article is intriguing; "...conjectured type of star..." Once the merge is complete I guess you could not call really it a "binary object", *but during the formation and orbital decay of the pair* it certainly is binary.I think this would be an excellent answer to my question, please consider writing an answer! (see also https://arxiv.org/abs/1806.07399)
uhoh
CC BY-SA 4.0
2019-02-28T03:18:17.980Z
sure! I'm glad I can help!
Max0815
CC BY-SA 4.0
2019-02-28T04:28:43.273Z
its late. I'll type the answer tomorrow.
Max0815
CC BY-SA 4.0
2021-06-29T15:16:21.400Z
different but related: [Binaries consisting of a black hole and a non-black hole?](https://astronomy.stackexchange.com/q/22963/7982)
uhoh
<black-hole><stellar-evolution><neutron-star><binary-star>
Binary pairings that haven't been discovered yet?
uhoh
# Binary pairings that haven't been discovered yet?

(Asked by: username_0 on 2019-02-24)

**Question:** Are there any kinds of binary pairings that haven't been discovered yet? Any that are particularly significant, or that might shed some light on binary stellar evolution or theories of capture processes, that are sought, but so far no examples have been found or at least suspected?

By "pairing" I mean a binary object where each is *a type of star* or a *black hole*.

Stars can be anything from brown dwarfs to neutron stars. Black holes should be very roughly comparable to stars in size. (I'm not asking about a star in orbit around a supermassive black hole in the center of a galaxy)

For example, [recent observations of gravitational wave events](https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations) have suggested a pair of black holes merging, and a pair of neutron stars merging.

**Tags:** black-hole, stellar-evolution, neutron-star, binary-star
> **Comments:**
> **username_1**: I doubt there are any. Other than if we discover the theoretical stars such as planck stars or quark stars.  
> **username_1**: there is a TZH binary system where a neutron star spirals into the core of a red giant. It demonstrates a new way of fusion...  
> **username_1**: Err, I means TZO. https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/  
> **username_0**: @Max0815 ah, [Thorne–Żytkow objects](https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object) I'd never heard of them, thank you very much! I noticed that the [Ars Technica article](https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/) has a big picture of one of my favorite objects, V838 Monocerotis for bling, but I'm not sure why. ([V838 Monocerotis “light-echo” images morphed into nice video, but why so few original images?](https://astronomy.stackexchange.com/q/14660/7982))  
> **username_0**: @Max0815 the first line of the Wikipedia article is intriguing; "...conjectured type of star..." Once the merge is complete I guess you could not call really it a "binary object", *but during the formation and orbital decay of the pair* it certainly is binary.I think this would be an excellent answer to my question, please consider writing an answer! (see also https://arxiv.org/abs/1806.07399)  
> **username_1**: sure! I'm glad I can help!  
> **username_1**: its late. I'll type the answer tomorrow.  
> **username_0**: different but related: [Binaries consisting of a black hole and a non-black hole?](https://astronomy.stackexchange.com/q/22963/7982)  

---

## 1 answers

### Answer by username_1 on 2019-02-28. Score: 1 | Accepted answer

**tl;dr** Yes there have been theorized, and ***extremely*** possible binary systems that have not been observed. One such thing is a TZO, or a Thorne–Żytkow\_object. This is a neutron star-red giant binary.

---

There are many kinds of binary pairings that have not been discovered yet, if they exist. A hypothetical star is the quasi-star, which you could say it is a binary between a black hole and an extremely massive star. You can also consider a Wolf–Rayet star, which can occur from a binary system. Other binary systems would include theoretical stars, also known as exotic stars.

