Young and getting cooler – the early life of a neutron star
Last Updated on Monday, 26 April 2010 16:49
Published on Thursday, 15 April 2010 19:55
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 will present the findings on Thursday April 15th at the RAS National Astronomy Meeting in Glasgow.
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 using data obtained by NASA’s Chandra X-ray Observatory between 2000 and 2009.
“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 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 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.
“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, we can use the observed temperature changes to probe what’s going on in the neutron star’s core. The structure of neutron stars determines how they cool, so this discovery will allow us to understand better what neutron stars are made of. Our observations of temperature variations already rule out some models for this cooling and has given us insights into the properties of matter that cannot be studied in laboratories on Earth,” said Dr Ho.
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. 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.
Dr Ho and Dr Heinke have submitted a paper on their discovery to the Astrophysical Journal.
Dr Wynn Ho
School of Mathematics
University of Southampton
Southampton, SO17 1BJ, UK
Dr Craig Heinke
Department of Physics
University of Alberta
Room 238 CEB
Edmonton, AB, T6G 2G7, Canada
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Image of Cassiopeia A taken by the Chandra X-ray Observatory (Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.):
Image of Cassiopeia A and an artist’s impression of the neutron star (Credit: NASA/CXC/Southampton/W. Ho et al.; Illustration: NASA/CXC/M.Weiss):
NASA's Chandra X-ray Observatory, which was launched and deployed by Space Shuttle Columbia in 1999 is the most sophisticated X-ray observatory built to date. Chandra is designed to observe X-rays from high-energy regions of the universe, such as the remnants of exploded stars.
For more details, see: http://chandra.harvard.edu/about/axaf_mission.html
THE RAS NATIONAL ASTRONOMY MEETING 2010
The RAS National Astronomy Meeting 2010 will take place from 12-16th April at the University of Glasgow. The conference is held in conjunction with the UK Solar Physics (UKSP) and Magnetosphere Ionosphere and Solar-Terrestrial Physics (MIST) meetings. NAM2010 (www.astro.gla.ac.uk/nam2010/) is principally sponsored by the Royal Astronomical Society (RAS) and the University of Glasgow.
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The Royal Astronomical Society (RAS: www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
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The conference comes to Glasgow during the 250th anniversary year of the founding of the Regius Chair of Astronomy at the University of Glasgow, first held by astronomer and meteorologist Alexander Wilson in 1760. The present incumbent is Prof. John Brown, 10th Astronomer Royal for Scotland.