In our rapidly expanding universe, the lives of stars follow well-trodden paths: first they fuse hydrogen and then helium before growing in size until their nuclear fuel is used up and they collapse because they can no longer resist gravity.
But some stars in the innermost region of our Milky Way, very close to the galactic center, may be forging their own path, exhibiting strange properties that do not fit our standard picture of stellar evolution.
Recent research suggests that these anomalies may not be caused by nuclear fusion, but primarily by dark matter – with this dark matter “renewing” stars and making them virtually ancient in comparison.
Using a computer model of stellar evolution, researchers from Stockholm University in Sweden and Stanford University in California simulated a population of stars orbiting the galactic center of the Milky Way, comparing how they evolved with and without the addition of dark matter.
Their model predicts the existence of a new class of heavy stars that are kept burning not only by the fusion of small amounts of atomic nuclei, but also by the annihilation of a “virtually infinite” amount of dark matter particles colliding with antimatter.
This exotic reaction releases a hot mess of photons and electrons and could generate enough pressure to prevent the star from collapsing. This could last up to 100 times longer than the lifetime of a more typical star, making it virtually immortal.
“The density of dark matter is high enough precisely in the innermost part of the galaxy that the destruction of dark matter can largely replace nuclear fusion as a stellar energy source, allowing stars to remain eternally young despite their great age,” write Isabelle John, an astrophysics student at Stockholm University, and her colleagues in their preprint, which has not yet been peer-reviewed.
The galactic center of the Milky Way is extremely bright, making the region very difficult to image. In addition, the stars there appear younger than their spectroscopic characteristics would otherwise suggest.
This youthfulness suggests that they formed locally rather than coming from elsewhere. However, our current models of star formation suggest that stars cannot merge within about a third of a light-year of the central black hole, which is puzzle number one.
There are also few old and evolved stars near the galactic center, and the stellar population is surprisingly top-heavy, with more massive stars than the outer regions of the galaxy. These inner stars also race around the galactic center at unexpectedly fast speeds. Puzzles two, three, and four.
One explanation for all these oddities could lie in dark matter, John and his colleagues suggest. Dark matter is one of the most elusive substances in our universe, an invisible mass that has never been directly detected but is thought to provide the extra gravity needed to hold fast-moving galaxies together.
John and his colleagues take the influence of dark matter into account in their models of stellar evolution and predict that a new class of stars could exist near the galactic center that exhibits “striking differences” and thus falls outside the range of so-called main sequence stars.
Main sequence stars, the most common type of star in the universe, are located in a broad band of the Hertzsprung-Russell diagram, which records the trajectories of stars over their lifetime, taking into account their initial mass, brightness and temperature.
The simulations by John and his colleagues suggest that a new branch of stars may need to be added to this scheme: more massive stars (at least five times as massive as our Sun) that accumulate dark matter from a virtually endless supply and burn at lower temperatures, reaching the same brightness as main-sequence stars.
“The density of dark matter in these stars is continuously renewed, giving these stars immortality and correcting numerous stellar anomalies,” the team writes.
Although their simulations suggest a new population of stars that we could search for signs of dark matter with our telescopes, the enormous brightness of the galactic center would make it difficult to observe.
“The number of stars known to date in the inner parsec [of the Milky Way] is limited and more stars are needed to reliably detect any effects of dark matter,” the team notes.
Other research published earlier this year also offered an alternative explanation for why stars at the center of our galaxy look so youthful. They suggest that they rejuvenate not with dark matter but with hydrogen that they absorb from their neighbors.
The research results were published in arXiv Preprint server in May.