“Every good thing comes to an end.” This saying is valid both in the universe and on Earth.
We realize that stars, like everything else, must die. When the fuel supply for nuclear fusion in their cores is exhausted, stars of all sizes collapse under their own gravity and die to form a dense cosmic remnant such as a white dwarf, neutron star, or black hole. Our star, the Sun, will meet this fate in about 5 billion years, first swelling into a red giant and destroying the inner planets, including Earth. After about 1 billion years, this phase will also end, leaving the Sun’s core as a white dwarf ember surrounded by a cloud of cosmic ash in the form of cooling stellar material.
Scientists have developed the Hertzsprung-Russell diagram, which shows the life of stars, afterlife and death. This diagram traces the evolution of stars of all masses, from main sequence hydrogen-burning stars to dense cosmic remnants.
But new research has revealed that some stars at the heart of our galaxy may be turning their noses up at our best models of stellar life and death. These stars may be feeding on dark matter, the most mysterious substance in the universe, giving themselves cosmic immortality, which may require the creation of a “dark Hertzsprung-Russell diagram.”
Relating to: Dark matter extinction everywhere in the universe may be heating dead stars
“The Galactic Center of the Milky Way is a very extreme environment and very different from where we are in the Milky Way,” research team leader Isabelle John of the Kavli Institute for Particle Astrophysics and Cosmology told Space.com. “The stars closest to the Galactic Center, the ‘S-cluster stars,’ are very surprising.
“They show a set of features found nowhere else: It is unclear how they got so close to the centre, where the environment is thought to be quite hostile to star formation.”
John added that these S cluster stars, located about three light-years away in the heart of our galaxy, appear to be much younger than would be expected if the stars had migrated to this region from elsewhere in the Milky Way. “What’s even more mysterious is that not only do the stars appear unusually young, but there are also fewer old stars than expected,” he continued. “There also seem to be an unexpectedly large number of massive stars.”
John and his colleagues suggest that the reason for these unusual features may be that these stars accumulate large amounts of dark matter, which is then destroyed within them. This process could provide them with a completely new and unexpected type of fuel.
“Our simulations show that stars can survive with only dark matter as fuel, and because there is such a large amount of dark matter near the Galactic Center, these stars become immortal,” John added. “This is quite impressive because our simulations show similar results to observations of S-cluster stars: dark matter as fuel will keep the stars young forever.”
“The idea of immortal stars,” John continued, “could simultaneously explain many of the unusual properties of S-cluster stars. If stars in the Galactic Center become immortal due to the high density of dark matter, this could explain the unusual situation.” “The abundance of apparently young stars in the Galactic Center also explains the absence of older stars.”
Dark matter is its own worst enemy
Dark matter is a problem for physicists because it makes up an estimated 85% of the universe and is invisible to us because it doesn’t interact with light. Dark matter also doesn’t seem to interact with “ordinary matter.” This everyday matter is made up of protons, neutrons, and electrons and includes all the stars, planets, moons, asteroids, comets, gases, dust, and living things in the universe.
Scientists can only infer the existence of dark matter because it interacts with gravity, and this interaction can affect ordinary matter and, in fact, light. However, if interactions do occur between dark matter and ordinary matter, they are rare and weak; scientists do not believe we have ever detected such an interaction.
What’s less certain is whether dark matter interacts with itself. To understand what this means, recall that ordinary matter particles all have an antimatter version of themselves. For example, for a negatively charged electron, there’s a positively charged antiparticle called a positron. And when matter and antimatter meet, they annihilate each other, releasing energy.
“Dark matter annihilation is similar to the annihilation of matter and antimatter: if a particle and its antiparticle meet, they annihilate and produce other particles, such as photons. Similarly, dark matter particles can annihilate in this way,” John said. “In many dark matter models, dark matter particles are considered to be their own antiparticles, meaning that any two dark matter particles can annihilate with each other.”
But we don’t see dark matter annihilation, so it must be pretty rare. John said it’s more likely to happen in an environment where a lot of dark matter can clump together. Perhaps the ultra-dense region at the heart of a star is where gravity is strongest, where dark matter interacts.
Could the sun also be immortal?
Main sequence stars burn hydrogen in nuclear fusion processes throughout their lifetimes. This creates helium, the majority of the star’s energy, and outward “radiation pressure” that balances the inward push of the star’s own gravitational forces. This cosmic tug of war between radiation pressure and gravity lasts for millions, even billions of years, and keeps these stars in stable balance.
“For most of a star’s life, these processes occur primarily in the star’s core, where gravitational pressure is highest,” John said. “We have shown that if stars collect large amounts of dark matter and this then disappears inside the star, this can also provide an outward pressure and stabilize the star due to the annihilation of dark matter rather than nuclear fusion. So stars can use dark matter as fuel instead of hydrogen.” .
“Stars run out of hydrogen, which eventually causes them to die. Dark matter, on the other hand, can continually accrete, making these stars immortal.”
So, can the Sun grant itself immortality by switching to this alternative fuel source? John thinks no. Located in the middle of one of the Milky Way’s spiral arms, the Sun is in just the wrong spot in our galaxy to tap into this dark fountain of youth.
“Stars need very large amounts of dark matter to effectively switch fusion. In most of the Milky Way, the density of dark matter is not high enough to significantly affect stars. But in the Galactic Center, the density of dark matter is very high, potentially billions of times greater than on Earth.” “This provides the amount of dark matter needed to make stars immortal,” Jon explained. “So our sun is not immortal.”
John added that the team’s findings could reveal many secrets about dark matter itself and the immortal stars it could power.
“Our findings tell us that dark matter can disperse with ordinary particles, which is necessary to slow down and capture dark matter particles inside the star – and that dark matter particles can also annihilate with each other,” he said. “By observing the distribution of immortal stars around the Galactic Center, we can also learn about the distribution and density of dark matter around the Galactic Center.”
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To confirm these findings, astronomers need more precise observations to determine whether the Milky Way’s innermost stars are located on a “dark main sequence,” which could hint at their immortality, John explained.
They also aim to determine the impact of dark matter extinction on different stars. Early simulations show that lighter stars will become “bloated” and shed their outer layers as they switch to this dark fuel. This may explain the nature of so-called “G-objects”, which are stellar bodies found in the Galactic Center that appear to be surrounded by clouds of gas.
“So far, our work has focused on main sequence stars. We also want to understand how dark matter affects stars in later stages of evolution, when they move away from the main sequence and undergo different nuclear fusion processes,” Johns said. “Our results are exciting because they show that stellar observations offer an additional and unique way to study and understand dark matter’s interactions with ordinary matter.”
A pre-peer-reviewed version of the team’s research is available on the arXiv paper repository.