About 170,000 years ago, a star exploded in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. When light and neutrinos from this explosion reached the Earth in 1987, SN1987A became the brightest supernova seen since 1604 — and the best studied ever.

Eleven years later, a second light show is beginning. The blast wave from the original explosion has finally reached a dense and mysterious ring of gas surrounding the star. Its first contact can be seen in these Hubble Telescope images, which show a small patch of the ring suddenly brightening.

What produced this ring? It may have been thrown off by a merger between the main star and a binary companion, some 20,000 years before the explosion.

That idea is appealing because it could also explain why the progenitor star was a blue supergiant, instead of the red supergiant that usually precedes this type of supernova. But the ring could instead be the waist of a more extended shell of gas, emitted as a dense stellar wind in a passing stage of the precursor star's evolution.

Within a year or two we may know better. As the shockwave passes through at 18,000 km s−1, it will heat the gas. The ring will then shine brightly, allowing highly detailed spectroscopy and imaging, and illuminating gas elsewhere in the system.

It is a familiar story — often, the best way to learn about an object is to watch what happens when something hits it.