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It Was Universe-Shaking Announcement. But What Exactly Are Neutron Star Collisions?

16 October 2017 2:15 PM
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A neutron star, as its name suggests, is a star that consists almost entirely of neutrons.

For most of their existence, stars emit light through fusion — the merging of hydrogen atoms into helium, which releases gargantuan amounts of energy. When a large star — probably at least six times the mass of the sun — exhausts its hydrogen, it begins to collapse. The collapse accelerates so quickly that it sets off cataclysmic explosion known as a supernova. What’s left over is an extremely dense cinder that is only about six miles wide, but packs in more mass than the sun. The pressure is so great that electrons and protons are squeezed together into neutrons.

A single thimbleful of a neutron star weighs as much as several million elephants.

A neutron star is a stellar cinder that stopped collapsing. But when even larger stars explode, the remaining core is so dense that the core continues collapsing until it turns into a black hole. Here’s our guide to black holes.

Welcome to the place of no return — a region in space where the gravitational pull is so strong that not even light can escape it. This is a black hole.

In the case of the discovery that was detailed on Monday, the merging objects were probably survivors of massive stars that had been orbiting each other and had each puffed up and then died in spectacular supernova explosions. Making reasonable assumptions about their spins, the astronomers calculated that these neutron stars were about 1.1 and 1.6 times as massive as the sun, smack in the known range of neutron stars.

As they approached each other, swirling a thousand times a second, tidal forces bulged their surfaces outward. Quite a bit of the material was ejected and formed a fat doughnut around the merging stars.

At the moment they touched each other, a shock wave squeezed more material out of their polar regions, but the doughnut and extreme magnetic fields confined the material into an ultra-high-speed jet emitting a blitzkrieg of radiation. That blast set off the gravitational waves detected by LIGO, as well as the light show spotted by a variety of telescopes.

Watch this video we made in 2016 when LIGO first detected them to learn more about these ripples in space-time that confirmed key aspects of Albert Einstein’s theories.

About a hundred years ago, Einstein predicted the existence of gravitational waves, but until now, they were undetectable.

It is not easy to explain how the universe made heavy elements like uranium. The Big Bang that started the universe created only the lightest elements — hydrogen, helium and a little bit of lithium. Just floating around in space, these light elements do not combine into heavier elements.

The insides of stars can fuse these lighter elements into heavier ones like carbon and oxygen, all the way to iron. That still left the mystery of the origins of elements heavier than iron like gold and platinum.

When two neutron stars collide, they expel neutrons into surrounding space, which slam into free-floating heavy nuclei in their stellar neighborhood. When enough of these collisions occurred, neutrons piled on top of more neutrons and created these heavier elements, notably gold.

These heavier elements drifted around, some accumulating in the gas and dust clouds that formed into new stars and planets including our solar system. The gold in a wedding ring is likely made of the leftovers from a long-ago neutron star collision somewhere in our galaxy.

A gamma ray is a particle of light, a photon, but it’s a very high energy form of light. It’s more energetic than an X-ray, which in turn is more energetic than ultraviolet light, which is more energetic than the visible light we see.

For years, astronomers have been fascinated and perplexed by bursts of gamma rays that they were spotting in the skies. These ultra-high-energy particles told of some distant, cataclysmic, unimaginably powerful events.

The detection of this particular gamma-ray burst led astronomers to point a multitude of other telescopes at the same spot, including the LIGO observatory that detects the vibrations in space-time known as gravitational waves. That is what scientists reported on Monday.

LIGO’s gravitational wave measurements helped astronomers pin down what they thought caused at least some of these outbursts: the merger of neutron stars.

Collisions of neutron stars are still something novel for scientists. The plethora of data will allow them to test and refine their theories and computer models of what happens. This is another a piece of the puzzle for trying to figure out how the galaxy came to be filled with the elements that became planets, people, plants and everything else.


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