Why seeing a star crash is a 'watershed moment in astrophysics'

Scientists have followed gravitational waves to something they've never seen before — the collision of two exotic objects called neutron stars.

By observing a fleeting star-like object in the sky in August, they've learned a lot of new things about the universe worth clinking glasses over.

Here are some amazing things about that discovery.

1. A sugar cube heavy as Everest

A neutron star is what's left after a massive star explodes in a supernova at the end of its life and then collapses into an extremely dense core.

How dense? According to NASA, a sugar-cube-sized portion of neutron star would weigh about a billion tonnes, or about the same as Mount Everest. A neutron star would typically be about 20 kilometres wide and weigh twice as much as the sun.

While they're relatively common, they're generally too small and dark to see even with telescopes.

Many stars are binary — two stars that orbit each other. In this case, both stars eventually exploded in supernovas and collapsed, producing a binary neutron star. The two neutron stars spiralled into each other until they collided and merged.

2. A fast-fading kilonova

As the name implies, a kilonova is similar to a supernova (and is sometimes considered a type of supernova) or a nova — the eruption of a white dwarf star.

All those objects appear in the sky as a "star" that quickly appears, then disappears.

Astronomers had predicted that a neutron star merger would produce something that was 1,000 times as bright as a nova. Hence the name kilonova.

A number of possible kilonovas have been spotted, including a faint red object captured by NASA's Hubble Telescope in 2013. But many scientists consider those inconclusive, as they were pretty faded by the time they were observed.

According to Dale Frail of the National Radio Astronomy Observatory, the kilonova spotted in August is the "first unambiguous detection of a merger of two neutron stars."

Kilonovas are hard to spot because they fade really fast. This one stopped producing detectable visible light within a few days, says Maria Drout, who remotely co-ordinated telescope observations of the kilonova at Las Campanas Observatory in Chile.

"It got fainter so much faster than anything we've seen."

Drout is a fellow with Dunlap Institute for Astronomy & Astrophysics in Toronto and the Carnegie Institution for Science in Washington, D.C. She added that even if this isn't the first kilonova spotted "it is certainly, certainly the first one we have any amount of good data on."

3. A Jupiter's worth of gold

What's the celestial factory that makes the precious metals of our universe? Why, colliding neutron stars, of course!

Astronomers had long debated the source of elements such as gold. But the light produced from this kilonova confirmed that it was blasting large amounts of freshly made heavy elements, such as gold, platinum, uranium and plutonium into space.

Each of those elements gives off a unique "fingerprint" of certain colours of light when it's heated. That allows them to be identified with telescopes and devices that can separate the colours.

According to calculations by theoretical astrophysicist Enrico Ramirez-Ruiz at the University of California Santa Cruz, a single neutron star merger like this one can produce enough gold to equal the mass of Jupiter. He estimates neutron star mergers are responsible for half of the elements heavier than iron in the universe.

4. A black hole?

"Very likely, the collision of two neutron stars, resulted in a new black hole," said Eleonora Troja, a research scientist with NASA Goddard Space Flight Center and the University of Maryland at a news conference on the new discoveries Monday.

But other scientists suggest they could also have merged into a single neutron star or something else.

"We don't actually know what happened to the two objects at the end," said David Shoemaker, spokersperson for the LIGO Scientific Collaboration, at the news conference. "Maybe some continued observation will lead to a really complete and final answer."

5. A new era in astronomy

Not only is the first time we've spotted a neutron star merger, but it's also the first time gravitational waves have been detected from anything other than black hole mergers, which are invisible to telescopes.

But the biggest first for astronomers – the reason some call it "a watershed moment in astrophysics" —  is that scientists have been able to observe the same event with gravitational waves and telescopes.

Chad Hanna, a physicist at Pennsylvania State University, likens it to going from silent movies to "talkies" — the start of a new era in astronomy.

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