During the roughly 13.6 billion year history of the Milky Way, billions of stars have formed, grown and finally died in spectacular supernova explosions. So where are all their bodies hiding?
In a new study published Aug. 25 in the Monthly Bulletins of the Royal Astronomical Society, astronomers set out to unearth (sort of) these long-lost stellar bodies. Using a computer simulation, the team modeled the initial positions of millions of stars in early times Milky Way (long before its iconic spiral arms evolved), then press a cosmic fast-forward button to show where the shriveled post-supernova remnants of these stars may have landed.
The resulting map revealed a “galactic underworld”. black holes and neutron stars (two forms of extremely dense stellar remnants) lurking in every corner of the Milky Way—and far beyond. According to the researchers, the galactic underworld stretches more than three times the height of the Milky Way itself, while up to a third of the galaxy’s dead stars have been blasted deep into space by the force of their own end-of-life explosions, never to return.
“Supernova explosions are asymmetric, and the remnants are ejected at high speeds — up to millions of kilometers per hour,” said the study’s lead author David Sweeney, a PhD student at the University of Sydney, in a expression. “A staggering 30% of objects were completely ejected from the galaxy.”
The fast and the dead
In their research, the team focused on two types of stellar debris: neutron stars — ultradense stellar cores that pack the mass of a sun into a sphere no wider than a city — and black holes, which are giant objects so dense that not even light can escape their attraction.
Both types of objects are formed when massive stars run out of fuel and shed their outer layers gas in titanic supernova explosions when their cores collapse inward. If the dying star had at least eight times the mass of Earth Sun, a neutron star is born; If the star measured more than 25 solar masses, a black hole is formed.
Astronomers have discovered both classes of stellar debris in our galaxy, though not nearly enough to explain the billions of dead stars in the Milky Way’s past. Finding these ancient remains is difficult for two main reasons: first, the shape of the Milky Way has changed significantly over the past 13 billion years, meaning the galactic underworld does not exactly overlap the current distribution of stars in our galaxy; and second, stars dying from a supernova can be “hurled” long distances in random directions by the force of the explosion and end up on the fringes of the galaxy or lost in intergalactic space.
The study authors created a computer simulation to account for this randomness, as well as the changing shape of the Milky Way and many other factors. Their results showed that the greatest concentration of stellar debris is near the galaxy’s center, where a supermassive black hole exerts an extremely strong gravitational pull. The rest of the dead stars are scattered wildly to all sides of the galaxy, in contrast to the spiral distribution of stars seen today.
“These compact remnants of dead stars show a fundamentally different distribution and structure than the visible galaxy,” Sweeney added.
The team also found that although the galactic underworld contains only an estimated 1% of the galaxy’s total mass, ancient stellar bodies are never far away. The nearest star remnant should only be around 65 light years from the Sun – or closer to us than the stars of the Big Dipper constellation. With a better idea of where to look for them, hopefully space surveys like the European Space Agency’s ongoing Gaia mission can help exhume the galaxy’s ancient dead in greater numbers than ever before.
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