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First Predicted Supernova Tests Nature’s Magnifying Glass
published during a new moon.
03/07/2016

When and where a star is going to bite the dust has always been essentially impossible for scientists to determine in advance, until recently with supernova Refsdale.

Supernova Refsdal

Supernova Refsdal appeared four times in this Hubble image, making it the first multiply-imaged supernova. Its unusual configuration allowed scientists to predict a fifth appearance, making it the first predicted stellar explosion. Credit: NASA/ESA/STScI/UCLA

When a massive star reaches the end of its lifetime, it can explode violently. These explosions occur with no warning signs, leaving scientists scrambling to study the bright burst before it begins to fade over a handful of days. When and where a star is going to bite the dust has always been essentially impossible for scientists to determine in advance, until recently.

The supernova (SN) Refsdale first appeared in the sky in 2014–not once but four times, in a configuration known as an Einstein cross. In that case, scientists were able to see Refsdale thanks to the help of a single galaxy in the foreground that acted as a natural magnifying glass by bending and distorting the light from objects located behind it. The multiple images were due to the warped light from the lensing galaxy containing SN Refsdal taking different paths and thus, arriving at different locations.

Such warping was predicted exactly 50 years before the appearance of the quadruple supernova, by Norwegian astronomer Sjur Refsdal in 1964. Multiple images of quasars–the brightest objects in the universe–have been captured, but similar supernovae have remained elusive.

Now in 2016, the quadruple-imaged Refsdal allowed scientists to make the first prediction of when a supernova would appear again in the sky—and the bright explosion showed up right on schedule.

“We’re getting a second chance,” Patrick Kelly of the University of California, Berkeley told NOW.SPACE. Kelly, who is the lead author of the team that initially detected the first multi-imaged supernova with NASA’s Hubble Space Telescope, reported that the explosion appeared again in the sky right on schedule this fall.

Theorists predicted that SN Refsdal would appear again in the sky because the elliptical galaxy containing it is part of an enormous cluster of galaxies. They determined that the timing of when the supernova would show up again would depend on how much the galaxy cluster warped space-time. To make an accurate prediction, scientists needed to make precise models of exactly how this warping occurred. Calculations revealed that Refsdale had shown up once in 1998 and been missed. Forecasts of future appearances varied based on the model, but most models favored that the supernova would reappear within a year and be significantly fainter than the original images. True to predictions, SN Refsdal showed up between the middle of November and early December 2015, the exact date coming between Hubble’s observations of the region.

“It’s like a double lens,” Kelly said. “We were double lucky.”

Not only did this mark a stellar day for supernova observation as scientists were able to study the before-and-after of the galaxy containing the explosion, but it also provided a results-driven opportunity for scientists to predict the effects of cosmic lensing due to the presence of dark matter. The presence of all matter, dark or regular, affects the path the light travels, so predictions about the supernova’s reappearance were based on the amount of dark matter along the way.

The research was published online in the Astrophysical Journal Letters.