The Milky Way Star Graveyard Unveiled

The galactic underworld does not take on the appearance a spiral like our galaxy, but it still roughly follows its shape.

A first map of the underground galactic world populated by dead stars of the Milky Way has been created from simulations carried out by Australian astrophysicists.

These star remnants show a different distribution and structure from those of the visible galaxy, David Sweeney, a doctoral student at the Institute of Astronomy Associated with the University of Sydney and lead author of the work, explains in a press release. published in the Monthly Notices of the Royal Astronomical Society (in English).

If the galactic world underground does not take on the spiral appearance of our galaxy, it nevertheless roughly follows its shape. More diffuse, this ghost galaxy extends over a region three times larger than the galactic disk.

• At the center of the galaxy is a bright nucleus, called a bulge, from which spiral arms extend into a huge flattened disk.
• At the center of the bulge is Sagittarius A*, a supermassive black hole.
• The bulge and disk are surrounded by a spherically shaped region called the halo.
• The galaxy includes over 100 billion stars, interstellar dust and gas.< /li>
• Our Sun is located on one of the spiral arms, about 27,000 light-years (1 AL = 9460 billion kilometers) from the center of the Milky Way.

The Milky Way observed by the Gaia telescope.

Map created by Australian astrophysicists shows where neutron stars and stellar black holes are found, which form when massive stars – more than eight times larger than our Sun – run out of fuel and collapse on top of them- themselves.

This runaway reaction causes the outer layers of massive stars to explode into supernovae, while their cores continue to compress on themselves until which, depending on their initial mass, they become a neutron star or a black hole, the statement explains.

Color representation of the visible Milky Way (right) and the galactic underworld (left).< /p>

It is largely its mass that determines whether a star will become a neutron star or a black hole.

In the first case, the nucleus is so dense that electrons and protons are forced to combine at the subatomic level to form neutrons, thus compressing its total mass into a sphere smaller than a city.

In the second case, when the mass of the original star is more than 25 times that of our Sun, its collapse continues until that the core is so dense that it turns into a black hole.

These two types of stellar corpses, which would represent barely about 1% of the the Milky Way, distort the space, time and matter around them.

Since the birth of the Milky Way, billions of stars have been born and died. The simulation indicates that nearly a third of these stars were thrown out of the galaxy and are now in the intergalactic medium.

These expulsions would represent about 40% of all neutron stars produced, but only 2% of stellar black holes during the entire evolution of the galaxy. […] In total, about 0.4% of the stellar mass of our Milky Way would have left it since its birth, the scientists note.

It is by carefully recreating the complete life cycles of extinct stars astrophysicists have compiled the first detailed map showing where their remains lie.

Initially, this work was not easy, since the scientists did not know where to look. The oldest neutron stars and black holes were created when the galaxy was younger and had a different shape. They were then subjected to complex changes spanning billions of years, says Professor Peter Tuthill, who is also authoring the study.

“The oldest neutron stars and black holes are like ghosts that still haunt a house demolished long ago. They are therefore more difficult to find. »

— Peter Tuthill, University of Sydney

PhD student David Sweeney adds that there is another difficulty that complicates our ability to establish the distribution of dead stars: supernova explosions are asymmetrical, so that the remains are ejected at high speed, up to millions of kilometers per hour, in random directions to each object.

“It's a bit like billiards. If you know which direction the ball is hitting, and with what force, then you can determine where it will land. But in space, objects and speeds are much larger. Also, the table is not flat, and star remnants follow complex orbits through the galaxy.

—David Sweeney

Also, there is no friction in space, so objects never slow down.

“Nearly all the remnants that formed there are still there, gliding like ghosts through interstellar space.

— David Sweeney

To successfully locate them, the team first used data collected from neutron stars and black holes recently discovered in the current galactic form.

She then coded the places of birth and death of older stars to estimate their dispersion during galaxy evolution.

The end result is a distribution map of the stellar necropolis of the Milky Way which surprises its creators since it does not quite correspond to the appearance of the visible Milky Way.

The Milky Way (top image) and the galactic underworld (bottom image).

In modeled maps (see images above), the characteristic spiral arms of the Milky Way (top image) have disappeared into the galactic underworld (bottom image ). These have disappeared entirely due to the age of most of the remains and the blurring effects associated with the explosions of the supernovae that created them.

David Sweeney now wants to use this new knowledge to develop even more accurate models of the galactic underworld.

The most exciting part of this research is still ahead of us. I bet the galactic underworld will not remain shrouded in mystery for long, rejoices the doctoral student.