How to kill a star? – 06/24/2023 – Science

Astronomers have detected an immensely energetic explosion emanating from an ancient galaxy, apparently triggered by a type of stellar destruction theorized for decades but never before observed. It could be called a stellar death by demolition contest.

The researchers said the gamma-ray burst could have been caused by two compact stars colliding in the densely populated and chaotic environment near a supermassive black hole at the center of this elliptical-shaped galaxy. They suspect that the two doomed stars were neutron stars, which pack roughly the mass of our Sun into a sphere the size of just one city.

“To explain the gamma-ray burst, it must have been a compact star, so not like the Sun,” said astronomer Andrew Levan of Radboud University in the Netherlands, lead author of the research published this week in the journal Nature Astronomy.

“Gamma-ray bursts are the most powerful in the universe. They release more energy per unit time than any other known cosmic phenomenon. So they are really superlative in their properties. They are named after the first type of light we see, rays range, but they actually emit across the entire electromagnetic spectrum,” said astrophysicist and study co-author Wen-fai Fong of Northwestern University in Illinois.

The immense gravitational forces exerted by the black hole at the center of the galaxy can wreak havoc, disrupting the motion of nearby stars and other objects and increasing the chances of collisions — looking, the researchers say, like a wrecking contest.

“Most stars in the universe die in a predictable way, which is just based on their mass,” Levan said. “This research shows a new route to stellar destruction.”

Very massive stars—more than ten times the mass of the Sun—die in a supernova explosion that leaves behind neutron stars or even denser black holes, whose gravitational pull is so strong that no matter or light can escape. Relatively low-mass stars like our Sun swell and explode their outer layers, turning into a stellar remnant called a white dwarf.

The new discoveries show another path to stellar demise.

“The idea that stars can also die through collisions in extremely dense regions has been around since at least the 1980s. We waited 40 years for clear signals to be found observationally,” said Levan.

The researchers used data from orbital and ground-based telescopes to study the gamma-ray burst in a galaxy about 3 billion light-years from Earth, roughly in the constellation of Aquarius. A light year is the distance that light travels in one year, 9.5 trillion km.

This ancient galaxy was mostly populated by multi-billion-year-old stars.

“The galaxy is what we call a ‘quiescent’ — a galaxy that is not actively forming stars at a high rate and has passed its zenith,” Fong said. “These quiescent galaxies are very massive and have built large supermassive black holes at their centers, making them a perfect field for stellar collisions.”

The distance between our Sun and the nearest star, Proxima Centauri, is about four light years. This same expanse of space would be filled with perhaps 10 million stars in a galactic core, with the destabilizing influence of the supermassive black hole stirring things up.

“You certainly wouldn’t want a front row seat to one of these events,” said Levan.

“But if you were close enough, you would see the two neutron stars getting closer and closer until their gravity deforms them and they start to tear apart,” Levan added. “Then the nuclei of stars would merge to form a black hole, surrounded by a disk of leftover material. A fraction of a second later, that material would flow into the black hole, and a jet of material moving at 99.99% of the speed of light would be released”, representing the burst of gamma rays.