Last night I was randomly surfing astronomy pages and came across this.I searched up Wikipedia and other sites,but it doesn't exactly give me enough information about their characteristics.

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    $\begingroup$ I've read the wikipedia page and I think I understand quite well what they should be and how they could appear observationally. Maybe you can reformulate your question to include what you don't understand about them? What are 'characteristics' in your eyes? $\endgroup$ – AtmosphericPrisonEscape Jun 8 '17 at 9:07
  • $\begingroup$ Thorne-Żytkow objects: When a supergiant star swallows a dead star: "One of the universe’s strangest stars is thought to form when a neutron star gets sucked into a red supergiant. But despite 45 years of searching, astronomers still aren’t sure if they’ve ever found one." $\endgroup$ – uhoh Mar 4 at 0:21

I'll try to add a bit of context to the Wikipedia article, though the major references are all available there. The article covers things like the potential formation mechanisms but I guess doesn't really put them into scientific perspective.

Consider, for a moment, a red giant of one solar mass, far up the red giant branch. It has a deep convective envelope, at the bottom of which there is a shell of hydrogen fusing into helium, all surrounding an inert, highly degenerate helium core. The core at this point is much like an isolated white dwarf. So, back in the late 1970's, Kip Thorne and Anna Żytkow tried to model (Thorne & Żytkow 1977) what would happen if you replaced the white dwarf with a neutron star. Such hypothetical objects thus became known as Thorne-Żytkow objects (TZOs).

TZOs have mostly remained theoretical exotica, at least in part because it's very difficult to distinguish them from "ordinary" red supergiants. The main giveaway would be the presence of unusual elements in the atmosphere, produced by the much hotter nuclear reactions happening in the fusion shell around the neutron star core. There have been just a handful of papers published about them, almost all by theoretical groups, in the intervening 40 years.

The only candidate that I think anyone takes seriously is HV 2112. As far as I know, Levesque et al. (2014) found the object serendipitously as part of a survey of red supergiants in the SMC.

PS, following comment: The consensus view is that a single star couldn't form a TZO because if the core collapses into a neutron star, models predict that the envelope will be ejected. That is, single star's only evolve into "naked" neutron stars. Under this assumption, the only way to form a TZO is to have a neutron star collide with or accrete enough material from another star. This could happen if two wandering stars collide. Alternatively, it could happen if one star in a binary collapses into a neutron star. In particular, it's known that the collapse process is probably not symmetric, so the neutron star gets a "kick" in some direction. If the neutron star is kicked into the companion, it will presumably undergo some viscous drag and settle into the centre.

  • $\begingroup$ Fantastic explanation, thanks. What, really is the term for the "normal" version? (ie: "a red giant twice as massive as the Sun, far up the red giant branch. It has a deep convective envelope, at the bottom of which there is a shell of hydrogen fusing into helium, all surrounding an inert, highly degenerate helium core") what 'is' that called? ie, in the sentence, "Well hell, it's not a Thorne–Żytkow object, it's an ordinary old _ _ _ _ " $\endgroup$ – Fattie Jun 8 '17 at 14:32
  • $\begingroup$ @Fattie That kind of object doesn't really have a distinct name. Just about any low-mass red giant will look like that. (Also, I realise I've made a mistake: higher mass red giants don't develop degenerate He cores, so really we'd want a lower mass red giant, say of one solar mass.) I say high up the red giant branch because the level of degeneracy increases as the star evolves up the red giant branch but really, in low-mass red giants the core is already quite degenerate once it reaches the start of the red giant branch. $\endgroup$ – Warrick Jun 8 '17 at 15:25
  • $\begingroup$ It's been a while since I looked at the papers but I think the idea is that TZOs would have to be quite a lot more massive so they'd look like red supergiants. Red supergiants don't necessarily have the same structure as the low-mass red giants: I don't think the He cores are degenerate. Some red supergiants are already burning helium (or even heavier elements) in the core but they have the same deep convective envelopes that TZOs would, hence their similar outward appearance. $\endgroup$ – Warrick Jun 8 '17 at 15:27
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    $\begingroup$ Anyway, to answer the original question, if you mean from an observation perspective, the answer would be a red supergiant. That is, if we find HV 2112 is not a TZO, then it's "an ordinary old red supergiant". $\endgroup$ – Warrick Jun 8 '17 at 15:29
  • $\begingroup$ I meant to know about their origins and and in which situations they form,like was it possible for them to be born in the early universe. $\endgroup$ – Meowthezombie Jun 8 '17 at 16:59

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