Dark matter makes up the vast majority of matter in the universe, but we can’t see it. At least, not directly. Whatever the dark matter is, it must interact with everything else in the universe through gravity, and astronomers have found that if too much dark matter collects inside of red giant stars, it can potentially cut their lifetimes in half.
When stars like our sun near the end of their lives, they stop fusing hydrogen in their cores. Instead, the fusion takes place in a shell surrounding a dense core of inert helium – the leftover ash from that nuclear reaction. Over the course of hundreds of millions of years, that core contracts (after all, there’s nothing inside of it generating energy to keep it inflated), heating it up.
Simultaneously, because of the increased core temperature, the rest of the star swells, ballooning to ridiculous proportions as a red giant star.
Astronomers can estimate the lifetimes of red giant stars by studying the complex physics of the core, tracing how long the helium can continue to heat until it reaches the critical threshold needed for it to undergo its own nuclear fusion, triggering the final end stages of the star.
It’s a pretty straightforward astrophysical calculation.
That is, it’s pretty straightforward unless something jams up the works.
A Dark Heart
Completely unrelated to red giants, astronomers are currently puzzling over the nature of dark matter, a substance that comprises roughly 80% of all the matter in the universe, yet is completely invisible. We’re not exactly sure what dark matter is, but we’re pretty confident that it is some sort of particle, as yet completely unknown to the standard model of particle physics.
Whatever the dark matter is made of, it must interact with normal matter through gravity, because that’s how we’ve been able to detect it so far. Beyond that, it may be possible for dark matter to form clumps, or regions of high density inside normal-matter objects like stars and planets.
Astronomers have already investigated the consequences of pooling dark matter into the hearts of normal stars, but new research has revealed what happens to red giant stars near the end of their lives.
Short version: it’s not pretty.
According to a paper recently appearing on the preprint journal arXiv, When too much dark matter sits inside a giant star, it causes the helium core to contract more than it normally would. That increased density raises the temperatures, which in turn raises the luminosity, which goes on to make the future evolution of the star that much shorter.
The effects are dramatic. If dark matter makes up a mere 10% of the mass of the red giant core, the temperatures jump by 10%, the luminosity doubles, and the lifetime of the red giant is cut in half.
We don’t know how much dark matter – if any – sits inside red giants, but future studies of this population of dying stars may reveal clues to one of the most enigmatic substances in the universe.
C. Sunny, et al. “Effects of Dark Matter in Red Giants“. arXiv.org preprint (2020)
Source: Universe Today, by Dr. Paul M. Sutter.