Neutrons Stars Have Crusts of Super-Steel
Written by Nancy Atkinson
Artist concept of a neutron star. Credit: NASA
Neutron stars are dying stars that are seemingly 'off the charts' in almost every category. They are small and extremely dense; about 20 km in diameter with masses of about 1.4 times that of our Sun, meaning that on Earth, one teaspoon of a neutron star would weigh about 100 million tons. They also rotate exceeding fast, about 700 times per second. And according to a new study, neutron stars have another almost super-hero like quality: the outer surface of these collapsed stars is likely to be 10 billion times stronger than steel or any other of Earth's strongest alloys.
Neutron stars are massive stars exhibiting extreme gravity. They have collapsed inward once their cores ceased nuclear fusion and energy production. The only things more dense are black holes.
Scientists want to understand the structure of neutron stars, in part, because surface irregularities, or mountains, in the crust could radiate gravitational waves and in turn may create ripples in space-time. Understanding how high a mountain might become before collapsing from the neutron star's gravity, or estimating the crust's breaking strain, also has implications for better understanding star quakes or magnetar giant flares.
Charles Horowitz, a professor in Indiana University conducted several large-scale molecular dynamics computer simulations and determined the crust of neutron stars is extremely strong.
"We modeled a small region of the neutron star crust by following the individual motions of up to 12 million particles," Horowitz said of the work conducted through IU's Nuclear Theory Center in the Office of the Vice Provost for Research. "We then calculated how the crust deforms and eventually breaks under the extreme weight of a neutron star mountain."
Performed on a large computer cluster at Los Alamos National Laboratory and built upon smaller versions created on special-purpose molecular dynamics computer hardware at IU, the simulations identified a neutron star crust that far exceeded the strength of any material known on earth.
The crust could be so strong as to be able to elicit gravitational waves that could not only limit the spin periods of some stars, but that could also be detected by high-resolution telescopes called interferometers, the modeling found.
"The maximum possible size of these mountains depends on the breaking strain of the neutron star crust," Horowitz said. "The large breaking strain that we find should support mountains on rapidly rotating neutron stars large enough to efficiently radiate gravitational waves."
Because of the intense pressure found on neutron stars, structural flaws and impurities that weaken things like rocks and steel are less likely to strain the crystals that form during the nucleosynthesis that occurs to form neutron star crust. Squeezed together by gravitational force, the crust can withstand a breaking strain 10 billion times the pressure it would take to snap steel.
The research will appear Friday (May 
in Physical Review Letters.
See an online version of the Horowitz's research paper, "The breaking strain of neutron star crust and gravitational waves."
Source: EurekAlert
Filed under: Astronomy
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May 6th, 2009 at 2:17 pm
Interesting that the peaks and valleys mentioned in the paper that may have gravitational wave signatures are on the order to femtometers (10^-15 meters) ! This just goes to show how dense and 'degenerate' neutron stars are.
May 6th, 2009 at 2:18 pm
Errata: read "of femtometers"
May 6th, 2009 at 5:16 pm
Might this crustal material be referred to as the mythical "neutronium?"
May 6th, 2009 at 7:18 pm
It is likely that the crust of a neutron star consists of iron atoms in a crystal held up by degenerate electron pressure. it would be very dense and hard.
As for gravity waves, for the metric h_{ab} correction the linearized wave equation is
&^2h_{ab} = (16 pi G/c^4)T_{ab}.
for & = partial. So the laplacian on the left brings down a frequence ~ k^2 or omega^2. We consider the stress-energy as density = 10^{13}g/cm^3. Then ball parking the numbers this would lead to an amplitude of about
h_{ab} ~ 10^{-28}x10^{13}x10^{2}eta_{ab},
or about 10^{-13}cm or a femtometer.
Lawrence B. Crowell
May 6th, 2009 at 7:24 pm
Andrew: No the 4 page paper linked to this story does not mention neutronium.
May 7th, 2009 at 12:10 am
"Rubbish", I thought, when I read about mountains on neutron stars…. but hey, yes, if one can call a 10^-13 to 10^-15 m unevenness a "mountain"… it's all a matter of scale and perspective, eh?
I've occasionally speculated: if I was to drop something, anything at all (if it can "survive" the temperature), onto a neutron star, would it spread out all over the surface?
May 7th, 2009 at 2:38 am
I have read that Stars at their core are essentially a Crystal lattice. A mild (creative) notion I had was that if a (white dwarf) or neutron star finally extinguished all its fuel and activity, if a gigantic crystal might remain?
The core of a star solidified.
Would make a good plot for a Sci-Fi. But is it even halfway plausible?
Damian
May 7th, 2009 at 2:54 am
Conceivably (at least to me) this awesome material can be referred to as "allotropic iron". I believe you would find it in its mythical form in E.E. "Doc" Smith's "Lensman" series.
May 7th, 2009 at 4:21 am
Or the Mythical Nutronium perhaps
Which just like Allotropic Iron is essentially a energy fluid state if I understand correctly.
I guess us earthlings like to think of things as solids we can use, but Its always fun to remember that all matter is energy anyway.
Must read the Lensman series one day.
May 7th, 2009 at 7:32 am
Does gravity somehow change when the star becomes a neutron star?
The way I understood things is that if our sun today somehow turned into a black hole (without obliterating our solar system in the nova event), the earth would continue to happily orbit.
