Astronomers have made a groundbreaking discovery that provides compelling, albeit indirect, evidence for the existence of a theoretical class of supernovas so immensely powerful they completely obliterate the universe's largest stars, leaving absolutely nothing behind. This defies the typical stellar remnants of neutron stars or black holes that usually form after such cosmic explosions.
The Search for Cosmic Cataclysms
While the explosive death of a star, known as a supernova, typically blasts material into space before collapsing into a compact stellar core, some of the cosmos's most massive stars may undergo a far more destructive fate. The existence of these ultra-powerful supernovas has been theorised since the 1960s, and new research now offers significant evidence through studies involving black holes and gravitational waves.
Massive Stars and Their Violent Ends
These cataclysmic events are predicted to occur in the most enormous stars, those with a mass approximately 140 to 260 times greater than our sun. The findings were detailed in a study published in the journal Nature, led by Hui Tong, a doctoral student in astrophysics at Australia's Monash University.
"Despite their enormous mass, they live relatively short lives, about a few million years," Tong explained. "For comparison, the sun will live for about 10 billion years, so these stars burn out roughly a thousand times faster - like a massive firework that burns intensely and briefly before exploding."
The Missing Black Hole Mystery
In this comprehensive study, researchers meticulously combed through data on 153 pairs of black holes, knowing their mass based on gravitational waves they emitted. They then separated out black holes that had formed through earlier mergers of two smaller black holes.
What the researchers discovered was particularly intriguing: an absence of black holes between about 44 and 116 times the mass of the sun, what they termed a "forbidden range." This absence, they argue, may best be explained if the largest stars, which might be expected to leave behind black holes in that mass range, instead were completely obliterated at the end of their lifespan in a rare type of explosion called a pair-instability supernova.
Understanding Pair-Instability Supernovas
"A pair-instability supernova is one of the most violently explosive types of stellar deaths," said astrophysicist and study co-author Maya Fishbach of the University of Toronto's Canadian Institute for Theoretical Astrophysics.
"For the most part, massive stars make black holes. The more massive the star, the heavier the black hole," Fishbach explained, until stars reach a certain mass threshold beyond which the physics of their explosive demise dictates that there is no stellar remnant left behind whatsoever.
The Physics of Complete Destruction
These enormous stars evolve similarly to other massive stars initially, burning hydrogen and helium and building up a large core made mostly of carbon and oxygen. For the core to remain stable, there needs to be a delicate balance between the inward pressure of gravity and the outward release of energy—in the case of these stars, high-energy photons, the particles that make up light.
However, at the extreme temperatures present inside these stars, some of the photons convert into pairs of subatomic particles called electrons and positrons, thus weakening the outward pressure that was helping to maintain the core's stability. These particle pairs and the instability they cause explain the name of this supernova class.
"The core becomes unstable, leading to a runaway collapse and then a violent thermonuclear explosion that blows the star apart," Tong described the process.
Rare Cosmic Events
While these supernovas were first predicted six decades ago, Fishbach noted that "they are rare and difficult to find and identify." Scientists have observed a type of stellar explosion called a superluminous supernova that is a candidate for being a pair-instability supernova. These explosions can be more than 10 billion times more luminous than the sun.
For now, the evidence presented in this study may be the best indication yet about the existence of pair-instability supernovas. As Tong eloquently summarized: "We are essentially using something invisible, black holes, as a record of some of the brightest explosions in the universe."
This research represents a significant step forward in understanding the most extreme stellar phenomena in our universe, revealing how the largest stars meet their spectacular ends without leaving any trace of their former existence.
