By Leah Crane
A pair of distant stars have a weird wobble to their orbits, which is probably caused by a strange effect predicted by Einstein’s theory of general relativity. This tumbling motion tells us that they formed in a very unusual way.
Pulsars are dense neutron stars that emit beams of light. As they spin, those beams sweep across the sky, so we only see pulsars when their beams pass over Earth. PSR J1141-6545 is one such object, and it orbits a stellar corpse called a white dwarf, which formed when a lower-mass star ran out of fuel and lost its outer layers.
Nearly 20 years of observations of this system have enabled astronomers to calculate the two stars’ orbits around one another with extreme precision. “What we have found is that the system’s orbit is tumbling in space,” says Vivek Venkatraman Krishnan at the Max Planck Institute for Radio Astronomy in Germany.
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That tumbling is due to a destabilising effect called frame dragging, which is predicted in the theory of general relativity and occurs when a fast-spinning object drags space-time around it. Imagine pressing your finger down on a piece of loose fabric and then twisting – the resulting bunching is similar to the shape of space-time around the rotating white dwarf.
Measuring the stars’ wobble due to frame dragging allowed Venkatraman Krishnan and his colleagues to calculate how fast the white dwarf was spinning. They found that it is spinning faster than the pulsar, which is twirling relatively slowly at about 2.5 rotations per second.
That indicates that this system seems to have formed in an order opposite from that we would expect. Usually, the pulsar forms first, and then as the white dwarf is losing its outer layers, the pulsar sucks up that material and starts to spin faster. In this case the white dwarf formed first, so there was no dust and gas to speed the pulsar up, and it is still spinning slowly compared with others in binary systems.
While the system is unusual compared with others we have observed, “everything about it completely agrees with the predictions of general relativity,” says Venkatraman Krishnan. He says that watching other binary systems that have faster-spinning neutron stars dragging space-time around them could let us finally understand their internal structure, which is a long-standing question in astronomy.
Journal reference: Science, DOI: 10.1126/science.aax7007
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