Bizarre Star Could Host a Neutron Star In Its Core

ananyo writes "Astronomers say that they have discovered the first example of a long-sought cosmic oddity: a bloated, dying star with a surprise in its core — an ultradense neutron star. Such entities, known as Thorne-Zytkow objects, are theoretically possible but would alter scientists' understanding of how stars can be powered. Since Thorne-Zytkow objects were first proposed in 1975, researchers have occasionally offered up candidates, but none have been confirmed."

Comments

Boo :-(

[Baby Duck]

A star IN a star? Now I can't make fun of Sunshine anymore.

Yo dawg, I herd yo like stars

[Anonymous Coward]

So I put a star in yo star so you can collapse into a black hole if your combined solar mass exceeds the Oppenheimer-Volkoff limit.

Yo dawg

[trmj]

I heard... you know what? It's too easy.

That's inexplicable, but not inexplicable enough!

[ackthpt]

Such entities, known as Thorne-Zytkow objects, are theoretically possible but would alter scientists' understanding of how stars can be powered.

How apt this phrase is now: There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

scan of the original article

[Trepidity]

"Stars with degenerate neutron cores", Astrophysical Journal, 1977.

Courtesy of the SAO/NASA Astrophysics Data System, an open-access digital library that other fields could do well to emulate...

Where is this claim?

[hubie]

I read TFA and I don't see where this comes from: but would alter scientists' understanding of how stars can be powered

It sounds like Thorne and Zytkow proposed the scenario and predicted what one would observe, followed up by people like the guy quoted in the brief article (Podsiadlowski), and these astronomers are putting forth a candidate based upon their observations being similar to what the theory suggests. I'm missing the part that alters the understanding. Podsiadlowski, by the way, has been thinking about these objects for a very long time.

Some more explaination

[Michael Woodhams]

While I've not heard of a Thorne-Zytkow object before, I can apply my general astronomical knowledge to explain a bit further.

The idea of an internally inert condensed object at the centre of a star is very standard: red giants have a white dwarf at their core, indeed this is how white dwarfs are formed. The weirdness is in having a neutron star instead of a white dwarf core.

The condensed object is supported by degeneracy pressure (electron degeneracy pressure for a white dwarf, neutron degeneracy pressure for a neutron star.) (Degeneracy pressure is a quantum mechanical effect. It is only appreciable at very high densities, and is not dependent on temperature.) The surface of the condensed object will be very hot, because nuclear burning is going on nearby and it is insulated from the coldness of space by the envelope of the star (i.e. the bits of star which are not the condensed object.) The density of gas just above the surface of the core will also be large, due to the high surface gravity plus the pressure of the weight of the envelope.

High temperature and high density leads to nuclear burning (combining light nuclei into heavier ones, releasing energy.) The nuclear reactions are generally very strongly dependent on temperature (e.g. one important reaction has a rate approximately proportional to temperature to the 17th power) so the burning happens in a thin layer. The 'burnt' material settles on the core, slowly enlarging it.

The gravitational attraction of the core pulling the envelope inward is largely balanced by gas pressure and radiation pressure. While stars like our sun are dominated by gas pressure, in this case radiation pressure will dominate. As the radiation escapes outward, mass is able to migrate inwards, to the thin burning layer. An equilibrium is reached between the burning/energy production rate and the mass inflow rate.

Because they are dominated by radiation pressure, it doesn't take much extra push for something at the surface of a red giant star to escape, so these stars have strong stellar winds and high mass loss rate to winds. So the envelope gets eaten from the bottom by burning and deposition onto the growing white dwarf, and from the top by mass loss. Eventually there is no envelope left and a bare white dwarf is exposed. (The final transition is quite spectacular and is called a planetary nebula.)

Heat transport in red giants is dominated by convection rather than radiation. (I think this is a general property of being dominated by radiation pressure, but I may be mistaken.) This allows material which has been close to the burning zone to mix through the star. Various secondary nuclear reactions occur there (e.g. s-process nucleosynthesis), so the products of this are mixed to the surface, where they can be observed in the spectrum. (I'm not sure whether partly-burnt material from the main burning shell can get mixed out or not.)

