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Paul M. Sutter is an astrophysicist at SUN Stony Brook and the Flatiron Institute, host of “Ask an astronaut“ and “Space radio, “and author of”How to die in space. “
The universe is good enough to break things together. All kinds of things collide – stars, black holes, and ultra-dense objects called neutron stars.
And when neutron stars doing so, collisions release a flood of elements necessary for life.
Let’s explore how astronomers have used subtle ripples in the fabric of space-time to confirm that neutron star collisions make life as we know it possible.
Related: When neutron stars collide: Scientists spot kilonova explosion in epic 2016 crash
Just about everything collided at one point or another in the history of the universe, so astronomers have long thought that neutron stars – superdense objects born during the explosive death of large stars – also crashed. But over the past decade, astronomers have realized that the collision of neutron stars would be particularly interesting.
On the one hand, a collision of neutron stars would go out with a flash. It wouldn’t be as bright as a typical supernova, which happens when large stars explode. But astronomers predicted that an explosion generated by a collision of neutron stars would be about a thousand times brighter than a typical nova, so they dubbed it a kilonova – and the name stuck.
As the name suggests, neutron stars are made up of a lot of neutrons. And when you put a bunch of neutrons in a high energy environment, they start to combine, transform, separate, and do all kinds of other wild nuclear reactions.
The birth of the elements
With all the neutrons flying and combining with each other, and all the energy needed to power nuclear reactions, kilonovas are responsible for producing huge amounts of heavy elements, including gold, money and xenon. Along with their cousins the supernovas, the kilonovas fill the periodic table and generate all the elements necessary to prepare the rocky planets to welcome living organisms.
In 2017, astronomers witnessed their first kilonova. The event occurred about 140 million light years from Earth and was first heralded by the appearance of a certain model of gravitational waves, or ripples in space-time, sweeping over the Earth.
These gravitational waves were detected by the Observatory of gravitational waves by laser interferometer (LIGO) and the Virgo Observatory, who immediately informed the astronomical community that they had seen the distinct ripple in space-time that could only mean that two neutron stars had collided. Less than 2 seconds later, the Fermi Gamma-ray Space Telescope detected a burst of gamma rays – a brief bright flash of gamma rays.
A wave of scientific interest followed, as astronomers around the world dragged their telescopes, antennas, and observatories into orbit with the kilonova event, scanning it through every wavelength of the electromagnetic spectrum. In total, about a third of the entire astronomical community around the world participated in the effort. It was perhaps the most widely described astronomical event in human history, with more than 100 articles on the subject appearing in the first two months.
Kilonovas have long been predicted, but with an occurrence rate of 1 every 100,000 years per galaxy, astronomers didn’t really expect to see one so soon. (By comparison, supernovas occur once every few decades in each galaxy.)
And the addition of gravitational wave signals provided unprecedented insight into the event itself. Between gravitational waves and traditional electromagnetic observations, astronomers got a complete picture from the start of the fusion.
This kilonova alone has produced over 100 pure, solid precious metals on Earth, confirming that these explosions are fantastic for creating heavy elements.
In short, the gold in your jewelry was forged from two neutron stars that collided long before the birth of the solar system.
The death of modified gravity
But that wasn’t the only reason the kilonova sightings were so fascinating. Albert Einstein’s theory general relativity predicts that gravitational waves travel at the speed of light. But astronomers have long tried to develop extensions and modifications of general relativity, and the vast majority of these extensions and modifications have predicted different speeds for gravitational waves.
With this one kilonova event, the universe gave us the perfect place to test this. The gravitational wave signal and the gamma-ray burst the signal from the kilonova arrived 1.7 seconds apart. But that was after driving over 140 million miles (225 million kilometers). To get to Earth so close to each other on such a long journey, gravitational waves and electromagnetic waves would have had to travel at the same speed at one part in a trillion.
This unique measurement was a billion times more precise than any previous observation, and thus has wiped out the vast majority of altered theories of gravity.
No wonder a third of astronomers around the world found it interesting.
Learn more by listening to the episode “What’s so groovy about gravitational waves? (Part 2)” on the “Ask A Spaceman” podcast, available on itunes and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on twitter @Spacedotcom and on Facebook.
