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Lopsided star deaths spew matter into the universe

by Lindsay Jolivet Feb 28 / 12

When supernovae explode and die they spew titanium unevenly in all directions, rather than in a perfect sphere around their cores, according to the first study to observe this important element in a star explosion.

A massive star (left), which has created elements as heavy as iron in its interior, blows up in a tremendous explosion (middle), scattering its outer layers in a structure called a supernova remnant (right).
Image courtesy of NASA/CXC/SAO/JPL-Caltech

A team of researchers including R. Howard Webster Foundation Fellow Victoria Kaspi (McGill University) studied the explosion of the massive star Cassiopeia A, which went supernova about 300 years ago, by observing how it dispersed of one of its elements — titanium.

The study, published in Nature, found that Cassiopeia A blasted out titanium asymmetrically, but in a way that was different and more significant than the lopsided dispersion of other elements.

The result suggests titanium is important for understanding how the elements that form our universe end up where they are.

Stars create all of the known matter that makes up the universe, from planets to humans, through nuclear fusion, says Kaspi.

These nuclear reactions also keep the star from collapsing under the pressure of gravity, but at the end of their lives the fusion stops. When this happens, the most massive stars go supernova.

The supernova could then turn into a black hole, or as in the case of Cassiopeia A, it can form a dense “nugget” in the centre and become a neutron star.

“You form the neutron star, then all the matter falls in, smacks the surface of this newly formed neutron star and bounces back, and sends all of the outer regions of the star, that didn’t form the neutron star, careening into space,” Kaspi says.

“This is the mechanism by which all of the material in these stellar factories gets distributed throughout the galaxy.”

The pattern of which elements the star disperses and which it sucks inward depends on the structure of the star before the explosion, which Kaspi compares to a Gobstopper candy, layered with different elements from the core to the outer surface. Sometimes, the neutron star even shoots off into space to a distance from where the star originally sat.

“You have this very delicate, beautiful structure inside the star that then just totally explodes,” she says.

This is the first time scientists have been able to study the trajectory of titanium from a supernova explosion using the sharper imagery offered by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), which launched in 2012.

Kaspi says future observation of the titanium from other objects in space could help scientists better understand how exactly it influences the way matter is blasted through outer space.

The findings could also help inform models of star explosions simulated on supercomputers, which have had difficulty getting stars to explode at all. When they do work, these simulations often estimate asymmetries inaccurately, Kaspi says, because scientists understand very little about the physics underlying the mysterious and spectacular deaths of giant stars.