Déjà supernova: star observed exploding four times

In a case of cosmic serendipity, a US astronomer has observed multiple images of the same star’s explosion. The supernova was discovered in November 2014 by Patrick Kelly, from the University of California, Berkeley, while searching infrared images taken by the Hubble Space Telescope for distant galaxies.

Over the years, astronomers have come to realise that the sky is filled with ‘magnifying glasses’ that allow the study of distant and faint objects barely visible with even the largest telescopes. One of these lenses - a red elliptical galaxy located within a cluster which is gravitationally bending and magnifying light - was found by Kelly to have created four separate images of a supernova located 9.3 billion light years away, near the edge of the observable universe.

“It really threw me for a loop when I spotted the four images surrounding the galaxy - it was a complete surprise,” said Kelly, who has published his discovery in the journal Science.

In this Hubble Space Telescope image, a large cluster galaxy (centre of the box) has split the light from an exploding supernova in a magnified background galaxy into four yellow images (arrows). Image credit: NASA, ESA, and S Rodney (JHU) and the FrontierSN team; T Treu (UCLA), P Kelly (UC Berkeley) and the GLASS team; J Lotz (STScI) and the Frontier Fields Team; M Postman (STScI) and the CLASH team; and Z Levay (STScI).

This multiplication effect has been previously predicted by Albert Einstein, whose General Theory of Relativity states that dense concentrations of mass in the universe will bend light like a lens, magnifying objects behind the mass when seen from Earth. Dr Brad Tucker from The Australian National University (ANU), a co-author on Kelly’s paper, noted that the discovery enables researchers to “test some of the biggest questions about Einstein’s theory of relativity all at once” - such as the strength of gravity and the amount of dark matter and dark energy in the universe.

“It kills three birds with one stone,” Dr Tucker said.

Astrophysicist Sjur Refsdal also hypothesised that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. Kelly explained that the researchers will “measure the time delays between [the supernova’s] arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses”.

Kelly should consider himself very fortunate, with co-author Professor Alex Filippenko noting that scientists have been “searching for a strongly lensed supernova for 50 years”. But luck was on his side, with Kelly stating that the combined effect of the red galaxy within a cluster of galaxies provided “a double lensing system”.

Astronomers have been closely following the supernova ever since its discovery, with the Hubble Space Telescope to remain focused on that area of sky for the next six months. Additionally, computer modelling has predicted that the supernova will be ‘replayed’ within the next four years, enabling researchers to once again observe the four images of the explosion. This is because light can take various paths around and through a gravitational lens, arriving at Earth at different times.

“The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay,” said Filippenko.