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What Are Dark Stars?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Ready for a new celestial object to add to the hierarchy that is stellar evolution? Yeah, I know it seems like it’s never ending. I therefore hope you will find this topic to be refreshing for its uniqueness. It combines the mystery of dark matter with stars to be known as…a dark star. Yes, let’s dig into this and see what we find.

Getting to the Matter

How did the first stars form? Roughly 200 million years post Big Bang, lots of particles floated about and would collide. This would reduce their energy and slow them down, therefore allowing slower particles to be captured by gravity and so based on the amount of material around at the time a clump would grow faster and faster until you had a large ball of gas. If there was enough matter present to crunch down on the center via gravity then nuclear fusion would occur and our star would be born.

But baryonic matter wasn’t the only other thing around. Dark matter played a role too, considering that supersymmetry predicts it to be a weakly interacting yet massive particles, or WIMPS. Having mass means that dark matter also had a role to play in the gravity game, and if so then how does that fit into our scenario? Well, the universe was much smaller than that it is now, so for a given volume of space more matter would be present, including dark matter. It would collect much like the baryonic matter but it would primarily interact via gravitational effects. But just as normal matter would undergo nuclear fusion and release energy, so would dark matter. According to the WIMP model, dark matter would be its own antiparticle because it has no charge and so would annihilate with itself. This would result in quarks and antiquarks which then would annihilate, generating radiation pressure outward and goofing up hydrostatic equilibrium. With current stars, they exist in this state where the pressure of radiation emanating outward is balanced by the force of gravity going inward. It’s what prevents stars from dissipating or from collapsing under it’s our gravitational field. The WIMP annihilation would generate radiation pressure to the point of halting star production, only being compensated for if a lot of normal matter was present (Johnson 20-1, University of Utah, Freese).

This would result in our dark star, which would be roughly the size of Saturn….’s orbit. Yes, it would be huge, with an average radius of nearly 900 million miles! To put that into perspective, the average star radius is the same as our Sun’s, or about 400,000 miles. The largest known star in the universe is UY Scuti with a radius of nearly 750 million miles. So our dark stars would be about 300 million miles wider than the largest star known and would be over 2,000 times the radius of the sun! And the largest ones the models predict would be up to 200,000 times wider than the sun! Dark stars, if real, would be a new standard for large celestial objects. This large size is a result of the low density of the star, resulting from the large outward radiation pressure the annihilation produces (Johnson 21-2, University of Utah, Parks).

Dark stars are predicted to be mainly hydrogen (the predominant element of the early universe) with about 0.1% of the total content being dark matter. With the cluster of dark matter in the center, particle annihilation would drive the radiation pressure of a traditional star, with little to no contribution coming from the hydrogen. Models show this method of generating pressure could be sustained for millions, perhaps billions, of years (Johnson 22).

A dark star in infrared, surrounded in hydrogen and helium.

A dark star in infrared, surrounded in hydrogen and helium.

On the Hunt

To find dark stars, we would need to look at galactic centers for they would be a location where dark matter concentrations are sufficiently high to fuel our stars. We also need to look at the past when the density of dark matter in the universe was high enough to allow the concentration we need. We can also see if other dark matter fueled stars are out there like white dwarfs or neutron stars. They could have a small amount of this fuel helping them along. But in general a dark star would be a hot star courtesy of that radiation, even at the end of its life. It therefore would appear to be younger than it actually is, outputting 1million to 1 billion solar units. That would place it above supergiants on the H-R diagram! And like most massive stars, the end of a dark star would be a result of the lack of fuel providing the balancing force to counter gravity, therefore collapsing the star into a black hole. By looking at the surroundings of a black hole or a star, we can find indirect detection methods. Dark stars are not only names for their fuel but also for their lack of emitting visible light. However, other spectrums would be detectable and for dark stars we would look for the remnants of the dark matter annihilation such as gamma rays, neutrinos, and antimatter emissions from “clouds of cold, molecular hydrogen gas” which have no business harboring such energetic material. Their temperature of 9.700 Celsius would also be a giveaway, for that thermal level would correspond to a less active star than our dark star (Johnson 22-3, University of Utah, Parks, Freese).

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Dark stars may solve the mystery of the early supermassive black holes seen to exist way too early in the universe. Theory predicts a much later starting point for supermassive black holes than observations have shown, but nothing massive should have been around to accumulate into a supermassive black hole. But if dark stars were present then in the smaller universe it would have been possible to have their eventual collapse into black holes then accumulate into supermassive black holes we have seen (Johnson 23, Parks, Tangermann).

While no candidates exist yet, it is only a matter of time until we know one way or another. More data will point towards this or perhaps to something even more interesting, which I know sounds impossible. But that is physics for ya.

Works Cited

Freese, Katherine et al. “The Effect of Dark matter on the first stars: a new phase of stellar evolution.” arXiv: 0709.2369v1.

Johnson-Groh, Mara. “Dark Stars Come into the Light.” Astronomy. Oct. 2018. Print. 20-3.

Parks, Jake. “The Early Universe May Have Been Filled With Dark Matter Stars.” Blogs.discovermagazine.com. Kalmbach Media, 22 Jul. 2019. Web. 23 Aug. 2019.

Tangermann, Victor. “These Bizarre Objects May Have ‘Seeded’ Supermassive Black Holes.” Futurism.com. Futurism, 22 Jul. 2019. Web. 23 Aug. 2019.

University of Utah. “The first dark stars.” Astronomy.com. Kalmbach Publishing Co., 04 Dec. 2007. Web. 23 Aug. 2019.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley

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