If you live in the Southern Hemisphere you can look up on any given starry night and just over from the Southern Cross you’ll see a fuzzy patch. It’s called the Large Magellanic Cloud (LMC) and is actually another galaxy quite close to our own Milky Way. To get a sense of size and distance our Milky Way is about 100,000 light-years across and the LMC is about 160,000 light-years away from us but only about 1/10th of the size. Right on our doorstep so to speak.

160,000 light-years is still a long way away though. A light-year is the distance light can travel in a year which equates to about 9,460,730,000,000 kilometres and this LMC is 160,000 of those.

There are different types of stars and they all go through different stages of their lives. Some stars burn up all their fuel and end up collapsing in on themselves at the same time as shedding an ‘outer shell’ and they go nuts burning new materials – these are called Supernovae, you’ve probably heard of them.

One day in 1987 a star in the LMC (previously catalogued as Sk-69 202 but now called SN 1987A) did just this and it was a spectacular sight. Observatories around the world kept an eye on it as it wrecked havoc in slow motion over the next few years.

One thing they noticed was that when the star in the middle brightened the main ring would brighten eight months later and when the star dimmed the ring would, again, dim eight months later. This means that the light that comes directly from the star to our eyes travels a shorter distance than the light that has to go out to the ring and bounce off toward us. Which also means that the ring has a radius of 0.658 light-years (or a diameter of 6,200,000,000,000 km if we’re feeling clever).

Knowing the size of an object is very useful if you want to be able to know the distance to that object. If you know that person is 2m tall and they look very small (this is really measured in angles) you know that they are a long way away.

Astronomers were able to measure the total angle that SN 1987A took up (a radius of 0.808 arcseconds – very small but still useful) and were able to calculate that it is 168,000±3.5% light-years away from us. Which means that what we saw in the skies in 1987 actually happened 168,000 years ago.