Evidently (according to the article) in Thorne-Zytkow objects these reactions are different from in a normal red giant and so mix different products to the surface. The star of the article has a spectrum rich in predicted reaction products of a Thorne-Zytkow object.

While white dwarf naturally grow inside stars, the process that generates neutron stars (supernovae) removes the stellar envelope, so finding a neutron star inside an envelope requires some rare post-supernova event to supply the neutron star with stellar-mass quantities of fresh gas.

Re:Some more explaination

[DirePickle]

THIS is why I still come to Slashdot. Thank you!

Re:Some more explaination

[able1234au]

requires some rare post-supernova event to supply the neutron star with stellar-mass quantities of fresh gas.

Such as a binary star where one goes supernova and somehow the binary is not destroyed? Perhaps a binary with a long period? Though imagine that the binary pair would be destroyed or ejected. What is another example that might do this?

Re:Some more explaination

[Anonymous Coward]

Whether a binary system survives a supernova event is a bit complex topic, actually.

First, the explosion is not likely to destroy the companion star, even on a fairly close orbit. Stars are really massive objects, and can readily absorb the fraction of supernova energy that happens to hit them. Second, if too much mass is ejected from the system in the explosion, the neutron star and its companion will become gravitationally unbound and drift off separately into space. Assuming a circular orbit and a perfectly symmetrical collapse of the exploding supergiant's core, this will happen if more than half of the total mass is ejected from the system. Now, there are good reasons to believe that the collapse is often slightly asymmetrical, and given the amount of energy released in the explosion, a 'slight' asymmetry might give the remaining neutron star enough 'kick' to propel it out of the host galaxy, let alone of a binary system.

That all said, there are objects such as Cygnus X-1 that are composed of a regular star and a degenerate object that is believed to be a supernova remnant (in case of Cygnus X-1, a black hole rather than a neutron star, but the principles are the same), so it appears that at least some supernova events do not disrupt binary systems. In that case, when the companion becomes a (super)giant itself later, if it is close enough to the neutron star to fill its Roche lobe (as in Cygnus X-1) and at some point the mass transfer onto the neutron star becomes fast enough, a common envelope will form around the binary system. Common envelope causes much drag and shrinkage of the orbit; in some cases it is believed to cause both objects inside it to merge.

Of course, much of that is rather speculative, mainly because the common envelope stage is short-lived and thus difficult to observe.

Re:Some more explaination

[NoImNotNineVolt]

I'm no astrophysicist, but the respectable Randall Munroe suggests that "However big you think supernovae are, they're bigger than that."

According to this What If, you'd get more photons hitting your eye from a supernova seen from as far away as the Sun is from the Earth than the detonation of a hydrogen bomb pressed against your eyeball... by nine orders of magnitude. So while stars are really massive objects, supernovae are apparently unimaginably energetic phenomena.

"a surprise in its core"

[Anonymous Coward]

Who else was thinking "nougat"?

Re:"a surprise in its core"

[PPH]

I'd mod you up, but my keyboard is covered with powdered sugar.

Take THAT, science!

[Swampash]

Another nail in the coffin of so-called "knowledge". How can you trust a belief system that CHANGES all the time?

Neutron Star Decay

[TrollstonButterbeans]

So let's say you have a neutron star.

How does it decay? Blackholes have hawking radiation, neutron stars don't need to decay that way of course but they must eventually decay

If a neutron star emits enough energy to pass below the limit that keeps the pressure as neutrons --- and neutrons have a half-life of 14 minutes outside an atom --- do the neutrons decay into a proton (hydrogen) and an electron and become hydrogen and then start fusing again into helium?

Re:Neutron Star Decay

[IndustrialComplex]

Why "must" it decay?

Neutron stars basically just cool off/slow down over time due to rather normal mechanisms of radiation. There's some other stuff going on, but very old neutron stars are basically just lumps of cold very old neutrons.