How do you weigh a star? Well, you can't put it on a scale, but you can use mathematics to determine the mass of one particular kind of star, say scientists in Britain who've been busily weighing pulsars.
Pulsars are rotating, highly magnetic neutron stars formed out of the debris left over when massive stars explode as supernovae.
The traditional way to determine the mass of a star, a planet or a moon is by studying the motion of the object in relation to other nearby bodies, calculating mass by the gravitational attraction each exerts on the other.
However, researchers at the University of Southampton writing in the journal Science Advances say they've come up with a method to measure the mass of a pulsar, even if it exists in open space with no other nearby objects.
"For pulsars, we have been able to use principles of nuclear physics, rather than gravity, to work out what their mass is – an exciting breakthrough which has the potential to revolutionize the way we make this kind of calculation," says university mathematician Wynn Ho.
The method makes use of a special property of pulsars: rotating electromagnetic beams of radiation sent out as they spin that can be observed using telescopes as the beams sweep by the Earth.
While the rotation rates of most pulsars are incredibly stable, young pulsars can occasionally exhibit "glitches" in which they briefly speed up.
Astronomers believe these glitches are the result of rapidly spinning superfluids inside the star transferring their rotational energy outward to the star's outer crust, affecting the rotation rate and the timing of the observed beams.
"Imagine the pulsar as a bowl of soup, with the bowl spinning at one speed and the soup spinning faster," explains Nils Andersson, a Southampton professor applied mathematics.
"Friction between the inside of the bowl and its contents, the soup, will cause the bowl to speed up," he says. "The more soup there is, the faster the bowl will be made to rotate."
The researchers say they've used X-ray and radio data on pulsars to create a mathematical model to determine the mass of a pulsar seen to exhibit glitches.
The frequency and magnitude of the pulsar's glitches depends on how much superfluid is in the star and its movement, and by combining data gathered by observations with known nuclear physics the mass of the rotating star can be determined, they say.
"Our results provide an exciting new link between the study of distant astronomical objects and laboratory work in both high-energy and low-temperature physics," says Andersson. "It is a great example of interdisciplinary science."
