Black neutron star confounds astronomers

30 June 2020

Interview with 

Ben McAllister, University of Western Australia

NEUTRON_STAR

Space

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A strange object that should not exist has been spotted by scientists, using gravitational waves to peer deep into outer space. Previously astrophysicists thought they had it all worked out: at the end of its life and depending upon its size, a star either fades into a remnant called a white dwarf, explodes and then shrinks into a dense neutron star, or collapses into a black hole. But now scientists have spotted a new object that doesn’t fit the mould. From the University of Western Australia, Ben McAllister…

Ben - Stars like our sun spend their lives fusing atoms together under extreme temperatures and pressures to generate light and heat. It’s this process that gives us sunshine and warmth on Earth. Stars are effectively the Universe’s nuclear power plants!

Eventually though, all stars run out of fuel, and then they die.

In the most massive stars, when the nuclear fire goes out, the core of the star collapses violently under its own gravity to form a “black hole”. This is an extreme region of space that is so dense, that its gravity is sufficiently strong to prevent even light from escaping, so it looks black.

But, if the star is a bit less massive, rather than collapsing to form a black hole, instead the protons and electrons are squeezed together to form neutrons, and a neutron star is born.

These are also extreme regions of space, with very strong gravity. They are also super dense: the average neutron star is about the same size of a small city, but just a teaspoonful would weigh as much as a million blue whales.

For decades, maths and physics predicted that black holes and neutron stars should exist; and since the 2015 detection of gravitational waves, we’ve been able to observe them by looking at the gravitational waves produced when they interact.

So far though, our best theories say that neutron stars can’t be heavier than about 2.4 times the mass of our Sun, and black holes haven’t been detected which are lighter than about 5 times the mass of the sun.

The gap between these two values is what astrophysicists call a “mass gap”. It’s a physics no man’s land in which no small, dense objects like neutron stars or black holes have ever been observed, and we aren’t sure if they even exist.
But this week new evidence from the world-leading gravitational wave observatory, LIGO, has got physicists scratching their heads!

A recent set of results suggest that far away in space a black hole 23 times the mass of the sun “swallowed” a lighter object.

When this occurred, the two objects merged to form a bigger black hole. A large amount of energy was simultaneously released as gravitational waves, which travelled through the Universe for 780 million years to reach the Earth.

But when the team at LIGO looked at the data to determine what the smaller object in the collision was, something didn’t add up...

LIGO has previously detected neutron stars colliding, and black holes colliding, and they’ve been eagerly awaiting the first detection of a black hole and a neutron star colliding, to test our theories. So the gobbling up of a much lighter object by a black hole looked promising.

LIGO can look at the data from the collision, and compare the shape of the gravitational wave signals with those predicted from models to determine the mass of the objects involved - kind of like a cosmic weight scale.

In this collision, the smaller object looked too big to be a neutron star, and too small to be a black hole - it seemed to have a mass of 2.6 times that of our sun - placing it firmly within the mass gap.

This makes the smaller object in this collision either the heaviest neutron star ever detected, or the lightest black hole. As Professor Vicky Kalogera, a co-author on the study, put it - “Either way, it breaks a record!”

This is fascinating, since it challenges our current understanding of black holes and neutron stars. If the object turns out to be a heavy neutron star, it will require a tweaking of our physical models to explain how such a thing could have formed. If it turns out to be a light black hole, it will be the lightest black hole ever observed by a large margin, almost as light as the heaviest neutron stars!  Either way, this event will change the way we understand these extreme objects, and lead to a deeper understanding of gravity, and the Universe. Who knows, it could even be a completely new kind of dense object altogether!

Maybe we’ll realize there is no mass gap after all, and this is just the latest example of our arrogant human brain’s desire to put messy, complex things into nice, well-defined boxes.

Either way, don’t touch that dial - it’s an exciting time to be listening to the vibrations of the cosmos.

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