What are the weirdest stars in the universe?

Exotic stars may be scattered throughout the cosmos, from boson stars that could look like black holes to dark stars that might be powered by dark matter.

Over the past month there have been several fascinating bits of news about some of the weirdest stars in the universe – even objects that seem to me like they shouldn’t really count as stars at all, such as “dark stars”, which are powered by dark matter. So, I thought this month we could dig into the wild and wacky world of exotic stars.

Stars can theoretically come in many exotic types
Shutterstock / sakkmesterke

There are plenty of stars that are just weird. Take, for example, the newly discovered two-faced star called Janus. Its surface is half helium and half hydrogen, split neatly down the middle. Sounds completely bizarre, right? But when it comes down to it, Janus is a pretty normal white dwarf that astronomers happened to catch at just the right moment, from just the right angle – many white dwarfs go through a transition between helium and hydrogen, and we just spotted this one right in the middle of the switch. Weird, but there are far weirder stars that might be out there.

One of my personal favourites is the hybrid star, also known as a Thorne-Żytkow object. It’s like a Russian nesting doll of stars – a neutron star within a regular one. This could happen in two ways: either the two stars begin as a binary and one expands, engulfing the other, or they collide and merge in a crowded environment. These stars wouldn’t exist for long, a million years max, because eventually they’d either collapse and become a black hole or complete their merger and become a single huge neutron star.

During their lifetime, if they exist, hybrid stars would look basically like normal stars, just particularly massive. The only way to conclusively find one would be with a combination of gravitational waves and measurements of the elements within the star. Over the last decade, astronomers have found a few candidates that look like they might be hybrid stars, but we haven’t definitively detected one yet.

Let’s go one step weirder: dark stars. These stars are expected to be made of hydrogen and helium like regular stars, but they’re not powered by fusion. Instead, the thing that keeps them from collapsing under their own gravity is dark matter. In some theories of dark matter, each dark particle is its own antiparticle – if the two ever collide, they should annihilate in an incredible burst of energy. So if there’s enough dark matter at the core of a gas cloud that would otherwise form a normal star or black hole, it can keep that gas from collapsing. Like hybrid stars, these should look very similar to normal stars, just brighter and cooler and they can get much, much bigger. Some of the distant galaxies that the James Webb Space Telescope has spotted could actually be supermassive dark stars instead, but we don’t have enough data to be able to tell for sure.

My first thought about dark stars was that there was no way they could technically count as stars. After all, stars are big, hot balls of fusion-powered fire while dark stars have no fusion or fire, and they’re relatively cool all the way through. Turns out I was wrong! Every researcher I spoke to about dark stars said that they do count, because they satisfy the four equations of stellar structure. These equations deal with hydrostatic equilibrium – meaning the force of gravity working to collapse the star is exactly balanced by the pressure from within – along with the mass and energy of the star, and how that energy is transported within the star. There isn’t a single official definition of what a star is, but it seems like astronomers agree on those conditions – as much as astronomers as a group can agree on pretty much anything.

Even dark stars aren’t the wildest and most exotic stars that might be out there. The strangest of all are the stars that wouldn’t even be made of hydrogen and helium. Those would be things like quark stars, which are like a neutron star whose neutrons have broken down into their constituent parts, making it even smaller and denser.

There are also the oddballs known as boson stars. These are particularly interesting to me because bosons are pretty ordinary – they make up a huge proportion of all the particles out there – but stars made of them would be really wild. The thing about bosons is that many of them can be in exactly the same state at the same time, so you can get a whole bunch of them to behave as a single particle. That means that if you slammed enough of them together, they’d just sort of hang out there with no fusion happening. They wouldn’t generate any light naturally, so they might look like black holes, making them extraordinarily difficult to find. That’s probably why we haven’t really found any good candidates yet. And if boson stars spin, simulations suggest they’d be donut-shaped instead of spherical. Finally, something that might break the rules and skirt the technical definition of a star.

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