The University of Southampton

Carbon atmosphere discovered on young neutron star

6 November 2009

A neutron star discovered with a carbon atmosphere is the youngest yet known in our galaxy, according to new research led by a University of Southampton scientist.

Dr Wynn Ho, of the University's School of Mathematics, investigated the remnant of the supernova Cassiopeia A. Scientists have puzzled over the nature of the remnant since its discovery at the centre of the supernova - or exploded star - in 1999.

In a paper published in the latest edition of Nature, Dr Ho and his colleague Craig Heinke of the University of Alberta, Canada, identify it as a neutron star with a carbon atmosphere. The discovery, made with NASA's Chandra X-ray Observatory, the world's most powerful X-ray telescope, resolves the 10-year mystery surrounding the remnant.

"The compact star at the centre of this famous supernova remnant has been an enigma since its discovery in 1999," says Dr Ho. "Now we finally understand that it can be produced by a hot neutron star with a carbon atmosphere."

When detected 10 years ago, the object was presumed to be a neutron star - the typical remnant of an exploded star - but researchers were unable to determine its properties. It did not show any X-ray or radio pulsations, as scientists would have expected. The new study shows that a neutron star with a carbon atmosphere would emit X-rays across its entire surface rather than in pulses, like a lighthouse.

Cassiopeia A, the supernova that created the remnant, may have been observed by Britain's first Astronomer Royal, John Flamsteed, in 1680. The estimated age of the neutron star in Cassiopeia A makes it about 10 times younger than other neutron stars. Its young age will offer scientists an important insight into the early life and evolution of neutron stars. Its youth may also explain why it is veiled in carbon, as an older star would have cooled enough to accumulate hydrogen and helium in its atmosphere, says the research.

Unlike most astronomical objects, neutron stars are small enough to understand on a human scale. They typically have a diameter of about 14 miles. The researchers calculate that the carbon atmosphere is only about four inches thick, because it has been compressed by a surface gravity that is 100 billion times stronger than on Earth.

"For people who are used to hearing about immense sizes of things in space, it might be a surprise that we can study something so small," says Dr Ho.

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