In 2016, astronomers will watch a distant supernova burst forth. This sort of thin usually comes as a surprise, because the subject goes from being a bright star  – something incredibly difficult to make out individually over intergalactic distances – to a brilliant point that outshines the host galaxy in a matter of seconds to hours. So how can astronomers know that one will be coming months in advance? Because they’ve already seen it.

Astronomers get once-in-a-lifetime opportunity to predict supernova

In short: in November, 2014, a supernova was discovered in a very distant (9 to 14.4 billion lightyears, based on redshift) galaxy. What made it unique is that the galaxy was directly behind a closer (about 5 billion lightyear) galaxy cluster. Normally the closer galaxy cluster would obscure the further one, but due to the effects of gravity lensing, the image of the distant galaxy was replicated at least six times. The six images of the galaxy are distorted and scattered around the periphery of the cluster. But given the *vast* distances involved, even the slightest difference in the path lengths lead to measurably different  trip times. Four of the images showed the supernova within a few days of each other; another image is though to have shown the supernova perhaps 20 years ago. But a sixth image is predicted to show the supernova burst forth in 2016.

Because reality is awesome, that’s why.