Saturday, January 14, 2012


Astrophile: Glimpse elusive matter in shattering star

From opera star to neutron star <i>(Image: Getty Images/Dorling Kindersley)</i>
From opera star to neutron star (Image: Getty Images/Dorling Kindersley)

Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Object type: Neutron star
Strength: 10 billion times the strength of steel
For a taste of some of the strangest and most exotic matter in the universe, try smashing open a neutron star. Forged in the violence of a supernova, the incredibly dense matter inside them would exhibit bizarre behaviour, impossible to recreate in Earth laboratories. Now it seems that these stars may sometimes shatter like a wine glass, explaining some weird cosmic flares and providing a way to dissect these strange objects from afar.
Made of countless atomic nuclei squished together and rich in the subatomic particles called neutrons, neutron stars are natural physics experiments. They pack the mass of the sun into a sphere just 20 kilometres across, making their interiors far denser than anything we can create on Earth – about what you would get "if you took every human on Earth and packed them into a space the size of a sugar cube", says Anthony Piro of the California Institute of Technology in Pasadena.
Under such conditions, the normal rules for matter as we know it break down. Elusive particles called strange quarks that exist only fleetingly in particle smashers on Earth may be long-lived and as common as dirt inside such stars, for example.
Because such densities are impossible to recreate in the lab, reliable predictions are hard to make. Short of a visit to a neutron star, what we need is a way to probe them remotely and persuade them to give up their secrets. Previous astronomical observations have provided some hints to conditions in neutron stars, but they are still tentative.

Shattered glass

New calculations by a team including Piro and led by David Tsang, also of Caltech, suggest that nature occasionally obliges by shattering neutron stars, allowing us to gain precious information about their innards.
It is not easy to shatter something with 10 billion times the strength of steel, but it seems to happen via the same trick that lets an opera singer shatter a wine glass. In that case, sound waves that equal the glass's resonant frequency are the culprit. A neutron star has a resonant frequency too – and Tsang's team have shown that it gets shaken at this frequency when a pair of such stars spirals closer together on the way to merging in a black hole.
Before the neutron stars touch, they exert strong gravitational tugs on each other. Just as the moon's gravity creates a tidal bulge in Earth's oceans, the gravitational forces between the neutron stars distort their shapes.

Periodic squeeze

This distortion squeezes and stretches the stars as they orbit each other faster and faster while spiralling in to merge. At the same time, the orbital frequency changes from less than one orbit per second to more than 1000 orbits per second.
A few seconds before the neutron stars merge, the orbital frequency will match the neutron star's vibration frequency, which could be from a few tens of hertz to a few hundred, the team calculates.
At that point, the vibrations get so large that the neutron star's hard outer crust shatters, suddenly forming cracks all over the surface. The star's gravity is too powerful to let the pieces fly away, so it holds itself together until the stars merge, forging a black hole that swallows the stars up and shoots out jets of matter and gamma rays.
The researchers reckon the shattering would produce a powerful X-ray flare. This could explain some puzzling X-ray flares seen by NASA's Swift satellite a few seconds before some short gamma-ray bursts thought to be caused by neutron star mergers.

Gravity window

Combining observations of X-ray flares with gravitational waves emitted by the neutron stars as they spiral together and merge could reveal the exact frequency at which the shattering occurs. This in turn could tell us more about neutron stars, such as the pressure in their interiors and whether they host strange quarks or other exotic stuff.
"It's really intriguing," says Benjamin Owen of Pennsylvania State University in University Park, who was not involved in the study. "By looking at something from two different angles you learn more about it."
Temper your excitement, though. We haven't actually detected a gravitational wave yet. Such a measurement will probably have to wait until at least 2015, when a powerful new gravitational-wave observatory called Advanced LIGO is scheduled to go into operation in the US. But it will be worth the wait to get such a unique window into neutron stars, says Tsang.
Recreating neutron star conditions in the laboratory is "way beyond" our technology, he says. "[But] if we can measure the frequency at which neutron stars shatter, then we can tell a lot about the properties of matter in such dense states."

Journal reference: Physical Review Letters, DOI: 10.1103/physrevlett.108.011102

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