However, I do know of a binary system that is composed of real observed types of stars or stellar remnants. This system is composed of a red giant and a neutron star, with the neutron star spiraling in towards the red giant's core. It starts off with a binary system with a star and a neutron star. When the star becomes a red giant, its atmospheric friction on the neutron star(the red giant envelops the neutron star) makes their orbit deteriorate, and the neutron star and the red giant's core will spiral into each other. After the neutron star meets the core, if the mass of the core does not overcome neutron degeneracy pressure, then the core is replaced with a neutron star. If it does, then a supernova resulting in a black hole occurs. This binary system is particularly significant because this can shed light onto how fusion works, and how binary systems develop, because in this model the neutron star does not absorb a lot of the red giant's mass. The surface of the neutron star would also be so hot that fusion can occur, demonstrating a new way of fusion.

Astronomers can observe these stars by their emitted wavelengths because the fusion creates heavy elements that may find their way to the surface of the giant:

> To look for a TZO, the team searched the spectrums for some of the elements predicted to be produced in high quantities in these objects: lithium, rubidium, strontium, vanadium, zirconium, and molybdenum. Not all of these are easy to image, so they focused on lithium, rubidium, and molybdenum. They compared the levels of these elements with elements that create spectral features nearby but aren't expected to be enhanced in TZOs: potassium, calcium, iron, and nickel.
>
> > <https://arstechnica.com/science/2014/06/red-supergiant-replaced-its-core-with-a-neutron-star/>

I would say that no true examples of this kind of binary pairing has been found. However, there are three possible candidates that are observed, though recently, these stars are under serious questioning on their validity(that may not answer part of your question on "at least suspected :("), according to

> [https://en.wikipedia.org/wiki/Thorne–Żytkow\_object](https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object)

---

There is another one mentioned by mistertribs(all credits to him), that a black hole and neutron star binary is not observed to exist. These type of systems are possibly the most interesting from a gravitational point of view. They should produce gravitational waves, which could be detectable by LIGO. LIGO has yet to observe such a system and confirm such, which seems a bit puzzling, although something like the blue supergiant in Cygnus X-3 could evolve into a neutron star in the future. This is a case where more data and observations are needed.

However, scientists were only able to constrain the merger rate enough to eliminate the most optimistic models, and that it would take several more observing runs with no results before a continued failure became problematic.

> **Comments:**
> **username_0**: Excellent, this is just the kind of thing I was thinking of, thanks!  
> **username_1**: @uhoh updated the answer  
> **username_1**: @uhoh what do you think? Possible to mark an answer as correct?  
> **username_0**: Your answer is long and interesting, but you don't really propose a binary object at all. A TZO is "[...a red giant or supergiant contains a neutron star at its core.](https://en.wikipedia.org/wiki/Thorne%E2%80%93%C5%BBytkow_object)". If you would like to call this a binary system, I think you'll have to make an argument for that. However, if you want to make it a lot clearer that you are naming a "neutron star (in the process of) spiraling in towards the red giant's core" as the object, and say that this has never been observed, then it may qualify as a type of binary object.  
> **username_1**: @uhoh yeah that's how they form I'll add it.  
> **username_0**: I often add a **tl;dr** to the beginning of longer answers in lieu of an abstract, to make it easier for readers to know where the answer is going (e.g. https://space.stackexchange.com/search?q=user%3A12102+%22tl%3Bdr%22). btw some questions have more than one excellent answer and occasionally an OP will be so torn that they will accept neither. It should not happen most of the time, but it does happen. Is a TZO *clearly more correct* than a BHNS?  
> **username_1**: @uhoh... you make a great point!  
> **username_1**: @uhoh I added the tl;dr.  Well, lets first see, your first question, "Are there any kinds of binary pairings that haven't been discovered yet?" is answered in both posts. "Any that are particularly significant, or that might shed some light on binary stellar evolution or theories of capture processes" is answered in mine, but not his, and "so far no examples have been found or at least suspected?" is answered in his, but not mine. I see why you would say this is a stalemate! Cheers!  
> **username_0**: There has been some news about BH + NS pairs being detected (posthumously) by gravitational wave detection, maybe you can add a little "update note" at the beginning or end if you have a chance.  

---