In the article.
" The crust could be so strong as to be able to elicit gravitational waves that could not only limit the spin periods of some stars… "
Would not the gravity of the star actually decreased due to mass loss during the nova event?
May 7th, 2009 at 11:42 am
dbdncr Says:
"The way I understood things is that if our sun today somehow turned into a black hole (without obliterating our solar system in the nova event), the earth would continue to happily orbit."
that's correct, I reckon – if the mass of both, Earth and Sun (now a black hole), remains the same.
You can look up Schwarzschild Radius – all massive objects have one, and it's a function of the mass. In a star, like the Sun, it lies well within the volume of the star, deep underneath the surface. For a Black Hole, it marks the event horizon.
The escape velocity from a massive object from within the Schwarzschild radius would have to be greater than the speed of light (that's roughly how the SR is defined), hence nothing can escape, even under acceleration.
Anyhow – the net result is: The orbit of the Earth wouldn't change – it lies well outside the SR.
May 7th, 2009 at 12:11 pm
You are right. The star loses most of its mass in the nova event. A neutron star has masses of about 1.4 to about 3.8 solar masses (if I remember correctly). It cannot have more mass, otherwise it would become a black hole.
But we are talking about an object of the size of a city and, as I said, with a mass of probably 2 solar masses. That's f***ing dense! So the gravity close to the surface of the neuron star is incredibly strong. On the other hand, a planet farther out (that somehow managed to survive the explosion) will increase its distance due to the lower gravity (due to the lower mass) of the central object.
May 7th, 2009 at 1:01 pm
Interested readers may want to check out the Wiki page on 'pulsar planets' here: http://en.wikipedia.org/wiki/Pulsar_planets . This page also notes suspected pulsar planets and pulsars with debris disks (and those disproven). Quite an interesting summary of our knowledge of planets orbiting (spinning) neutron stars.
May 7th, 2009 at 4:08 pm
If you think of there being a Gaussian surface at the surface of the star, the gravitational potential there is the same if the star is imploded into a black hole, or any other compressed body.
The 10^{-15}m for gravity waves refers to the amplitude of the gravity wave, not the size of the asymmetry (mountain) on the neutron star which would generate them.
Neutronium is just a term for the matter state of neutrons in a neutron star. It is not often used in astrophysics literature. I suppose there is too much reference in science fiction and StarTrek which prevents its use.
I am not sure of the exact state of this "neutronium." I would imagine after implosion it is a gas. Just as density and pressures are enormous so are temperatures. If it gets cold enough, say within 10^{100} years as the neutron star comes to thermal equilibrium with a near zero temperature universe, that it might exhibit a phase transition to a solid. I am unaware of research into the phase structure of neutron matter — neutronium.
Lawrence B. Crowell
May 7th, 2009 at 4:33 pm
I know the general gist. I was just having fun with the term.
May 7th, 2009 at 5:05 pm
Feenixx–if I was to drop something, anything at all (if it can “survive” the temperature), onto a neutron star, would it spread out all over the surface?
I recall reading somewhere that if you stood on the surface of a neutron star, "you would cover the entire surface, several molecules thick."
May 7th, 2009 at 5:30 pm
Well, the Cheela have to live on something!
May 7th, 2009 at 9:48 pm
So we are to believe that a sphere rotating with an equatorial velocity of ~ 44,000 Km/s, and an unevenly distributed crust will remain intact? Considering the material falling onto it that cannot be evenly distributed or a crust that breaks. The perturbations, resulting in vibration, that would be caused by any amount of imbalance would tear that thing apart in any reality outside of an ill conceived mathematical concept. I didn't notice that they included an imbalance in their equations.
May 8th, 2009 at 4:35 am
Things are pretty extreme on and in neutron stars. Far more so with black holes.
Neutron stars with significant instabilities, in particular magnetic instabilities, can force an Earth equivalent amount of crust hurling out at 1/2 the speed of light by magneitc force. This is then quickly forced back by gravity to the neutron star. Extreme stuff.
Lawrence B. Crowell
May 10th, 2009 at 5:24 am
Nice graphic – anyone know the source? Be there a higher resolution version available?
May 10th, 2009 at 7:45 am
solrey, which reality are you talking about?
But I have another story for you: Have you ever heard of the supernova SN 1987A?
The same time the light of the explosion reached us we also detected a hugh wave of neutrinos. These were the only non-solar neutrinos ever detected coming from outer space. Where did they come from? I think it's reasonable to link them to the SN. But how did it creat such high numbers of neutrinos?
Probably (and most likely) via "inverse beta-decay". An electron and a proton combine to form a neutron. In order to conserve the lepton number, an electron-neutrino must be sent out.
Hmmm….. So, if hugh numbers of neutrons were created during the SN, we should've detect high numbers of neutrinos. But it's not that easy to force electrons into a proton – in order to achieve such high numbers you need a very strong force. A gravitational collapse can provide exactly that.
So to sum up: We have a SN and we detect a hugh flow of neutrinos. The only way to creat that flow is with the inverse beta-decay: Electrons and protons creat neutrons during a gravitational collapse. Since neutrons are fermions the matter will degenerate and become stable (if the core is not too massive) – a neutron star is born.
Do you have a better explanation?
May 10th, 2009 at 10:15 pm
solrey's response:
*CRICKETS*