Some of the Universe's Heavier Elements Are Created By Neutron Star Collisions (sciencemag.org) 53
sciencehabit shares a report from Science Magazine: Scientists have long suspected that neutron stars, the superdense remnants of burned out suns, are needed for this sort of rapid neutron capture. But until 2 years ago, they had never witnessed such an event. That's when the GW170817 merger happened. Taking place 140 million light-years away, astronomers first detected it from the gravitational waves generated by the stars crashing together. Computer modeling revealed that strontium in the expanding ball of gas would absorb light at wavelengths of 350 and 850 nanometers. When they looked again at the X-shooter spectra, they found dips in the spectra at those wavelengths. The end result: five Earth masses worth of strontium. The work confirms that at least some of the heavier elements are produced by merging neutron stars, and that neutrons stars really are made of neutrons. The findings have been published in the journal Nature.
It's right there in the name (Score:2)
neutrons stars really are made of neutrons
It's nice to get more confirmation that our models are correct... but what else would neutron stars be made of? Is it conceivable that such a dense mass of matter consists of anything other than neutrons?
Re:It's right there in the name (Score:5, Informative)
Is it conceivable that such a dense mass of matter consists of anything other than neutrons?
Yes. There are also quark stars [wikipedia.org].
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Re:It's right there in the name (Score:5, Informative)
There are also black holes. But the question was what neutron stars were made of.
Neutron stars and quark stars exist on a continuum, with larger overdense neutron stars [wikipedia.org] having an outer layer of neutrons and an inner core of quarks, and possibly even a thin layer of ordinary nuclei on the surface.
Black holes are completely different.
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Re:It's right there in the name (Score:5, Funny)
There are also black holes. But the question was what neuron stars were made of.
Neurons, smart guy.
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There might also be even denser things than quark stars. We currently have no reason to believe there's any smaller sub-quark particles that quarks could collapse into but the energies in the core of a quark star make the LHC look like a pea shooter, so who knows?
There also might be Strange Stars - primoridal quark stars whose matter was converted to strange quarks, which are about 30x more massive than the normal Up and Down quarks. I guess there's some reason to believe that such a conversion couldn't
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Probably not top stars. At some density the star becomes a black hole, even if there is some form of degenerate matter that prevents a collapse.
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If it becomes a black hole, nothing can prevent collapse: once you cross the event horizon causality can only flow inward (speed of light = speed of causality), which means there's no way for material further inward to support you. Unless of course there's some sort of exotic undreamt-of physics that takes over beyond that point.
Of course, we still have precious little direct evidence that event horizons actually exist - and relatively minor reformulations of Relativity can render them impossible: e.g. if
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"*otherwise* prevents a collapse"
Yours is a bit of a strong statement though. Our theories predict weird things inside black holes. Inspection of spacetime diagrams would suggest that the radius and time axes flip. I'm not going to think too hard about what radial quantum degeneracy pressure (or gravity for that matter) would look like in such a case because it's generally accepted that such weirdness is a sign the theory is inapplicable in that domain.
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That would depend entirely on the mass of the star. So long as the degenerate matter stellar object is just slightly larger than the event horizon for a black hole of the same mass would be, you won't get a black hole.
And as I recall the "critical density" beyond which an object will definitely collapse, or alternately the average density of a black hole, varies wildly with mass.
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I was thinking of making a joke about the desnity of politicians, then realised we would need to send an awful lot of them out into space to make up the mass of a neutron star... so we can send all the lawyers out there too!! /.
That then reminded me of this "what would a mole of moles look like" thought experiment by Randall Munroe at XKCD, which is something of an obligatory nerd reference that needs to be linked periodically on
https://what-if.xkcd.com/4/ [xkcd.com]
Re:It's right there in the name (Score:5, Informative)
I was thinking of making a joke about the desnity of politicians, then realised we would need to send an awful lot of them out into space to make up the mass of a neutron star
The minimum size of a neutron star is about 1.4 solar masses or 2.8e30 kg. If the average politician has a mass of 75 kg, you would need 3.7e28, or about 50 moles of politicians.
But it wouldn't work. The problem is that politicians, like other people, contain a lot of hydrogen and other fusible nuclei. So instead of collapsing into degenerate neutronium, they would instead ignite and form a star. It would likely take a few billion years to burn off enough energy to finally collapse.
Re:It's right there in the name (Score:5, Funny)
The problem is that politicians, like other people, contain a lot of hydrogen and other fusible nuclei. So instead of collapsing into degenerate neutronium, they would instead ignite and form a star. It would likely take a few billion years to burn off enough energy to finally collapse.
I can wait.
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The minimum size of a neutron star is about 1.4 solar masses
1.4 M_sun is the maximum mass ("Chandrasekhar limit") of a white dwarf.
This is often used as a "canonical" mass for neutron stars - i.e. an educated guess when you
don't have an actual measurement.
However, neutron stars may have lower minimum masses, maybe ~1 M_sun.
e.g.
https://www.eso.org/sci/public... [eso.org]
https://arxiv.org/abs/1808.023... [arxiv.org]
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Not just may, neutron stars have been observed that have a pretty good mass estimate close to 1 M_sun.
