When a massive star reaches the end of its life, it explodes in a powerful event called a supernova.
These explosions are among the most energetic events in the universe. However, not all supernovae behave the same way.
Some fade away fairly quickly, while others remain incredibly bright for months or even years. For decades, astronomers have wondered why.
A new study by researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) may finally have the answer.
Instead of looking at a single dying star, the team focused on pairs of stars that orbit each other.
Their findings suggest that many of the brightest supernovae are actually created by two stars working together.
Most massive stars are not alone. They are born with a companion star, and the two spend millions of years circling each other under the pull of gravity. During most of their lives, they quietly orbit together. But as one star grows old, the relationship becomes much more dramatic.
Near the end of its life, the larger star expands until it becomes hundreds or even thousands of times bigger than our Sun.
As it swells, some of its outer gas begins to flow toward its companion star. While the companion captures part of this material, much of the gas escapes into space instead. Over time, this creates a large cloud, or cocoon, of gas surrounding both stars.
Only a few thousand years later, which is a very short time in the life of a star, the larger star explodes as a supernova. The explosion sends material racing outward at thousands of kilometers per second. When this fast-moving debris crashes into the nearby cocoon of gas, an enormous amount of energy is released as light. This extra energy allows the supernova to shine much more brightly and for much longer than a typical stellar explosion.
To better understand this process, the research team ran hundreds of computer simulations. They found that the timing is the key. If the gas had been released millions of years earlier, it would have drifted too far away before the explosion happened. Instead, the final exchange of material takes place only a few thousand years before the star dies, leaving the gas close enough for the supernova to collide with it.
The researchers estimate that this process may explain about one out of every eight core-collapse supernovae. This means these dramatic explosions may be much more common than scientists once believed.
The study may also explain unusual supernovae such as SN 2014C. This explosion first looked like a normal supernova but unexpectedly became brighter months later.
The new model suggests that its expanding debris eventually struck a shell of gas that had been created during the star’s final interaction with its companion centuries or even thousands of years earlier.
The findings show that many massive stars do not end their lives alone. Instead, their final moments are shaped by a close companion that has shared their journey for millions of years.
In many cases, the brightest supernovae are not the story of one dying star, but the spectacular final performance of two stars completing one last cosmic dance together.
