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Why scientists believe collision of two neutron stars is a ‘eureka moment’

Scientists detected the first “ripples in space” or gravitational waves produced by the collision of two ancient remnants of stars called neutron stars.

science Updated: Oct 17, 2017 11:45 IST
HT Correspondent
An illustration by the National Science Foundation show an artist's vision of two merging neutron stars.
An illustration by the National Science Foundation show an artist's vision of two merging neutron stars. (AFP Photo)

Physicists and astronomers announced Monday the merger of two neutron stars, providing a spectacular evidence of gravitational waves in the Universe that were first predicted by Albert Einstein.

Scientists described the cosmic clash as “truly a eureka moment”, “everything I ever hoped for” and “a dream come true” as its evidence hurtled through space and reached Earth on August 17 at exactly 12:41 GMT.

It set in motion a secret, sleepless, weeks-long blitzkrieg of star-gazing and number-crunching involving hundreds of telescopes and thousands of astronomers and astrophysicists around the world.

The stellar smash-up made itself known in two ways -- it created ripples called gravitational waves and lit up the entire electromagnetic spectrum of light, from gamma rays to radio waves.

But what is this and what does it mean? We’ve simplified the answers for you:

What is a gravitational wave?

Gravitational waves are ripples that are created as an object, which has mass, moves through space and time, reported Space.com. They were first predicted by Albert Einstein in 1916 but even he was unsure about them because the waves are weak, making them difficult to detect.

Have gravitational waves been detected before?

Yes, and the three American scientists were awarded the Nobel Prize for Physics earlier this month for it.

Gravitational waves have been recorded four times before but each of those events were generated by the collision of black holes, a region in space where the pulling force of gravity is so strong that even light cannot escape its grip. These signals lasted just a few seconds and were invisible to satellites on Earth and in space.

Why was the neutron star collision different?

The neutron star collision was different.

It generated gravitational waves -- picked up by two US-based observatories known as LIGO, and another one in Italy called Virgo -- that lasted an astounding 100 seconds. Less than two seconds later, a NASA satellite recorded a burst of gamma rays.

“It is the first time that we’ve observed a cataclysmic astrophysical event in both gravitational and electromagnetic waves,” said LIGO executive director David Reitze, a professor at the California Institute of Technology (Caltech) in Pasadena.

This handout image obtained from the European Southern Observatory is a mosaic showing how the kilonova in NGC 4993 brightened, became much redder in colour and then faded in the weeks after it exploded on 17 August 2017. (AFP Photo)

Why was the merger a big deal?

Scientists were able to find the location of the long-ago crash and see the end of it play out. Measurements of the light and other energy that the collision produced helped them answer some cosmic questions.

It confirmed that Gamma ray bursts and heavier elements such as gold, platinum and uranium came from neutron stars. The discovery also meant both light and gravitational waves can now be used to study celestial events together, marking the beginning of a new era called “multi-messenger astronomy”, according to Astronomy.com.

But what are neutron stars?

Patrick Sutton, head of Cardiff University’s gravitational physics department, who contributed to the discovery said: “You can think of them as the collapsed, burnt-out cores of dead stars.”

When large stars reach the end of their lives, their core will collapse and the outer layers of the star have been blown off. What’s left is an extremely exotic object, this neutron star.

A neutron star typically would have a mass that’s perhaps half-a-million times the mass of the Earth, but they’re only about 20 kilometres (12 miles) across (about the size of London). A handful of material from this star would weigh as much as Mount Everest.

They are very hot, perhaps a million degrees, they are highly radioactive, they have incredibly intense magnetic fields... They are arguably the most hostile environments in the Universe today.

Why do neutron stars merge?

It’s very common for stars in the Universe to actually be formed in pairs by a given gas cloud.

If the stars are large enough, then at the end of their life they explode and they leave behind neutron star cores, and the neutron stars will continue orbiting each other.

As they orbit, they give off gravitational waves and the waves carry away energy and so the stars slowly fall closer and closer together. As they get closer together, they orbit faster and faster and the gravitational wave emission speeds up.

Eventually they will merge.

What happens then?

Because we don’t understand exactly the mechanics of how these neutron stars work on the interior, it’s not certain what the final fate is, said Sutton.

If the stars are heavy enough, we’re sure they will collapse to form a black hole and some of the remaining matter will form what is called an accretion disk orbiting just around the black hole.

It may be that if the stars are light enough, that they will actually form a single, very heavy neutron star instead of a black hole. That may be stable and stay as a neutron star forever, or it may be unstable and eventually collapse into a black hole.

(With AFP inputs)