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.

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