Measuring the mass of stars, planets and moons in the universe usually relies on studying how their motion relates to other bodies nearby. For some objects that are on their own in space, however, this method is impossible. But research published in the journal Science Advances describes a new technique that could prove useful.
Scientists led by the University of Southampton have found that they can measure the mass of an ultra-dense star known as a pulsar – a rapidly rotating neutron star only 25 kilometers (15 miles) in diameter but with more mass than the Sun – by measuring "glitches" in its spin. As pulsars rotate, sometimes at hundreds of times per second, they emit a beam of electromagnetic radiation. This can be detected on Earth when it sweeps in our direction, and they are known to have extremely stable rates of rotation.
Occasionally, though, young pulsars – which form as the remnants of supernovae – speed up for a very short period of time. This "glitch" is thought to be caused by superfluid – a fluid state of matter with zero viscosity – within the star transferring rotational energy to the star’s crust. The size and regularity of the glitch depends on the amount of superfluid in the star, and measuring this can allow its mass to be determined.
"Imagine the pulsar as a bowl of soup, with the bowl spinning at one speed and the soup spinning faster," said Nils Andersson, a Professor of Applied Mathematics at the University of Southampton and a co-author on the study, in a statement. "Friction between the inside of the bowl and its contents, the soup, will cause the bowl to speed up. The more soup there is, the faster the bowl will be made to rotate."
In their research, the scientists were able to create a mathematical model that used radio and X-ray data to measure the mass of pulsars as a result of the glitch. Using upcoming radio telescopes like the Square Kilometre Array (SKA) and the Low Frequency Array (LOFAR), scientists will now be able to study more pulsars in even greater detail.
"The accuracy of our method is comparable to the traditional method of measuring a pulsar's mass by the gravitational effect it has on a nearby object, such as a planet or another star," lead author on the study Dr Wynn Ho from the University of Southampton told IFLScience. He noted that while the traditional method can be more accurate in some cases, it could not resolve the mass of solitary pulsars like the new method. He added, though, that their model "only works for pulsars because it requires the star to be partly made of a superfluid, which is only possible in compact stars like pulsars."
Thus, in the statement he heralded the new method an "exciting breakthrough" which could revolutionize these types of calculations, using nuclear physics rather than gravity to work out a pulsar's mass.