How do you weigh a star?

The measurement of the mass of stars and other objects in space depends largely on observing their gravitational effect on their cosmic neighbours. But what if that object is alone in space? Scientists have developed a new method to determine the mass of pulsars that could solve this problem.

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Still image from an animation of a pulsar. Image Credit: NASA

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Scientists have developed a method enabling them to measure the mass of pulsars: spinning neutron stars formed from the remains of massive stars that have exploded and become supernovae.

The most common method of determining a star’s mass is to measure its gravitational pull in relation to other nearby objects, but this is ineffectual if the star is alone in space.

A team of researchers at the University of Southampton have determined a way to measure the mass of pulsars using nuclear physics rather than gravity.

Pulsars emit a rotating beam of electromagnetic radiation that can be detected by telescopes when the beam passes by Earth.

The rate of rotation is mostly regular, but young pulsars can sometimes experience sudden brief bursts in the speed of the rotation.

It is thought that these bursts in speed are a result of a superfluid within the star spinning incredibly fast and transferring its energy to the star’s crust.

The team at Southampton have developed a method to use new radio and X-ray data to measure the mass of pulsars that ‘glitch’ in this way.

The magnitude and frequency of the glitches depend on the amount of superfluid in the star and the mobility of the superfluid vertices within.

Observational information can then be combined with the nuclear physics involved to determine the mass of the star.

Professor of Applied Mathematics at Southampton Nils Andersson explains: “Imagine the pulsar as a bowl of soup, with the bowl spinning at one speed and the soup spinning faster.

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.

“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.

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It is a great example of interdisciplinary science.”