“All good things must come to an end.” This adage is true in the cosmos as well as on Earth.
We know that stars, like everything else, must die. When they run out of the fuel needed for nuclear fusion in their cores, stars of all sizes collapse under their own gravity, dying to form a dense cosmic remnant such as a white dwarf, neutron star, or black hole. Our own star, the Sun, will meet this fate in about 5 billion years, first turning into a red giant and annihilating 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 matter.
Scientists have developed the Hertzsprung-Russell diagram, a chart of the life, afterlife, and death of stars. This diagram tracks stars of all masses throughout their evolution, from hydrogen-burning main-sequence stars to dense cosmic remnants.
However, new research has revealed that some stars at the heart of our galaxy may be flouting our best models of stellar life and death. These stars could be feeding on dark matter, the most mysterious substance in the universe, to effectively grant themselves cosmic immortality, necessitating the creation of a “Hertzsprung-Russell dark diagram.”
Related: In the universe, the annihilation of dark matter could heat up dead stars
“The galactic center of the Milky Way is a very extreme environment and very different from our location in the Milky Way,” Isabelle John, leader of the research team at the Kavli Institute for Human Astrophysics, told Space.com. particles and cosmology. “The stars closest to the Galactic Center, called “S cluster stars,” are very confusing.
“They show a series of properties that are not found anywhere else: we do not know how they got so close to the center, where the environment is considered rather hostile to star formation.”
John added that these stars in the S cluster, which lie about three light years from the very heart of our galaxy, also appear much younger than would be expected if they had migrated to this region from other parts of the Milky Way. “What’s even more mysterious is that not only do the stars appear unusually young, but there are also fewer older stars than expected,” she continued. “Plus, there seem to be an unexpected number of heavy stars. »
John and his colleagues speculate that these unusual features could be explained by the fact that these stars accumulate large amounts of dark matter, which then annihilates within them. This process could provide them with an entirely new and unexpected form of fuel.
“Our simulations show that stars can survive solely on dark matter as fuel, and because there is an extremely large amount of dark matter near the Galactic Center, these stars become immortal,” John added. “This is quite fascinating because our simulations show similar results to observations of S cluster stars: dark matter, as a fuel, will keep stars eternally young.”
“The idea of immortal stars,” John continues, “can explain many unusual properties of S cluster stars at once. If stars in the galactic center become immortal due to the high density of dark matter, this can explain the unusually high abundance of apparently young stars at the galactic center while simultaneously explaining the absence of older stars. »
Dark matter is its own worst enemy
Dark matter is a problem for physicists because, making up about 85% of the universe, it is invisible to us because it does not interact with light. Furthermore, dark matter does not appear to interact with “ordinary matter.” This ordinary matter is composed of protons, neutrons, and electrons and includes all the stars, planets, moons, asteroids, comets, gas, dust, and living things in the universe.
Scientists can only infer the presence of dark matter because it interacts with gravity, and this interaction can affect ordinary matter and even light. If interactions between dark matter and ordinary matter do occur, however, they are rare and weak; scientists do not believe we have ever detected such an interaction.
What is less certain is whether dark matter interacts with itself. To understand what this means, remember that ordinary matter particles all have an antimatter version of themselves. For example, there is a positively charged antiparticle called a positron for a negatively charged electron. And when matter and antimatter meet, they annihilate each other, releasing energy.
“Dark matter annihilation is analogous to the annihilation of matter and antimatter: if a particle and its antiparticle meet, they are destroyed and produce other particles, such as photons. Similarly, dark matter particles could annihilate in this way,” John said. “In many models of dark matter, dark matter particles are considered to be their own antiparticle, meaning that two dark matter particles can annihilate with each other.”
However, we don’t see dark matter annihilation, so it must be quite rare. That means, John says, that it would be more likely to happen in an environment where huge amounts of dark matter can be packed together. Perhaps the ultradense region at the core of a star is where gravity, with which dark matter interacts, is strongest.
Could the sun also become immortal?
Main sequence stars burn hydrogen throughout their lives in nuclear fusion processes. This creates helium, the majority of the star’s energy, and the outward “radiation pressure” that balances the inward push of the star’s gravitational forces. This cosmic tug-of-war between radiation pressure and gravity lasts for millions or even billions of years and keeps these stars in stable equilibrium.
“For most of a star’s life, these processes occur primarily in the core of the star, where the gravitational pressure is greatest,” John said. “We show that if stars collect a large amount of dark matter, which then annihilates inside the star, this can also provide outward pressure, making the star stable due to the annihilation of dark matter rather than nuclear fusion Thus, stars can use dark matter as fuel instead of hydrogen.
“Stars consume their hydrogen, which will eventually cause them to die. In contrast, dark matter can be collected continuously, making these stars immortal. »
So could the sun grant itself immortality by switching to this alternative fuel source? John doesn’t think so. Located halfway along one of the Milky Way’s spiral arms, it’s in the wrong place in our galaxy to access this dark fountain of youth.
“Stars need very large amounts of dark matter to effectively replace fusion. In most of the Milky Way, the density of dark matter is not high enough to affect stars significantly. But at the Galactic Center , the density of dark matter is very high, potentially several billion times higher than on Earth, which 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 as well as the immortal stars it might power.
“Our results tell us that dark matter can scatter with ordinary particles, which is necessary to slow down the dark matter particles inside the star in order to capture them. In addition, dark matter particles can annihilate each other,” she said. “By observing the distribution of immortal stars around the galactic center, we would also gain information about the distribution and density of dark matter around the galactic center.”
John explained that to verify these findings, astronomers need more precise observations of the Milky Way’s innermost stars to determine whether these stars are in a “dark main sequence,” which could indicate their immortality.
They also intend to determine the effect of dark matter annihilation on different stars. Early simulations indicate that brighter stars would become “bloated” and lose their outer layers as they transition to this dark fuel. This could explain the nature of the “G objects” discovered at the galactic center, which are stellar bodies that appear to be surrounded by clouds of gas.
“Our work so far has focused on main sequence stars. We also want to understand how dark matter affects stars at 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 the interactions of dark matter with ordinary matter. »
A pre-peer-reviewed version of the team’s research is available on the arXiv paper repository.