How stars live and die: Kepler captures rare supernova blast

When NASA’s Kepler telescope looked into space, it was also looking back in time.

Locked into a heliocentric orbit, Kepler was set to gradually trail the Earth giving the telescope a unique view of the universe.

Which is how we know that a billion years ago, a yellow supergiant, a star 100 times larger than the Sun, collapsed onto itself and then bounced back, sending out a shockwave and debris as it expanded in a cataclysmic explosion.

“The light we were seeing had actually left that star a billion years ago,” Patrick Armstrong, a PhD student at the Australian National University and the lead author of a study published this month in the Monthly Notices of the Royal Astronomical Society told Al Jazeera.

He adds that scientists were lucky Kepler was looking in that direction at that exact moment. While stars live for billions of years, they often die in a matter of weeks, with the actual explosion and shockwave themselves visible only for a matter of days.

The ground-breaking data from Kepler comes three years after the telescope was decommissioned in 2018 when it ran out of fuel after nine years in operation.

As NASA’s first mission to take a survey of exoplanets in our galaxy, Kepler leaves behind an extraordinary legacy, having identified thousands of exoplanets orbiting stars, many of which existed in arrangements that had not been conceived of before, including planets that orbited around two stars. Kepler also found planets that were likely to have water or were close to the size of the Earth.

Precision photometry

If Kepler had a superpower, however, it was its ability to measure the brightness of a star to a tiny fraction of a percent – it was equipped with precision photometry to allow it to track the tiny dimming in a star’s glare caused by the passing of a planet in front of it.

And having it stare at single patches of space for long periods provided the happy bonus of unlocking a vast trove of other cosmic treasures – including historically hard to track phenomena like supernovae, which pop rapidly in and out of view.

Brad Tucker, one of the co-authors of the study and Armstrong’s supervisor at ANU, has been poring over what Kepler sent back since 2013.

“A star explodes about every 100 years in your average galaxy, and Kepler allowed us to stack the deck by being able to monitor tens of thousands of galaxies,” said Tucker, adding that he is confident the telescope still has much to offer, with new research on supernovae based on Kepler data likely to be published even in the coming months.

“Kepler just gives us so much data, and in such a unique way, it takes a long time to pore over and analyse and study it. And so, I think we will be turning to Kepler even in the future.”

The supernova data is unprecedented, the first to offer a clear view of the progression of the shockwave that travels through a star at the end of its life – beginning with the earliest moments of the explosion.

As part of the Kepler 2 survey, the telescope was trained on a single patch of sky for about 80 days. Every 30 minutes it took a picture of what it saw. In contrast, a ground-based telescope would only have been able to make observations at night.

“The difference between looking through a ground-based telescope and Kepler is the difference between looking at a slideshow and watching a movie,” Armstrong explained, adding, “So we were really excited by the high quality of the data we were seeing.”

Armstrong and his team used the data to test multiple models and examined the “shock cooling light curve” which measured the change in the amount of light emitted by the supernova over time.

Now SN2017jgh, the name given to the supernova, promises to help improve scientists’ understanding of how stars live and die.

“We usually don’t capture a supernova until a few days or even a few weeks afterwards – it’s still rare to see those initial moments,” said Tucker. “Now we know which model to use, and so we can improve the use of all those other observations of supernovae we have to understand other stars as well.”

Astronomers from @scienceANU have led an intl. team of researchers to make the first observations of the light emitted just as a supernova explosion occurred! 💥https://t.co/GLxBmRK2xW#SpaceAustralia @ANUmedia @btucker22

📸 NASA pic.twitter.com/U4JHVTJ7dQ

— SpaceAustralia.com (@SpaceAusDotCom) August 12, 2021

Answering the big questions

Studying a supernova can reveal many details about a star, including its size and composition. The explosion itself creates a primordial soup of protons and neutrons and can eventually lead to the birth of new planets and stars.

However, researchers are also intrigued by supernovae because studying them helps answer some of the big questions about the universe.

Armstrong explains that analysing the light of specific kinds of supernovae can allow researchers to identify how fast the universe is expanding and accelerating. “All of this ties into our understanding of where the universe came from and what it’s made of and things like that,” he said.

Now the researchers are looking forward to the data from the Transiting Exoplanet Survey Satellite (TESS), which was launched in 2018 and completed its primary mission in 2020 before beginning an extended mission phase.

While Kepler’s mission was primarily statistical – to discover whether Earth-size exoplanets were common – TESS is designed to identify specific exoplanet systems that should be examined further.

Tucker explains that TESS just sees more volume and that by delivering more observations than Kepler, TESS will feel like going from a 1080p display to a 4k one. Such tools make this an exciting time for astronomy, the researchers say.

“We’re starting to literally see the universe in a way we never did before,” said Tucker. “We had this view that the universe is a somewhat static place with lots of things not changing or things just lasting billions of years, but the more we look, the more we realise just how dynamic and evolving our universe really is.”