Around 11.5 billion years ago, a giant red star collapsed and exploded, creating a spectacular supernova in the early universe.
Light from the star's cataclysmic death made its way through space and time to eventually be captured by the Hubble Space Telescope in 2010.
But it was not discovered until a team of scientists, led by Wenlei Chen of the University of Minnesota, trawled through Hubble's archives.
"This could be the earliest core-collapse supernova [yet discovered]," Dr Chen said.
Hubble caught the moments just after the star exploded in a series of three images, the team reports today in the journal Nature.
"You can see it evolving over a period of hours and days so it's a baby supernova," said Patrick Kelly, the study's co-lead author, also from the University of Minnesota.
"This is really the first detailed look at individual supernova explosions from when the universe was a fraction of its age."
Nature's magnifying glass captures the past
Detecting a supernova right at the time it explodes is hard enough in the nearby universe, but it is even more challenging for those that blew up not long after the Big Bang.
That's because light coming from objects is stretched and shifted to the red end of the spectrum — known as redshift — by the expansion of the universe.
The redder the light becomes, the harder it is to detect by telescopes such as Hubble, which have limited ability to see objects at those longer wavelengths.
So astronomers turn to galaxy clusters, which warp space-time, bending and magnifying light light coming from stars and galaxies much further away.
The effect produces multiple versions of the same object as light travels along different paths.
Dr Chen discovered the supernova, which had a redshift corresponding to 11.5 billion years old, magnified by a bunch of closer galaxies known as Abell 370 in the constellation Cetus.
The supernova was what's known as a core-collapse or Type II supernova, which happens when a massive star burns up all its fuel, puffs up and becomes dim. As the temperature and pressure mounts, it buckles under its own weight, sending out a shock wave.
"We are not able to see this process, because the star's outer envelope is still there, blocking the radiation from inside," Dr Chen said.
When the shock wave reaches the star's surface, it triggers a bright high-energy explosion that expands then fades over time as it cools.
Hubble captured this "shock cooling" phase at three different points in time, starting about five to six hours after the explosion.
At first the supernova appeared blue, then became redder as it rapidly cooled over eight days.
Dr Chen used the brightness of the supernova and the rate it cooled to estimate the size of the original star.
According to his calculations, the star was about 530 times the size of our Sun, making it a type of star known as a supergiant red star.
"Wenlei's measurement of the size is really amazing, because the individual star was so far away," Dr Kelly said.
Live fast, die young
The discovery of the distant supernova and seeing it in its early phase was exciting, said Brad Tucker, an astronomer at the Australian National University who has studied much closer cosmic explosions.
While hard to capture this far away, Dr Tucker said the effects of time dilation caused by the expansion of the universe meant the initial phase could be observed for longer than in a nearby supernova, where shock cooling was over in a day or two.
"The universe has expanded so much, it's literally taking longer for this process to be seen," Dr Tucker said.
And it tells us that the same process that happens in these "vanilla core-collapse supernova" in our local universe, also happened back towards the beginning of time.
Red supergiants, which live fast and die young, seed the evolution of the cosmos.
"This just shows that these stars, which we know produced some of the heavier elements in the universe, did exist 11-and-a-half billion years ago," Dr Tucker said.
The hunt is now on to find more of these stars in the early universe using the James Webb Space Telescope (JWST), which observes objects in infrared wavelengths of light, and is more sensitive to redshifted light.
Both Dr Chen and Dr Kelly have projects with the JWST.
"Given the sensitivity of James Webb, we expect to start seeing many more of these [early] supernovas than people have got before," Dr Kelly said.
