In summary

  • In two separate events, the death spiral and merger of a neutron star and a black hole have been observed
  • Black holes and neutron stars are two of the most extreme objects ever observed in the Universe
  • The discovery allows researchers to further understand the nature of the space-time continuum and the building blocks of matter

A new phenomenon in the Universe has been revealed – the death spiral and merger of the two most extreme objects in the Universe; a neutron star and a black hole. The two events have been officially announced by the Laser Interferometer Gravitational-Wave Observatory (LIGO), in the US, and the Virgo gravitational-wave observatory in Italy.

A milestone for gravitational-wave astronomy, the discovery allows researchers to further understand the nature of the space-time continuum and the building blocks of matter.

First merger detected

The first observation of the neutron star-black hole merger was made on 5 January 2020 when gravitational waves -- tiny ripples in the fabric of space and time -- were detected from the collision event by LIGO and Virgo.

When masses collide in space, they shake the whole Universe, sending out gravitational waves, like ripples on the surface of a pond. Detailed analysis of the gravitational waves reveal that the neutron star was around twice as massive as the Sun, while the black hole was around nine times as massive as the Sun. The merger itself happened around a billion years ago before the first dinosaurs existed, but the gravitational waves only just reached Earth.

Second merger detected

Remarkably, on 15 January 2020 another merger of a neutron star and a black hole was observed from gravitational waves. This neutron star and black hole also collided around a billion years ago, but it was slightly less massive: the neutron star was around one and a half times as massive as the Sun, while the black hole was around five and a half times as massive.

Australian scientists played leading role

"From the design and operation of the detector, to the analysis of data, Australian scientists are working at the frontiers of astronomy," says Dr Rory Smith, an astrophysicist at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University, who co-led the international team of scientists in this discovery.

The SPIIR pipeline, at the University of Western Australia (UWA) -- Australia's only real-time gravitational-wave search pipeline -- detected a neutron star-black hole event in real-time for the first time. SPIIR is one of five pipelines that alerts astronomers around the world within seconds of gravitational events, so they can try to catch the potential flash of light emitted when a neutron star is torn apart by its companion black hole

The Universe’s most extreme objects

Black holes and neutron stars are two of the most extreme objects ever observed in the Universe—they are born from exploding massive stars at the end of their lives. Typical neutron stars have a mass of one and a half times the mass of the Sun, but all of that mass is contained in an extremely dense star, about the size of a city. One teaspoon of a neutron star weighs as much as all of humanity.

Neutron stars and black holes orbit around each other at around half the speed of light before they collide and merge. This puts the neutron star under extraordinary strain, causing it to stretch and deform as it nears the black hole. How much a neutron star can stretch depends on what kind of matter it’s made of. The amount the star stretches can be decoded from the gravitational waves, which in turn tells us about the type of stuff they’re made of.

Black holes are even more dense objects than neutron stars: they have a lot of mass, normally at least 3 times the mass of our Sun, in a tiny amount of space. Black holes contain an “event horizon” at their surface: a point of no return that not even light can escape. 

Neutron star-black hole merger
Neutron star-black hole merger

Animation by Carl Knox, OzGrav-Swinburne University of Technology

Pairs of neutron stars and black holes have been predicted to exist by theorists for decades, but had long avoided detection. Since their first detection in 1975, many pairs of neutron stars have been found, but never a neutron star orbiting a black hole.

"This is a confirmation of a long-standing prediction from binary stellar evolution theory which predicted these systems should exist," explains Dr Simon Stevenson, OzGrav Postdoctoral Researcher at Swinburne University of Technology.

"We find that roughly one pair of neutron star-black holes merges for every ten pairs of neutron stars. This raises the possibility of observing a neutron star-black hole containing a pulsar -- a rapidly rotating neutron star pulsing radio waves -- in our own Milky Way using radio telescopes like the Australian Parkes radio telescope and the future Square Kilometre Array," says Dr Stevenson.

Sometimes colliding neutron stars and black holes can produce some of the brightest and most powerful explosions in the Universe. Astronomers across the globe used telescopes, like the SkyMapper telescope in central NSW, to scour the night sky for any light flashes associated with these two events -- sadly, none were found this time.

The research was published in The Astrophysical Journal Letters

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