GW170817

Two neutron stars spiralled together and merged; the collision was felt as ripples in space before it was ever seen.

What it is

Kilonova: Two neutron stars merging. This is where most of the gold in your house was made. Every kind of thing there is →

Can you see it?

No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.

Worth knowing about GW170817

Far outside our own galaxy. On the 100-meter-track scale model of the Milky Way, this would be 130 kilometres away -- an hour's drive past the end of the track. Light takes 130.0 million years to get here from GW170817, so what you are looking at is GW170817 as it was 130.0 million years ago.

What it is made of: Two neutron stars -- the dense dead cores of two stars that had each already exploded -- circling each other for millions of years, closer and closer, until they touched and merged.
What makes it worth knowing: A kilonova shines roughly a thousand times brighter than an ordinary nova -- which is where the name comes from -- and fades away in a couple of weeks.
Felt before it was seen: The merger shook space itself. Detectors on two continents measured the ground stretching and squashing by less than the width of an atom's nucleus, and that shaking is what announced the event. A flash of gamma rays arrived 1.7 seconds later, and telescopes found the glowing point in the galaxy about eleven hours after that.; How we know: Gravitational-wave detectors (LIGO and Virgo) caught the wave; about seventy observatories then found the same spot in every kind of light, the first time one event was both felt and seen.
Where gold comes from: Smashing two neutron stars together makes elements that ordinary stars cannot: gold, platinum, uranium. The glow that followed was the right colour, and faded at the right speed, for a cloud of freshly made heavy metal. Much of the gold anywhere on Earth was made in collisions like this one, long before the Sun existed.
What was left: Almost certainly a black hole. The merged object was too heavy to hold itself up as a neutron star, and collapsed within about a second.