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Our galaxy is surrounded by a graveyard of dead stars


Everything in the universe has a beginning and an end. And even stars need a resting place when they die. But where are all those stellar corpses? A team of scientists from the University of Sydney has investigated where this graveyard of stars may exist in the Milky Way, and has successfully managed, for the first time in history, to identify and map a stellar graveyard that far exceeds the plane of our own galaxy.

A star dies when it consumes all the hydrogen it has – in a few million years – and finally explodes as a supernova while its core remains collapsed into a neutron star or black hole. This is what remains in the graveyard of stars.


The first map of the ‘galactic underworld’

Researchers have more than aptly named this graveyard of stars the “galactic underworld.” Both objects, neutron stars and stellar black holes are difficult to detect, since the former have very few kilometers in diameter (about 15) and stellar black holes are even smaller and, furthermore, to increase the detection obstacles, They do not emit their own light.

“These compact remnants of dead stars show a fundamentally different distribution and structure than the visible galaxy,” explains David Sweeney of the Sydney Institute for Astronomy at the University of Sydney and lead author of the study published in the journal Monthly Notices of the Royal Astronomical. Society. “The ‘height’ of the galactic underworld is more than three times higher in the Milky Way itself and an astonishing 30 percent of objects have been ejected from the galaxy entirely.”

Although these cosmic objects should have formed billions of years ago since the galaxy was young, these exotic corpses were flung out into the darkness of interstellar space by the supernova that created them and thus slipped past sight and the knowledge of astronomers. Both black holes and neutron stars are therefore formed in the wake of a supernova, a cataclysmic explosion that occurs when massive stars collapse in on themselves at the end of their lifetimes. The powerful explosion launches these stellar remnants into dark regions of space, where they are elusive to astronomers. Yes indeed; We can already say that: until now.

How have they achieved it?

Using computer simulation , astronomers modeled the initial positions of millions of stars in the early Milky Way (long before its iconic spiral arms developed), and algorithmically recreated their future to show where the debris might have ended up. of those stars after having reached their useful life and thus they could predict where their corpses ended up.

The resulting map revealed a “galactic underworld” of black holes and neutron stars lurking in every corner of the Milky Way and even far beyond.

“The hardest problem I had to solve in looking for their true distribution,” Sweeney said, “was accounting for the ‘kicks’ they get at the violent moments of their creation. Supernova explosions are lopsided and debris is ejected at great speed , up to millions of kilometers per hour, and, what is worse, this happens in an unknown and random direction for each object”.

Looking at this graveyard of stars, scientists were amazed: “I expected the galactic underworld to be subtly different, but broadly similar,” said Sanjib Sharma, a researcher at the University of Sydney who helped map out the graveyard. “I didn’t expect such a radical change in form.”

The distribution of these stars is in a plane three times thicker than that of the visible Milky Way . About a third of these old dead stars are being ejected from the galaxy and, according to the model, will escape the gravitational pull of the Milky Way. In addition, another of the details revealed by the map of the ‘galactic underworld’ was that these stellar remains, which barely contain 1% of the total mass of the galaxy , are fairly uniformly distributed throughout the Milky Way; in essence, most stars should have a stellar corpse about a hundred light-years away.

“Statistically, our closest remnant should be only 65 light-years away : more or less in our backyard, in galactic terms,” experts say.

Referencia: David Sweeney et al, The Galactic underworld: The spatial distribution of compact remnants, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2092

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