If the universe has one lesson for humanity, it is that everything ends. That includes stars, which too must die, albeit on timescales of billions of years. But new research suggests that when some stars die, they do not do so alone, potentially solving a long-standing mystery around a particular class of cosmic explosion called an interacting supernova.
When stars much more massive than the sun reach the ends of their lives, their cores collapse, sending shockwaves blasting out into their outer layers, triggering explosions called supernovas and leaving behind stellar remnants in the form of neutron stars or black holes. Interacting supernovas differ because the shockwave generated by these explosions crash into a pre-existing cocoon of material. The big mystery has always been: where does this cocoon of gas and dust come from?
Humanity is somewhat biased when it comes to stars; after all, the sun dominates our existence, and it is a solitary stellar body. But contrary to this, the majority of stars aren’t so antisocial, existing in binary partnerships bound together by gravity. This new research suggests that these stars don’t just live together; they can die together too. Understanding this dual existence could be key to solving the origins of dust shrouds in interacting supernovas.
“Our study suggests that many stars do not die alone,” team member Ke-Jung Chen, of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), said in a statement. “Their final appearance may be shaped by a long and intimate partnership with a companion star.”
How somes stars become a drain
Before stars reach the ends of their lives, they enter a relatively short-duration red giant phase. This can see them swell out to hundreds or even thousands of times their original radius.
For a binary stellar pairing, this leads to a situation called “roche lobe overflow,” which basically sees the swollen-out star spilling material onto its companion. However, not all of that material is captured by the companion star, escaping to form a vast cocoon around the binary stars.
When the evolved and swollen star reaches the end of its life and “goes nova,” the shockwaves ripple forward and slam into this cocoon of matter at speeds of thousands of miles per second. The kinetic energy becomes light, creating a strange and intensely bright interacting supernova.
That leaves an obvious question, however. If stellar binaries are so common, and become even more common for stars massive enough to go supernova, why aren’t interacting supernovas more common?
Turns out, just like with comedy, the secret is … timing.
Chen and colleagues ran hundreds of computer simulations of mass transfer between binary stars and discovered that the key to generating an interacting supernova is when this mass transfer occurs late in the stars’ lives.
If mass transfer occurs too early, say millions of years before the final supernova blast, the team found that the material spreads far away from the binary stars, dissipating the surrounding cocoon. For the cocoon to hang around for shockwaves to strike, mass transfer has to occur just a few thousand years before the final explosive death throes of one of the binary stars.
“We found that binary stars can prepare the stage for interacting supernovas with remarkable timing,” team member Sung-Han Tsai of ASIAA said. “The companion star helps create a dense cocoon around the dying star just before the explosion, providing the fuel that powers these cosmic fireworks.”
The team’s research shows that there are many ways for stars to die, and these explosive fates are determined by the way they lived.
The team’s research was published on June 30 in The Astrophysical Journal Letters.