What is GW170817?

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

Can you see it?

No small telescope will show you this one; it is here to be understood, not spotted.

Its light took about 130 million years to get here.

Why it stands out: The glow that followed was a kilonova; 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.
What it is: Two neutron stars — the dense dead cores of two stars that had each already exploded — circling each other for probably billions of years, closer and closer, until they touched and merged.
Felt before it was seen: The merger shook space itself. Detectors on two continents measured the distance between mirrors kilometres apart stretching and squashing by about a thousandth of the width of a proton, and that shaking is what announced the event. A flash of gamma rays arrived 1.74 seconds later, and 10.87 hours after that telescopes found the glowing point in the galaxy NGC 4993.; 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: Its light turned red and faded exactly as it would if the debris held a few hundredths of the Sun’s mass of freshly made heavy elements — gold and platinum among them.
What was left: Probably a black hole. The merged object had too much mass to hold itself up as a neutron star, and collapsed within about a second.

The numbers

Distance from us	about 130 million light-years
Discovered	2017

It happened in the galaxy NGC 4993.