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Type 1a supernovae are used by scientists to measure vast distances across space and helped reveal that the expansion of the Universe is accelerating. Yet for more than 50 years, astronomers have debated exactly what kind of star system causes these explosions.

Type 1a supernovae are incredibly valuable to astronomers because they act as reliable cosmic distance markers. 

These explosions reach almost the same peak brightness every time, meaning scientists know how bright they should appear. By comparing that known brightness with how bright the explosion looks from Earth, astronomers can calculate how far away it is, much like estimating the distance of a lighthouse by comparing its expected brightness with what you can see from shore. 

By measuring thousands of these explosions across the Universe, scientists discovered that the Universe is expanding at an accelerating rate, which led to the 2011 Nobel Prize in Physics for Prof. Brian Schmidt. 

In led by 91ɫ��Ƭ Canberra PhD candidate Priyam Das, astronomers examined the remains of a Type 1a supernova that exploded around 310 years ago in the Large Magellanic Cloud; a nearby galaxy. 

By mapping the motion of the debris in three dimensions, they were able to reconstruct the original stellar system that existed right before the explosion - the so-called progenitor system - which is something that has not been done before. 

The team's results show the explosion likely came from a binary system consisting of two white dwarf stars orbiting each other. A white dwarf star is the dense remnant, or exhausted central ‘core’ of a star, that is left behind when Sun-like stars run out of fuel and reach the end of their lives.

In a key breakthrough, the researchers identified what appears to be the predicted – but until now, never observed — "shadow of the companion". 

They found a gap in the expanding debris where the second white dwarf appears to have blocked part of the explosion. This fossil-like imprint remained visible centuries later and allowed the team to work backwards to determine the stars' positions, orientation and motion just before the blast.

Mr Das, said the findings provide some of the clearest observational evidence yet that at least some Type Ia supernovae are caused when two white dwarfs orbit closely together and explode just before or as they merge, triggering a catastrophic explosion.

"This is the first time astrophysicists have been able to reconstruct the complete three-dimensional configuration of a Type Ia supernova progenitor system from the remnant left behind by the explosion," Mr Das said.

Co-author Prof. Brian Schmidt, said the discovery was groundbreaking.

"Type Ia supernovae were used by my team to discover the accelerating Universe, but how nature creates them has remained shrouded in mystery. This work cracks the case, providing forensic evidence that not only demonstrates the explosion mechanism, but exposes the physical attributes of the two white dwarf stars involved,” Prof. Schmidt said.

Dr. Ivo Seitenzahl, a core member of the research team from the Australian National University said the discovery cannot be understated.

“This is amazing because we are seeing the orbital motion of a star at the moment it destroyed itself,” Dr Seitenzahl said. 

“The exploding white dwarf was moving away from us at roughly a thousand kilometres per second in a tightly bound orbit around its companion, and hundreds of years later that motion is still imprinted in the iron-rich material. We are lucky that this remnant is close enough for our instruments to detect it.” 

“Even though the plasma is millions of degrees hot, we detect the signal as faint optical light from atomic transitions of highly-ionized iron.”

Co-author, Dr Ruediger Pakmor from the Max Planck Institute for Astrophysics in Germany said the discovery resolves a decades-long mystery. 

"Type 1a Supernovae are some of the most frequent, brightest, and biggest explosions in the Universe,” Dr Pakmor said.

“Despite decades of detailed observations, we couldn't figure out how they explode or even what exactly exploded. 

“With the discovery, we finally not only can directly infer which stars exploded - two white dwarfs - but also consistently solve for all properties of the system prior to the explosion. 

“And the result is quite different from the current textbook answer of single-degenerate Chandrasekhar-mass explosions, which says a white dwarf star steals matter from a companion star until it reaches a critical mass and explodes."

The breakthrough was made possible by deep observations using the MUSE instrument on the European Southern Observatory's Very Large Telescope (VLT), which allowed scientists to map faint iron-rich debris from the explosion in unprecedented detail.

The discovery brings astronomers closer to solving a decades-old mystery about the origin of Type 1a supernovae and provides new insight into the stellar explosions that help us measure and understand the Universe.

The project involved researchers from the University of New South Wales Canberra, the Australian National University, the University of Manitoba, RIKEN Centre for Interdisciplinary Theoretical and Mathematical Sciences, Max Planck Institute for Astrophysics, Heidelberg Institute for Theoretical Studies, and the Centre for Astronomy at Heidelberg University.

The published research paper can be accessed .