Categories: Dark MatterStars

If dark matter is a particle, it should get inside red giant stars and change the way they behave

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.

Paul M. Sutter

Astrophysicist, Author, Host | pmsutter.com

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  • That was an interesting reference, since the abstract refers to weakly Interacting particles but then it models pure additional mass effects. I take it from another astrophysicist that a solar mass system volume has about an average mass asteroid worth of dark matter differences [ https://www.forbes.com/sites/startswithabang/2018/03/24/ask-ethan-if-dark-matter-is-everywhere-why-havent-we-detected-it-in-our-solar-system/ ].

    "So now, we come to the big question. What about dark matter's effect on the Solar System? A huge part of what you're probably thinking is true: we should have dark matter particles flying through space everywhere, including throughout our Milky Way. It means there should be dark matter in our Solar System, in our Sun, passing through our planet, and even in our bodies. The big question you need to ask is this: compared to the masses of the Sun, the planets, and the other objects in our Solar System, what is the relevant, interesting mass due to dark matter?"

    "Because we know the mass of the Milky Way, the relative densities of normal and dark matter, and we have simulations that tell us how the dark matter density ought to behave, we can come up with some very good estimates. When you do these calculations, you find that about 10^13 kg of dark matter ought to be felt by Earth’s orbit, while around 10^^17 kg would be felt by a planet like Neptune."

    If I run the same density argument on Sun, it would feel an additional mass of ~ (10^6/10^8)^3 = 10^-6*10^13 kg dark matter or 10^7/10^30 = 10^-23 parts. Even with the luminosity going as 10^7, the luminosity difference between a pure baryon star and Sun would be ~7**10^-23 or 10^-24. The same difference when it pass into the low-mass red giant stage would be more like 10^13/10^30*10 = 10^-18.

    "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."

    There is that. "Using NASA's Hubble Space Telescope and a new observing technique, astronomers have found that dark matter forms much smaller clumps than previously known. This result confirms one of the fundamental predictions of the widely accepted "cold dark matter" theory. All galaxies, according to this theory, form and are embedded within clouds of dark matter. Dark matter itself consists of slow-moving, or “cold,” particles that come together to form structures ranging from hundreds of thousands of times the mass of the Milky Way galaxy to clumps no more massive than the heft of a commercial airplane. (In this context, "cold" refers to the particles' speed.)"
    [ https://www.nasa.gov/feature/goddard/2020/hubble-detects-smallest-known-dark-matter-clumps/ ]

    Maybe someone has time to run the Hubble results against the paper model and see if it works out. But unless nature is very clum(p)sy, I wouldn't worry yet.

    Else I like where this is going. Thermal generated weakly interacting massive particles [WIMPs] during the hot big bang would give a perfect fit to current cosmology. But now we have 3 results that reject that - LHC collider absence of thermal WIMPs, ACME electron sphericity absence of thermal WIMPs, Fermi-LAT Milky Way core absence of annihilation radiation from thermal WIMPs - so it looks like a dead end. The recent Fermi-LAT constraint on dark matter mass is the best, so dark matter could be a gravitationally acting particle with mass above the standard matter masses up to the inflation field particle mass - a huge range.

  • [it's been a day -I'm reposting the queued comment without link references. And also try to fix the paragraph separation again, the original text run together for some reason.]

    That was an interesting reference, since the abstract refers to weakly Interacting particles but then it models pure additional mass effects.

    I take it from another astrophysicist [Siegel @ Forbes] that a solar mass system volume has about an average mass asteroid worth of dark matter differences.

    “So now, we come to the big question. What about dark matter’s effect on the Solar System? A huge part of what you’re probably thinking is true: we should have dark matter particles flying through space everywhere, including throughout our Milky Way. It means there should be dark matter in our Solar System, in our Sun, passing through our planet, and even in our bodies. The big question you need to ask is this: compared to the masses of the Sun, the planets, and the other objects in our Solar System, what is the relevant, interesting mass due to dark matter?” “Because we know the mass of the Milky Way, the relative densities of normal and dark matter, and we have simulations that tell us how the dark matter density ought to behave, we can come up with some very good estimates. When you do these calculations, you find that about 10^13 kg of dark matter ought to be felt by Earth’s orbit, while around 10^^17 kg would be felt by a planet like Neptune.”

    If I run the same density argument on Sun, it would feel an additional mass of ~ (10^6/10^8)^3 = 10^-6*10^13 kg dark matter or 10^7/10^30 = 10^-23 parts. Even with the luminosity going as 10^7, the luminosity difference between a pure baryon star and Sun would be ~7**10^-23 or 10^-24. The same difference when it pass into the low-mass red giant stage would be more like 10^13/10^30*10 = 10^-18.

    “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.”

    There is that. “Using NASA’s Hubble Space Telescope and a new observing technique, astronomers have found that dark matter forms much smaller clumps than previously known. This result confirms one of the fundamental predictions of the widely accepted “cold dark matter” theory. All galaxies, according to this theory, form and are embedded within clouds of dark matter. Dark matter itself consists of slow-moving, or “cold,” particles that come together to form structures ranging from hundreds of thousands of times the mass of the Milky Way galaxy to clumps no more massive than the heft of a commercial airplane. (In this context, “cold” refers to the particles’ speed.)” [NASA]

    Maybe someone has time to run the Hubble results against the paper model and see if it works out. But unless nature is very clum(p)sy, I wouldn’t worry yet.

    Else I like where this is going. Thermal generated weakly interacting massive particles [WIMPs] during the hot big bang would give a perfect fit to current cosmology. But now we have 3 results that reject that – LHC collider absence of thermal WIMPs, ACME electron sphericity absence of thermal WIMPs, Fermi-LAT Milky Way core absence of annihilation radiation from thermal WIMPs – so it looks like a dead end. The recent Fermi-LAT constraint on dark matter mass is the best, so dark matter could be a gravitationally acting particle with mass above the standard matter masses up to the inflation field particle mass – a huge range.

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