Neutron stars are extremely dense objects that are formed by the collapse of massive stars. They are typically only about 10 kilometres in diameter, but are at least 40% heavier than the Sun, which means that their core density is several times that of the density of an atomic nucleus. However, it is possible that neutron stars could also contain other particles such as strange quarks, pions or kaons.

It is relatively straightforward to determine the mass of a star but it is much more difficult to measure the radius – especially if the star is only kilometres across and more than 1016 kilometres away. One approach is to use is high-resolution spectroscopy to measure the redshift caused by the strong gravitational field at the surface of the star. This redshift depends on the ratio of the star’s mass to its radius. However, the magnetic field around a star can greatly modify the lines in the spectrum, making them difficult to interpret.

To overcome this problem, the researchers studied EXO0748-676, a low-mass neutron star that has a weak surface magnetic field (about 107-109 gauss). Although this field is immense compared to the Earth’s (1 gauss), it is too weak to have a significant effect on atomic spectra.

The NASA-Columbia-SRON research group observed the star during a series of 28 X-ray bursts. They found three strong spectral lines of iron and oxygen and identified three sets of redshifts, all with a value of 0.35. This value is in the range expected for a star made of normal neutron matter. In addition, the results do not agree with the models for strange-matter stars.

The group now plans to perform more detailed calculations in order to better quantify and interpret their results. “The next step will be to calculate the properties of the neutron star atmosphere so that we can fully utilise the information available in our spectra,” Jean Cottam of the NASA Goddard Space Flight Center told PhysicsWeb. “We have only begun to extract all the information available in our data, but the next steps will require more theoretical work.”