Astronomers may have finally solved the mystery of how extremely dense dead stars called white dwarfs keep their heavy metal shells fresh by cannibalizing what’s left of their planetary systems.
Our Sun will turn into a white dwarf in about five billion years after exhausting the hydrogen that fuels nuclear fusion in its core. So the new research may give us a clue as to what might happen to the rest of the solar system after this transformation occurs.
White dwarfs form when stars like the Sun die, forming stellar remnants with masses about the same as the Sun and about the same width as the Earth. They are the most common stars in the Milky Way, accounting for 97% of stellar masses. However, despite their similarities in our galaxy, the chemical composition of white dwarfs remains a mystery. That’s because the surfaces of these stellar remnants are studded with elements heavier than helium, which astronomers call “metals.”
Now a team of scientists has discovered that heavier metals such as silicon, magnesium, and calcium find their way to the surface of white dwarfs as these zombie stars swallow small rocky bodies orbiting them, such as comets and asteroids. The researchers also identified the mechanism that white dwarfs use to feed themselves by accumulating planetoids.
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“The vast majority of planets in the universe will orbit a white dwarf,” study team member Ann-Marie Madigan, a professor of astrophysics and planetary sciences at the University of Colorado Boulder, said in a statement. “50% of these systems, including our own solar system, may have been eaten by their own stars. We now have a mechanism to explain why this would happen.”
How do white dwarfs remain heavy metals?
The initial discovery of heavy metals on the surface of white dwarfs was puzzling. Because when these stars turn from main sequence stars into white dwarfs, they must collapse and heavy metals must sink into the interior of these stellar remnants.
“If these heavy metals are present on the surface of the white dwarf, we know that the white dwarf is dense enough for these heavy metals to collapse towards the core very quickly,” said study team member Tatsuya Akiba, a doctoral candidate at the university. He’s from Colorado. “So unless the white dwarf is actively eating something, you shouldn’t see any metal on the surface of the white dwarf.”
This raises the question: How do these zombie stars snack on their environments in such a way that heavy metals are constantly replenished on their surfaces?
To investigate this, the team created computer simulations that recreated a white dwarf receiving the “birth hit” that these remnants received during their formation as a result of loss of material in a preferred direction. This changes the motion of the white dwarf and the dynamics of the material surrounding it.
“Simulations help us understand the dynamics of different astrophysical objects,” Akiba said. “So, in this simulation, we throw a bunch of asteroids and comets around the significantly larger white dwarf and see how the simulation evolves and which one of these asteroids and comets the white dwarf eats.”
In 80% of the team’s tests, the white dwarf’s prenatal kick altered the orbits of asteroids and comets by up to 240 times the distance between Earth and the sun. These changing orbits became further elongated and aligned with each other. They also found that 40% of planetoids eaten by a white dwarf had retrograde orbits, meaning they circled the stellar remnant in the opposite direction of its rotation.
The team let the simulation run for 100 million years and found that planets close to the white dwarf, about 30 times the distance between Earth and the sun (roughly the orbital distance of Neptune), remained in long orbits and began to move as a unit. .
“I think this is something unique about our theory: we can explain why accumulation events are so long-lasting,” Madigan said. “While other mechanisms could explain an original accretion event, our simulations of the kick show why it is still happening hundreds of millions of years later.”
The findings suggest that white dwarfs have heavy metals on their surfaces as these zombie stars move mindlessly, constantly consuming everything in their path, like the undead creatures in a George Romero movie.
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In the future, the team hopes to scale up their simulations to see what would happen if objects larger than comets and asteroids, such as planets, interacted with white dwarfs.
Until then, these findings reveal what’s going on around the most common stars in the Milky Way and serve as a crystal ball for peering into the solar system’s future.
“Planet fragments can give us insight into other solar systems and planetary compositions beyond where we live in our solar region,” McIntyre concluded. “White dwarfs are not just a lens looking into the past. They are also a lens looking into the future.”
The team’s research was published last month in The Astrophysical Journal Letters.