A peculiar cluster of stars swirling around the violent center of the Milky Way could become "immortal" by continuously capturing and destroying dark matter particles in their cores, a new study suggests.
Using computer simulations of stellar evolution, researchers found that dark matter particles captured by these stars' gravity may frequently collide with and "annihilate" each other inside the star, transforming into ordinary particles while releasing a significant amount of energy.
This additional energy source could maintain the star's stability and potentially make it immortal, even after its regular supply of nuclear fuel has run dry, the researchers suggest.
"Stars burn hydrogen in nuclear fusion," lead study author Isabelle John, a doctoral candidate in astroparticle physics at Stockholm University, told Live Science via email. "The outward pressure from this balances out the inward pressure from the gravitational forces, and keeps the stars in a stable equilibrium."
However, many stars spotted near the Milky Way's central black hole seem to be far younger than theories of stellar evolution predict. To investigate this mystery, the researchers tested whether the stars could be drawing energy from the plentiful supply of dark matter thought to exist at the galactic center.
"Our simulations show that if stars can collect large amounts of dark matter, which annihilates inside the star, this can provide a similar outward pressure, making the star stable due to dark matter annihilation rather than nuclear fusion — so stars can use dark matter as a fuel instead of hydrogen," John said. "The important difference is that stars use up their hydrogen, which will eventually cause them to die. On the other hand, stars can collect dark matter continuously."
The study, published to the preprint server arXiv in May, has not been peer reviewed yet.
Stars defying theory
Stellar evolution is a well-studied subject. Relationships among a star's age, luminosity, size and temperature have been derived with high precision both with theory and astronomical data. However, recent observations have shown that the properties of stars near the center of the Milky Way defy the generally accepted theory of stellar evolution.
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"The innermost stars of our Galaxy, the S-cluster stars, show a series of properties that [are] 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 explained. "They also seem to be much younger than what would be expected if the stars had moved there from somewhere else. Additionally, it seems like there are unexpectedly many heavy stars."
These strange properties of the S-cluster stars could be explained by the presence of an additional source of energy within them. For instance, this extra energy source could allow the star to burn hydrogen — the usual energy source — at a lower rate, causing it to age more slowly and appear younger than it actually is.
In their recent study, John, along with Tim Linden of Stockholm University and Rebecca K. Leane of the SLAC National Accelerator Laboratory at Stanford University, suggested that this source could be the annihilation of dark matter particles. This explanation aligns with the fact that greater amounts of dark matter are believed to lurk at the galaxy's center, right where the oddball stars were observed.
"Throughout most of the Milky Way, the dark matter density is not high enough to affect stars," John said. "But at the Galactic Center, the amount of dark matter is very high, potentially many billion times higher than on Earth."
Virtual annihilation
To test their hypothesis, the researchers conducted a computer simulation of the life cycle of a star surrounded by a dark matter cloud with a density matching that of the galactic center. They assumed dark matter consists of weakly interacting massive particles, one of the primary candidates for dark matter components.
Since dark matter particles have not yet been found in laboratory experiments, the strength of their interaction with ordinary matter and the rate at which they annihilate each other are not known. But the study showed that for certain values of these quantities, the dark-matter-based mechanism of energy production perfectly explained the observed properties of the S-cluster stars.
However, to confirm their explanation, the authors believe that more stars need to be discovered near the galactic center. Additionally, more precise measurements of the parameters of known stars must be conducted to reliably compare observations with theoretical predictions. Hopefully, such observations will be possible in the near future using the Very Large Telescope in Chile or the Keck Observatory in Hawaii, the researchers said.
"More precise observations of the S-cluster stars will provide us with more information about these stars and ongoing processes," John said. "This will show if the observations are consistent with our simulations, or if other explanations of their unusual properties become more favorable."