Theoretically, neutron stars can remain stable down to about 0.1 M_sun, the point where they don't have enough gravity to prevent the neutrons from decaying. You couldn't produce one of these from a supernova, but you might be able to get one some other way. A chunk spit off the merger of two neutron stars perhaps.
https://www.eso.org/sci/public... [eso.org]
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theoretically:
A neutron star cannot be more massive than 3. solar masses, a limit set by general relativity. It is likely that the maximum neutron-star mass is determined by the stiffness of the EOS, and is expected to be about .5 solar masses. Neutron stars also
have a minimum mass limit. The minimum stable neutron-star mass is about 0.1 solar masses, although a more realistic minimum stems from a neutron star’s origin in a super- nova.
That's from the eso paper, above.
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" It would likely take a few billion years to burn off enough energy to finally collapse."
That's a hell of a filibuster.
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Not nearly as long as black hole evaporation-- 10^74 years.
Hawking evaporation time scale of topological black holes in anti-de Sitter spacetime [sciencedirect.com]
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Is it conceivable that such a dense mass of matter consists of anything other than neutrons?
Maybe neutrons stars have a side order of Dark Matter . . . ?
Five earth masses of stronzo (Score:2)
The stars can have strange quark matter at their core
https://en.wikipedia.org/wiki/... [wikipedia.org]
I was affraid that the collisioin produced five Earth masses of stronzo !
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If they turned out not to be, would they rename them? For example, if they turned out to be made of "poorly understood exotic plasma", would they rename them "PUEP stars"?
ESO has the full paper, minus the figures (Score:4, Informative)
Identification of strontium in the merger of two neutron stars [eso.org]
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Strontium (Score:5, Interesting)
strontium in the expanding ball of gas would absorb light at wavelengths of 350 and 850 nanometers.
This almost exactly matches the range of visible light.
Strontium is used in flares and machine gun tracer ammunition because of its efficient emission of visible light.
Next time I see a flare, I will think of the neutron stars that made it possible.
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Not quite.
Visible light is 380 to 740 nm, so 850 nm is in the Near Infrared.
Moreover, this is an absorption spectrum (not emission), and it is in those two waveleng
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Our LTE models with abundances from a solar-scaled r-process and metal-poor star abunances all show that Sr produces a strong feature centred at an observed wavelength of 800 nm, as well as features shortward of 400 nm, for our adopted blueshift (Fig. 3, see also Extended Data Fig. 3). The restframe wavelengths of the longer wavelength features are 1,000–1,100 nm. It is worth noting that Sr is typically considered an s-process element because only about 30% of the cosmic (solar) abundance is produced by the r-process18,19. For this reason it has not always been considered in kilonova simulations. However, it is one of the more abundant r-process elements, accounting for at least a few percent by mass of all r-process elements19. Of all the r-proces elements Sr displays by far the strongest absorption features in this region of the spectrum (Ex-tended Data Figs. 2 and 3). The lanthanide elements, and especially Ba, produce strong absorption but only in the optical region shortward of about 650 nm. The spectral features we observe can therefore only be Sr, an element produced near the first r-process peak.
How many ways to make heavy elements? (Score:3)
I used to think only supernovas can create heavy elements. So neutron star colitions are another mechanism. Any other?
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Our milky way has a few supernovas per century. I don't know how many neutron star collisions there have been but you would need a ratio for production of some of the heavy elements between neutron star/supernova in order to make sense of it. If neutronstar collisions happen 1000 times less but produce 1000000 times more gold or strontium for instance (making up those numbers) then the supernovas become irrelevant - except for creating the neutron stars in the first place of course.
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But if you take this example https://www.skyandtelescope.co... [skyandtelescope.com]
then maybe most neutron star collisions are not so much formed in the process of two healthy supernovas but more in the context of one supernova combined with another stunted supernova, because the first neutron star interfered with the remaining star.
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Yup (well, a special sub-case):
"Some 80 per cent of the heavy elements in the universe likely formed in collapsars, a rare but heavy element-rich form of supernova explosion from the gravitational collapse of old, massive stars typically 30 times as weighty as our sun"
Link [phys.org]
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This is a rare type Ia supernova. Usually a type Ia is a white dwarf sucking matter out of a normal star.
https://en.wikipedia.org/wiki/... [wikipedia.org]
Three pathways (Score:2)
R-Process: https://en.wikipedia.org/wiki/R-process [wikipedia.org]
S-Process: https://en.wikipedia.org/wiki/S-process [wikipedia.org]
p-Process: https://en.wikipedia.org/wiki/P-process [wikipedia.org]
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That was the list I was looking for thanks.
Well that's just fucking great (Score:5, Funny)
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I keep trying to entertain dreams of reaching immortality, and the default setting of the universe is apparently set on "recycle".
So you've already got immortality. You should be working on the opposite problem: how to stop being recycled into existence.
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Re: Well that's just fucking great (Score:2)
So which elements? Where is the line? (Score:2)
ISTR hearing that Fe and heavier require a supernova.
So which elements are too heavy to be produced in a supernova, and require a neutron star collision?
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Here, let me google that for you. "Origin of Elements":
https://www.sciencealert.com/t... [sciencealert.com]
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Well, there's my favorite isotope: Hydrogen-10^10,000 :-D
Seriously though, I don't think it's a matter of *requiring* a neutron star collision, so much as "also get produce in them".
Which makes sense - two neutron stars collide they're likely to throw off a whole bunch of material in the impact And a lump of Neutronium that's even a few Earth masses probably won't have enough gravity to maintain the pressure to keep Neutronium stable, so a bunch of those neutrons would start decaying into protons and elect
Re:So which elements? Where is the line? (Score:4, Informative)
A lasbeled periodic table is lined at Wikipedia's article on the R-process. [wikipedia.org]
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+1 informative. I especially enjoy the laser labeling.