A new study reveals the first map of the “galactic underworld”—the corpses of suns that have collapsed into black holes and neutron stars—a graveyard stretching three times the height of the Milky Way, of which it is estimated that nearly a third of the objects have been flung out of the galaxy entirely.
For his part, David Sweeney, doctoral student at the Institute of Astronomy of the University of Sydney, and lead author of the article, published in the Royal Astronomical Society of London, explained: “These compact remnants of dead stars show a distribution and structure fundamentally different from those of the visible galaxy.
“The ‘height’ of the galactic underworld is more than three times higher than the Milky Way itself,” he added. “And a staggering 30 percent of the objects have been ejected from the galaxy entirely.”
In its communication, the University of Sydney described that neutron stars and black holes form when massive stars, more than eight times greater than our Sun, run out of fuel and suddenly collapse.
This, experts say, triggers a runaway reaction that blows up the outer parts of the star in a titanic supernova explosion, while the core continues to compress on itself until, depending on its initial mass, it becomes a star. of neutrons or in a black hole.
In neutron stars, the core is so dense that electrons and protons are forced to combine at the subatomic level into neutrons, compressing their total mass into a sphere smaller than a city, the study details.
Scientists point out that if the mass of the original star is greater than 25 times that of our Sun, this gravity-driven collapse continues until the core is so dense that not even light can escape. Consequently, both types of stellar corpses warp space, time, and matter around them.
“Although they must have fosigned mile“In the millions since the galaxy was young, these exotic corpses were flung into the darkness of interstellar space by the supernova that created them, thus slipping beyond the sight and ken of astronomers – until now.”
By carefully recreating the entire life cycle of ancient dead stars, researchers have constructed the first detailed map showing where their corpses lie.
“One of the problems with finding these ancient objects is that, until now, we had no idea where to look,” said Peter Tuthill, a professor at the Institute for Astronomy in Sydney, who co-authored the paper. “The oldest neutron stars and black holes were created when the galaxy was younger and had a different shape, and then underwent complex changes spanning billions of years. It has been an important task to model all this to find them.”
The University of Sydney explains that newly formed neutron stars and black holes fit into the current galaxy, so astronomers know where to look. But the oldest neutron stars and black holes are like ghosts still haunting a house long since demolished, so they are harder to find.
“It was like trying to find the mythical elephant graveyard,” said Professor Tuthill, referring to a place where, according to legend, old elephants go to die alone, away from their herd. “The bones of these rare massive stars had to be out there, but they seemed to be shrouded in mystery.”
Sweeney added: “The hardest problem I had to solve in pursuing their true distribution was accounting for the ‘kicks’ they get in the violent moments of their creation. Supernova explosions are asymmetric and the debris is expelled at high speed, up to millions of kilometers per hour, and what is worse, this happens in an unknown and random direction for each object.
Experts say that nothing in the universe sits still for long, so even knowing the likely magnitudes of the explosive kicks wasn’t enough: The researchers had to go deep into cosmic time and piece together how they behaved over thousands of years. millions of years.
“It’s a bit like pool,” Sweeney said. “If he knows which way the ball is hit and with what force, then he can figure out where it will end up. But in space, objects and speeds are much larger. Also, the table is not flat, so the stellar debris follows complex orbits through the galaxy.”
“Finally, unlike a pool table, there is no friction, so they never slow down. Almost all of the debris that formed is still out there, gliding like ghosts through interstellar space.”
The intricate models they built, together with University of Sydney researcher Dr Sanjib Sharma and Monash University’s Dr Ryosuke Hirai, encoded where stars were born, where they met their fiery end and their eventual dispersal as that the galaxy evolved.
The final result is a distribution map of the stellar necropolis of the Milky Way.
“It was a bit of a shock,” Dr. Sharma said. “I work every day with images of the visible galaxy we know today, and I expected the galactic underworld to be subtly different, but broadly similar. I did not expect such a radical change.”
In the generated maps (see images above), the characteristic spiral arms of the Milky Way (top image) have faded into the ‘galactic underworld’ (bottom image). These are completely obliterated due to the age of most of the remnants and the blurring effects of the energetic pulses from the supernovae that created them.
Perhaps most surprisingly, the side view shows that the galactic underworld is much more swollen than the Milky Way, as a result of the kinetic energy injected by supernovae lifting them into a halo around the visible Milky Way.
“Perhaps the most surprising finding of our study is that the kicks are so strong that the Milky Way will miss some of this debris entirely,” said Dr Hirai. “They are kicked so hard that about 30 percent of neutron stars are flung out into intergalactic space, never to return.”
Tuthill added: “For me, one of the best things we found in this work is that even the local stellar neighborhood around our Sun probably has these ghostly visitors passing through. Statistically, our closest remnant should be only 65 light-years away: more or less in our backyard, in galactic terms.”
“The most exciting part of this research is still ahead,” Sweeney said. “Now that we know where to look, we are developing technologies to look for them. I bet the ‘galactic underworld’ won’t remain shrouded in mystery for much longer.”