On December 11 scientists confirmed their predictions about where and when they would next see a particular star explode nine billion light years after it occurred. Not only that, but it wasn’t even the first time astronomers have seen this same star blow up.
In 2014, astronomers noticed the star, nicknamed Refsdal, when its explosion resulted in what looked like four separate images, configured in a cross-like pattern that is known as an Einstein cross. When looking at the NASA/ESA Hubble Space Telescope photograph of the cross, it is easy to imagine that the galaxy in the image has five nuclei. In reality, however, the galaxy’s core is the faintest part of the image and three of the dots are the same image of just one supernova.
This celestial mirage of one star appearing as four is due to gravitational lensing, a process described by Einstein 100 years ago in his general theory of relativity. Einstein explained how massive objects could pull and bend light, meaning that strong gravitational fields from massive objects have the potential to act as lenses, redirecting and magnifying light as it travels through the universe.
By studying positions of the star’s images and how long it took their light to travel to Earth, scientists were able to predict, to the day, when the cross would recur. So, back in 2014, scientists used lens models of the cluster to forecast that the supernova would reappear about one year later, offset by about eight inches from the earlier images. And it did, earlier this month, right on schedule.
Even though the star exploded nine billion years ago, gravitational lensing allows astronomers on Earth to watch reruns of its explosion — the first time scientists have ever successfully predicted the particular time and location of a supernova. An image from National Geographic shows where the supernova occurred in 2014 compared to 2015, and where, if scientists had known to look, it would have been visible in 1998.
Besides being spectacular to witness, this supernova also is beneficial to science. Normally, supernovas get bright and then dim in a predictable fashion, but Refsdal wasn’t following the pattern, which could give clues to how stars behaved nine billion years ago. Also, studying the delay between “show times” can help scientists study the rate at which the universe is expanding and how dark matter is distributed.
The name Refsdal is after Sjur Refsdal, the Norwegian astrophysicist who predicted in 1964 how lensed supernovae could be used to investigate cosmic expansion.
Image Credit: NASA / ESA