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Inside-out exploding star

Cassiopeia A: artist's impression of the precursor star and real X-ray image of the remnant

Before and after the explosion. Colour coded elements in artist's impression of the star's core match up with real observations of Cassiopeia A. Iron, shown in blue, has somehow gone from being in the middle of the star to the outskirts of the supernova remnant.

IN THE NORTHERN CONSTELLATION Cassiopeia lies a famous supernova remnant, a gas cloud that represents the shattered remains of a once-titanic star.

Known as Cassiopeia A, it is hard to see at normal visible light wavelengths but shines strongly at radio and X-ray wavelengths.

A supernova of this kind occurs when a massive star runs out of nuclear fuel and can no longer produce energy. Without an outflow of energy to keep the star “inflated”, it’s huge mass crushes inwards … producing an implosion followed by an explosion that tears the star apart.

In the millions of years leading up to the explosion, the star’s innards become layered with different elements as a result of different nuclear fusion processes. In the core there is a globe of iron. Surrounding that are layers of sulphur, silicon, magnesium, neon and oxygen.

It would seem to make sense that when the star explodes, the outer layers would end up on the outskirts of the resulting gas cloud, with the iron concentrated toward the middle.

But by compiling a staggering one million seconds worth of X-ray observations by satellite telescopes, astronomers have found that this is not the case for Cassiopeia A.

The observations show that the distribution of sulphur, silicon, magnesium and neon is about as expected. But the iron is spread around the outer part of the supernova remnant, and there’s none in the middle.

The astronomers think there must have been some sort of “instability” in the supernova explosion process, which turned the star inside out.

Story by Jonathan Nally. Images courtesy (illustration) NASA / CXC / M.Weiss; (X-ray) NASA / CXC / GSFC / U.Hwang & J.Laming.

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‘Weird science’ of neutron stars

Cassiopeia A and an artist's impression of a neutron star

Nebula Cassiopeia A, the remains of a massive star that exploded as a supernova. At it's heart is a neutron star (inset, artist's impression), where densities increase from the crust (orange) to the core (red) and finally to the core region where a "superfluid" exists (inner red section).

ASTRONOMERS HAVE GLIMPSED the inner workings of a neutron star and found a unique world where the physics can only be described as “weird.”

A neutron star is the extremely dense, collapsed core left behind from an exploding star, or supernova.

University of Alberta astronomer Craig Heinke’s team found that the neutron star’s core contains a superfluid … a friction-less liquid that could seemingly defy the laws of gravity.

“If you could put some of this superfluid in a jar it would flow up the walls of the container and over the edge,” said Heinke.

Heinke says the core of the neutron star also contains a superconductor, a perfect electrical conductor.

“An electric current in a superconductor never loses energy—it could keep circulating forever.”

Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material weighs six billion tonnes.

“Depending on their composition, superconductors created in laboratories on Earth stop working at anything warmer than -100 to -200 degrees Celsius,” says team member Wynn Ho of the University of Southampton. “In contrast, the incredible densities in neutron stars allow superconductivity at close to a billion degrees Celsius.”

Chandra X-ray telescope

Artist's impression of NASA's Chandra X-ray space telescope.

Cooling down

The discoveries were made when the researchers used NASA’s Chandra X-ray space telescope to investigate a sudden temperature drop on one particular neutron star 11,000 light years from Earth.

Heinke says this neutron star, known as Cassiopeia A, offered the researchers a great opportunity.

“It’s only 330 years old,” said Heinke. “We’ve got ringside seats to studying the life cycle of a neutron star from its collapse to its present, cooling off state.”

The researchers determined that the neutron star’s surface temperature is dropping because its core recently transformed into a superfluid state and is venting off heat in the form of neutrinos … sub-atomic particles that flood through the universe.

Here on Earth our bodies are constantly bombarded by neutrinos from space, with 100 billion neutrinos passing harmlessly though our eyes every second.

They also found that the neutron star’s core is a superconductor … the highest temperature (millions of degrees) superconductor known.

This research helps us to better understand the life cycles of stars, as well as the behaviour of matter at incredibly high densities.

Adapted from information issued by University of Alberta and NASA. Image credits: X-ray, NASA / CXC / UNAM / Ioffe / D. Page, P. Shternin et al.; optical, NASA / STScI; illustration, NASA / CXC / M. Weiss.

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Dead stars get the chills

Image of Cassiopeia A and an artist's impression of the neutron star

Background: An image of the Cassiopeia A supernova explosion remnant taken by the Chandra X-ray Observatory. Inset: An artist's impression of the neutron star that lives at the heart of Cassiopeia A.

Observations of how the youngest-known neutron star has cooled over the past decade are giving astronomers new insights into the interior of these super-dense dead stars.

Dr Wynn Ho presented the findings at the Royal Astronomical Society (RAS) National Astronomy Meeting in Glasgow last week.

Neutron stars are composed mostly of neutrons crushed together by gravity, compressed to over a million million times the density of lead. They are the dense cores of massive stars that have run out of nuclear fuel and collapsed in supernova explosions.

The Cassiopeia A supernova explosion, likely to have taken place around the year 1680, would have heated the neutron star to temperatures of billions of degrees, from which it has cooled down to a temperature of about two million degrees Celsius.

Dr Ho, of the University of Southampton, and Dr Craig Heinke, of the University of Alberta in Canada, measured the temperature of the neutron star in the Cassiopeia A supernova remnant nebula using data obtained by NASA’s Chandra X-ray Observatory between 2000 and 2009.

An artist's impression of a neutron star

An artist's impression of a neutron star

“This is the first time that astronomers have been able to watch a young neutron star cool steadily over time. Chandra has given us a snapshot of the temperature roughly every two years for the past decade and we have seen the temperature drop during that time by about 3%,” said Dr Ho.

Neutron stars’ cooling cores

Young neutron stars cool through the emission of high-energy neutrinos—particles similar to photons but which do not interact much with normal matter and therefore are very difficult to detect.

Since most of the neutrinos are produced deep inside the star, scientists can use the observed temperature changes to probe what’s going on in the neutron star’s core.

Initially, the core of the neutron star cools much more rapidly than the outer layers. After a few hundred years, equilibrium is reached and the whole interior cools at a uniform rate.

At approximately 330 years old, the Cassiopeia A neutron star is near this cross-over age. If the cooling is only due to neutrino emission, there should be a steady decline in temperature.

However, although Dr Ho and Dr Heinke observed an overall steady trend over the 10-year period, there was a larger change around 2006 that suggests other processes may be active.

“The neutron star may not yet have relaxed into the steady cooling phase, or we could be seeing other processes going on,” said Dr Ho. “We don’t know whether the interior of a neutron star contains more exotic particles, such as quarks, or other states of matter, such as superfluids and superconductors.”

“We hope that with more observations, we will be able to explain what is happening in the interior in much more detail,” said Dr Ho.

Adapted from information issued by NASA / CXC / Southampton / W. Ho et al / NASA / CXC / M.Weiss / MIT / UMass Amherst / M.D. Stage et al.