Star crust 10 billion times stronger than steel

BLOOMINGTON, Ind. -- Research by a theoretical physicist at Indiana University shows that the crusts of neutron stars are 10 billion times stronger than steel or any other of the earth's strongest metal alloys.

Charles Horowitz, a professor in the IU College of Arts and Sciences' Department of Physics, came to the conclusion after large-scale molecular dynamics computer simulations were conducted at Indiana University and Los Alamos National Laboratory in New Mexico. The research will appear Friday (May 8) in Physical Review Letters.

Exhibiting extreme gravity while rotating as fast as 700 times per second, neutron stars are massive stars that collapsed once their cores ceased nuclear fusion and energy production. The only things more dense are black holes, as a teaspoonful of neutron star matter would weigh about 100 million tons.

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.

"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. An online version of the research paper, "The breaking strain of neutron star crust and gravitational waves," can be found at http://arxiv.org/PS_cache/arxiv/pdf/0904/0904.1986v1.pdf.

"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.

Earlier this year, Horowitz was elected a fellow of the American Physical Society, the preeminent organization of physicists in the United States, for his contribution to research in dense nuclear matter. His most recent work on neutron stars was supported by a grant from the U.S. Department of Energy and through Shared University Research Grants from IBM to IU. Working with Horowitz were Don Berry, a principal systems analyst with the High Performance Applications Group in University Information Technology Services at Indiana University, and Kai Kadau at Los Alamos National Laboratory.

Star Birth Gone Wild in 'Cosmic Hurricane'

	Star Birth Gone Wild in 'Cosmic Hurricane'

A shower of hot gas spewed from a galaxy loaded with pockets of intense star formation offers a window to the more violent early universe.

The rapid-fire star birth in M82 was triggered by a collision with another galaxy, and the tremendous activity fuels a "cosmic hurricane is travelling at more than a million miles an hour [447 kilometers per second] into intergalactic space," said Linda Smith of the University College London.

The gas travels in two opposite directions and extends thousands of light-years. Traced back to their sources, the two plumes are revealed to originate in the many separate clumps of star formation and the quick, explosive deaths of massive stars that generate new elements.

"Our goal here is to understand the structure of the wind's plumes, which are key factors in the evolution of this galaxy and the eventual pollution of nearby intergalactic space with new chemical elements," Smith said.

An image of the scene was released Friday. It was created by combining Hubble Space Telescope observations that detail the inner part of the galaxy with a view from the WIYN Telescope on Kitt Peak in Arizona, which showed the extended winds, explained Mark Westmoquette, also of the University College London.

It is not unusual to see jets or plumes of material escaping along the rotation axis of stars, a black hole or an entire galaxy. But M82 is noted for its "superwinds," as astronomers call the bipolar outflows.

"The M82 wind is made up of gas jets from multiple chimneys, each of which is relatively distinct," said Jay Gallagher of the University of Wisconsin-Madison, another member of the study team. "We hypothesize that these originate from individual star-forming clumps within M82."

Some of the clusters contain as much mass as a million Suns packed within 30 light-years of space, Gallagher said earlier this month in discussing his group's work at an astronomy meeting at the Space Telescope Science Institute.

M82 is about 10 million light-years away, which is relatively close in space and time. Gallagher said the scene can help astronomers understand what occurred in the early universe, when star birth was rampant. Because primordial galaxies are incredibly far away -- billions of light-years -- detailed examination of their structures is not practical with current telescopes.

Yet astronomers have seen enough to know that there are big differences between early galaxies and most of the mature galaxies closer by.

"Observations of the distant universe have really shown us now -- and we have to confront this -- that star formation in early epochs was really intense," Gallagher said. "The universe has gone from an intense mode of star formation in galaxies to a lazier mode nowadays."

So it is imperative, he said, to understand the mechanics of so-called starburst galaxies like M82.

In particular, Gallagher told SPACE.com, the distinct clumping of star formation in M82 is thought to be similar to how it worked when some of the earliest galaxies were under construction.

The impetus for star formation in M82 came from a collision with another galaxy, M81, about 300 million years ago, astronomers say. Collisions were common when the universe was younger and smaller, and are thought to have played an important roll in star birth. Here's what happens in a typical collision:

"Huge amounts of gas are funneled into dense regions faster than the galaxy can get rid of it," Gallagher explained. "The galaxy overheats and explodes into stars."