In our rapidly expanding universe, the lives of stars follow well-worn tracks, fusing hydrogen and then helium before growing in size until they exhaust their nuclear fuels and collapse, no longer able to resist gravity.
But some stars in the innermost part of our Milky Way, very close to the galactic center, could chart their own path and exhibit strange properties that don’t fit our standard view of stellar evolution.
New research suggests that these anomalies may be largely caused by dark matter rather than nuclear fusion – where that dark matter ‘replenishes’ the stars and makes them practically old by comparison.
Using a computer model of stellar evolution, researchers from Stockholm University in Sweden and Stanford University in California have simulated a population of stars orbiting the center of the Milky Way. They compared how these stars evolved with and without injection of dark matter.
Their model predicts the existence of a new class of massive stars kept ‘burning’ not only by small amounts of fusion of atomic nuclei, but also by the destruction of an ‘effectively infinite’ supply of dark matter particles colliding with antimatter.
By releasing a hot mess of photons and electrons, this exotic reaction could exert enough pressure to prevent the star from collapsing, potentially taking up to a hundred times as long as a more typical star, making them practically immortal.
“The inner galaxy is precisely where the dark matter density is high enough for the destruction of dark matter to substantially replace nuclear fusion as the stellar energy source, allowing stars to remain young forever despite their old age,” says Isabelle John, a graduate astrophysics student at the University of California. Stockholm University and colleagues write in their preprint, which has not been peer-reviewed.
The Milky Way’s galactic center is extremely bright, making the region difficult to image, and the stars there appear younger than their spectroscopic features would otherwise suggest.
This youthfulness indicates that they originated locally and were not brought in from elsewhere. Yet 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 poses conundrum number one.
There are also few old and evolved stars near the galactic center, and the stellar population is surprisingly top-heavy, consisting of stars with higher masses than the outer regions of the Milky Way. These inner stars also zoom around the galactic center unexpectedly quickly. Riddles two, three and four.
An explanation for all that strangeness could lie in dark matter, John and 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 contribute to the excess gravity needed to hold fast-moving galaxies together.
Incorporating the influence of dark matter into their stellar evolution models, John and colleagues predict that a new class of stars could exist near the galactic center, with “striking differences” that place them outside the realm of so-called main sequence stars.
Main sequence stars, the most common type of star in the universe, are located in a thick band of the Hertzprung-Russell diagram, which maps the life trajectories of stars taking into account their initial mass, luminosity and temperature.
John and colleagues’ simulations suggest that a new branch of stars may need to be added to that plot: higher-mass stars (five times or more massive than our Sun) that collect dark matter from a virtually endless reserve and store it throughout the period burn at lower temperatures. the same brightness as main sequence stars.
“The density of dark matter in these stars is continuously replenished, making these stars immortal and resolving multiple stellar anomalies,” the team wrote.
Although their simulations point to a new population of stars that our telescopes could focus on to look for signs of dark matter, the enormous brightness of the galactic center would make them difficult to observe.
“So far the number of known stars in the innermost parsec [of the Milky Way] is limited, and more stars are needed to robustly detect any effect of dark matter,” the team notes.
Other research published earlier this year also proposed an alternative explanation for why stars near the center of our Milky Way look so youthful, suggesting that they rejuvenate themselves not with dark matter, but with hydrogen slurped up by their neighbors.
The survey was posted on the arXiv preprint server in May.