“All good things must come to an end.” That adage applies both in the cosmos and on Earth.
We are aware that stars, like everything else, must die. When the fuel supply needed for nuclear fusion in their cores runs out, stars of all sizes collapse under their own gravity and die out, forming a dense cosmic remnant such as a white dwarf, a neutron star, or a black hole. Our own star, the Sun, will suffer this fate in about 5 billion years, first dying out as a red giant and destroying the inner planets, including Earth. After about 1 billion years, this phase will also end, leaving the core of the Sun as a white dwarf surrounded by a cloud of cosmic ash in the form of cooling stellar material.
Scientists have developed the Hertzsprung-Russell diagram, a map of the life, afterlife and death of stars. The diagram tracks stars of all masses as they evolve from main-sequence hydrogen-burning stars to dense cosmic remnants.
But new research has revealed that some stars at the heart of our galaxy are turning their noses up at our best models of stellar life and death. These stars may be feeding on dark matter, the most mysterious stuff in the universe, effectively granting themselves cosmic immortality, necessitating the creation of a “dark Hertzsprung-Russell diagram.”
Related: Across the universe, the destruction 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,” 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 range of properties not found elsewhere: it’s not clear 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, which are within about three light-years of the heart of our Milky Way, also appear much younger than would be expected if the stars had migrated into this region from elsewhere in the Milky Way. “Even more mysterious is that not only do the stars look unusually young, but there are also fewer older stars than expected,” she continued. “Moreover, it seems like there are an unexpected number of massive stars.”
John and colleagues hypothesize that one reason for these unusual features could be that these stars are accumulating large amounts of dark matter, which is then destroyed within them. This process could provide them with an entirely new and unexpected form of fuel.
“Our simulations show that stars can survive on dark matter alone 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 very fascinating because our simulations show similar results to the observations of S-cluster stars: dark matter as fuel will keep stars young forever.”
“The idea of immortal stars,” John continued, “could explain many of the unusual properties of the S cluster stars at once. If stars in the Galactic Center become immortal due to the high density of dark matter, this could explain the unusually large abundance of apparently young stars in the Galactic Center, which at the same time explains 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. Furthermore, dark matter does not 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, gas, dust, and living things in the universe.
Scientists can only conclude the presence of dark matter because it interacts with gravity, and this interaction can affect ordinary matter and even light. However, if interactions between dark matter and ordinary matter do occur, they are rare and weak; scientists do not believe that we have ever discovered such an interaction.
What is less certain is whether dark matter interacts with itself. To understand what this means, consider that ordinary matter particles all have an antimatter version of themselves. For example, there is a positively charged antiparticle called a positron to a negatively charged electron. And when matter and antimatter meet, they destroy each other, releasing energy.
“The annihilation of dark matter is analogous to the annihilation of matter and antimatter: when a particle and its antiparticle meet, they annihilate and produce other particles, for example photons. In the same way, dark matter particles can annihilate in such a way,” John said. “In many dark matter models, the dark matter particles are considered to be their own antiparticles, meaning that two dark matter particles can annihilate with each other.”
However, we don’t see any dark matter annihilation, so it must be quite rare. That means, John says, it’s more likely to occur in an environment where huge amounts of dark matter can be crammed together. Perhaps the ultra-dense region at the heart of a star is where gravity, with which dark matter does interact, is strongest.
Can the sun also become immortal?
Main sequence stars burn hydrogen in nuclear fusion processes during their lifetime. This creates helium, the bulk of the star’s energy, and the outward “radiation pressure” that offsets the inward pressure of the star’s own gravity. 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 highest,” John said. “We show that if stars accumulate a large amount of dark matter, which then annihilates within the star, it can also provide an outward pressure, making the star stable through dark matter annihilation rather than nuclear fusion. Stars can thus use dark matter as fuel instead of hydrogen.
‘Stars use up their hydrogen, which will eventually cause them to die. On the other hand, dark matter can be continuously accreted, making these stars immortal.’
So could the sun grant itself immortality by switching to this alternative fuel source? Jan doesn’t think so. Located halfway up one of the Milky Way’s spiral arms, it is in exactly the wrong place in our Galaxy to access this dark fountain of youth.
“Stars need very large amounts of dark matter to efficiently replace nuclear 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 many billions of times higher than on Earth, which is the amount of dark matter needed to make stars immortal,” Jon explains. “So our sun is not immortal.”
John added that the team’s findings may point to many secrets about dark matter itself and the immortal stars it could power.
“Our findings tell us that dark matter can scatter with ordinary particles, which is necessary to slow down the dark matter particles in the star to capture them — also that dark matter particles can annihilate with each other,” she said. “By observing the distribution of immortal stars around the Galactic Center, we might also get some information about the distribution and density of dark matter around the Galactic Center.”
RELATED STORIES
— For the first time, dark matter has been detected dangling from the cosmic web
— Exotic ‘Einstein Ring’ Suggests Mysterious Dark Matter Interacts With Itself
— Small black holes left over from the Big Bang could be the prime suspects for dark matter
John explained that astronomers need more precise observations of the Milky Way’s inner stars to verify these findings, so they can determine whether these stars are on a “dark main sequence,” which could indicate their immortality.
They also plan to determine the effect of dark matter annihilation on different stars. Initial simulations show that lighter stars become “puffy” and shed their outer layers as they switch to this dark fuel. This could explain the nature of the so-called “G objects” found in the Galactic Center; these are stellar bodies 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 at later evolutionary stages, when they have moved away from the main sequence and are undergoing various nuclear fusion processes,” Johns said. “Our results are exciting because they show that stellar observations provide 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 in the arXiv paper repository.