“All good things must come to an end.” This adage is true in space as well as on Earth.
We are aware 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 and die to form a dense cosmic remnant like a white dwarf, neutron star, or black hole. Our own star, the Sun, will meet this fate in about 5 billion years, first erupting as a red giant and obliterating the inner planets, including Earth. After about 1 billion years, this phase will also end, leaving the Sun’s core as a carbon white dwarf surrounded by a cloud of cosmic ash in the form of cooling stellar material.
Scientists developed the Hertzsprung-Russell diagram, a chart of stellar life, afterlife, and death. This diagram traces stars of all masses through their evolution from hydrogen-burning main-sequence stars to dense cosmic debris.
But new research has revealed that some stars at the heart of our galaxy may be defying our best models of stellar life and death. These stars could feed on dark matter, the most mysterious matter in the universe, to effectively ensure cosmic immortality, necessitating the creation of the “dark Hertzsprung-Russell diagram”.
Related: Across the universe, the destruction of dark matter could heat dead stars
“The galactic center of the Milky Way is a very extreme environment and very different from our location 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 so-called ‘S cluster stars’, are very mysterious.
“They show a number of properties not found anywhere else: It is not clear how they got so close to the center, where the environment is thought to be rather hostile to star formation.”
John added that these S-cluster stars, which lie about three light-years from the very heart of our galaxy, also appear to be much younger than would be expected if stars had migrated into the region from elsewhere in the Milky Way. “Even more puzzling, not only do the stars look unusually young, but there are fewer older stars than expected,” she continued. “Also, there seem to be an unexpected number of heavy stars.
John and colleagues hypothesize that the reason for these unusual phenomena may be that these stars collect a large amount of dark matter, which then annihilates them. This process could provide them with an entirely new and unexpected form of fuel.
“Our simulations show that stars can only survive 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 stars in the S cluster: Dark matter as fuel keeps the stars young forever.”
“The idea of immortal stars,” John continued, “can explain many of the unusual properties of stars in the S cluster at once. the large number of apparently young stars in the galactic center while also explaining the lack 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 does not interact with light. Additionally, dark matter does not appear to interact with “ordinary matter”. This everyday matter consists 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 indeed light. However, if interactions between dark matter and ordinary matter do occur, they are rare and weak; scientists don’t believe we’ve ever seen such an interaction.
Less certain is whether dark matter interacts with itself. To understand what this means, remember that all ordinary matter particles have a version of antimatter 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 and release energy.
“The annihilation of dark matter is analogous to the annihilation of matter and antimatter: if a particle and its antiparticle meet, they annihilate and create other particles, such as photons. Similarly, dark matter particles could annihilate in such a way,” John said. . “In many dark matter models, dark matter particles are treated as their own antiparticles, meaning that any two dark matter particles can annihilate each other.”
However, we don’t see dark matter annihilation, so it must be quite rare. This means, John says, that it would be more likely to occur in an environment where huge amounts of dark matter can be squeezed in. Perhaps the ultradense region at the heart of the star is where the gravity with which dark matter interacts is strongest.
Could even the sun become immortal?
Main sequence stars burn hydrogen in nuclear fusion processes during their lifetime. This creates helium, most of the star’s energy, and an outward “radiation pressure” that balances the inward pressure of the star’s own 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 a stable equilibrium.
“For most of a star’s life, these processes take place primarily in the star’s core, where the gravitational pressure is highest,” John said. “We show that if stars accumulate a large amount of dark matter, which then annihilates inside the star, it can also provide an external pressure, making the star stable due to dark matter annihilation rather than nuclear fusion. So stars can use dark matter as fuel instead of hydrogen.
“Stars use up their hydrogen, which eventually causes their death. On the other hand, 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 thinks not. Located in the middle of one of the Milky Way’s spiral arms, it is in the wrong place in our galaxy to reach this dark fountain of youth.
“Stars need very large amounts of dark matter to effectively replace fusion. In most of the Milky Way, the dark matter density is not high enough to significantly affect the stars. But in the galactic center, the dark matter density is very high, potentially many billions of 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 potentially hint at many secrets about dark matter itself, as well as the immortal stars it might power.
“Our findings tell us that dark matter can be scattered by ordinary particles, which is necessary to slow down dark matter particles inside the star to capture them – also that dark matter particles can annihilate each other,” she said. “By observing the distribution of immortal stars around the galactic center, we would also get some 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 lie on the “dark main sequence,” which could indicate their immortality.
They also intend to determine the effect of dark matter extinction on various stars. Initial simulations suggest that lighter stars would become “bloated” and shed their outer layers as they transitioned to this dark fuel. This could explain the nature of the so-called “G-objects” found in the Galactic Center, which are stellar bodies that appear to be surrounded by clouds of gas.
“Until now, our work has focused on main-sequence stars. We also want to understand how dark matter affects stars at later evolutionary stages, when they have moved away from the main sequence and gone through various nuclear fusion processes,” Johns said. “Our results are exciting because they show that observing stars offers another and unique way to study and understand the interactions of dark matter with ordinary matter.”
A preliminary peer-reviewed version of the team’s research is available on the arXiv paper repository.