'Black neutron star' discovery changes astronomy
The collaboration between LIGO and Virgo uses some of the most exquisite scientific instruments ever built
Scientists have discovered an astronomical object that has never been observed before.
It is more massive than collapsed stars, known as "neutron stars", but has less mass than black holes.
Such "black neutron stars" were not thought possible and mean that ideas about how neutron stars and black holes form must be reconsidered.
The discovery was made by an international team using gravitational wave detectors in the United States and Italy.
Charlie Hoy, a PhD student from Cardiff University, UK who participated in the study, said the new discovery would change our understanding.
"We cannot rule out opportunities," he told BBC News. "We don't know what it is, and that's why it's so exciting because it really changes our field."
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This event concerned an object that was more massive than known neutron stars, but less massive than known black holes. It existed in the so-called "mass gap"
Mr. Hoy is part of an international team that works for scientific collaboration between Ligo and Jungfrau.
The international group, strongly supported by the UK Science and Technology Facilities Council, has several kilometers of laser detectors that can detect tiny waves in space-time caused by the collision of massive objects in the universe.
The data collected can be used to determine the mass of the objects involved.
Last August the instruments detected the collision of a 23-fold black hole with a sun object with 2.6 solar masses.
This makes the lighter object more massive than the heaviest type of dead star or neutron star that was previously observed - a little more than two solar masses. But it was also easier than the lightest black hole previously observed - about five solar masses.
Astronomers searched for such objects in the so-called "mass gap".
The research team writes in The Astrophysical Journal Letters that the object is most likely a bright black hole of all possibilities, but does not rule out other possibilities.
Gravitational waves are a prediction of general relativity
The development of the technology for direct recognition took decades
They are waves in the structure of spacetime that are generated by violent events
Accelerating masses generate waves that propagate at the speed of light
Verifiable sources include merging black holes and neutron stars
Ligo / Virgo fire lasers in long, L-shaped tunnels; The waves disturb the light
The recognition of the waves opens the universe for completely new investigations
After the collision with the large black hole, the object no longer exists. However, according to Prof. Stephen Fairhurst, also in Cardiff, there should be further opportunities to learn more about these mass gap objects in future collisions.
"It is a challenge for us to find out what this is," he told BBC News. "Is this the lightest black hole ever or is it the heaviest neutron star ever?"
If it is a bright black hole, there is no established theory of how such an object could develop. However, Prof. Fairhurst's colleague, Prof. Fabio Antonioni, has suggested that a three-star solar system could lead to the formation of bright black holes. After the new discovery, his ideas received increasing attention.
However, if this new object class is a heavy neutron star, theories about its creation may need to be revised, says Prof. Bernard Schutz from the Max Planck Institute for Gravitational Physics in Potsdam.
"We don't know much about the nuclear physics of neutron stars. People who look at exotic equations, explain what goes on in them may think, 'Maybe this is proof that we can get much heavier neutron stars.' . "
A scientific visualization of a fusion that generates gravitational waves in which one object is 9.2 times as massive as the other
It is believed that both black holes and neutron stars form when the stars run out of fuel and die. If it is a very large star, it collapses into a black hole, an object with such a strong gravitational force that even light cannot escape its grip.
If the starting star is below a certain mass, one possibility is that it collapses into a dense sphere made entirely of particles called neutrons, which are in the heart of atoms.
The material from which neutron stars are made is so densely packed that a teaspoon would weigh 10 million tons.
A neutron star also has a strong gravity that pulls it together, but a force between the neutrons, caused by a quantum mechanical effect known as degeneration pressure, pushes the particles apart and counteracts the gravitational force.
Current theories suggest that if the neutron star were much larger than two solar masses, gravitational force would overcome the degenerative pressure - and drop it into a black hole.
According to Prof. Nils Andersson from Southampton University, theorists have to rethink what happens in these objects when the mysterious object is a heavy neutron star.
"Nuclear physics is not a precise science in which we know everything," he said.
"We don't know how nuclear forces work under the extreme conditions you need in a neutron star, so any current theory we have about what's going on in a star has a degree of uncertainty."
Prof. Sheila Rowan, director of the Institute of Gravitational Research (IGR) at the University of Glasgow, said the discovery challenges current theoretical models.
"More cosmic observations and research need to be done to determine whether this new object is actually something that has never been observed before, or instead is the lightest black hole ever discovered."
How and interferometer works
A laser is fed into the machine and its beam is split into two paths
The separate paths jump back and forth between muted mirrors
Finally, the two parts of light are recombined and sent to a detector
Gravitational waves that run through the laboratory should disrupt the structure
The theory is that they should very subtly stretch and squeeze their space
This should show up as a change in the length of the light arms
The photodetector detects this signal in the recombined beam
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