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SYFY WIRE Black Holes

See-through stars could be glowing eerily all over space

By Elizabeth Rayne
NASA image of stars

Stars are those orbs of burning hydrogen that the naked eye can see from millions of light-years away, but what if space is haunted by another type of almost invisible star?

Things start to get weird when you venture into the realm of quantum physics. Spaceships could pass through wormholes, black holes might not obliterate everything, and boson stars may exist. These hypothetical stars are thought to be made of subatomic particles known as bosons. Even stranger, they would share features with black holes and may actually be the galactic cores that are thought to be supermassive black holes. At least astrophysicist Hector Olivares believes it's possible.

“If boson stars exist, it would have consequences for cosmology and for the evolution of galaxies in general,” Olivares, who led a recently published study on the possibility of these objects floating out there somewhere, told SYFY WIRE. “It would mean that light bosons would form boson stars with the mass of supermassive black hole candidates and play a role in the formation of structures in the universe.”

Meaning, it would basically change everything we think we know about galaxies — and far beyond.

Bosons either have positive or negative integer spins unless they have a spin of zero. Most exist in just one quantum state, which can explain how substances like superfluids behave in such bizarre ways. Because boson stars cannot be seen and have never been proven to exist, Olivares created a simulation to observe them further and compare them to black holes. Each phenomenon needed a different version of background spacetime, since spacetime for boson stars would depend on their structures. They then simulated the behavior of plasma and the paths of light it emitted in each version.

Plasma light is bent by gravity, which ultimately determines which boson stars and black holes would be seen, if they could be seen, in ultra hi-res.

“We blurred the images by taking into account most of the effects involved in the observations, from the location of each of the EHT radio-telescopes on the globe, to thermal noise and interstellar scattering. In this way we tried to produce synthetic images that looked as realistic as possible,” explained Olivares.


Black holes are invisible, and boson stars would emit no radiation because there is no nuclear fusion going on. The only light surrounding a black hole is the star stuff in its accretion disc that keeps spiraling into its gaping maw. Boson stars (if they exist) would have a similar plasma ring glowing around them and be able to bend light like black holes. Also like black holes, they would be able to grow into monsters with millions of solar masses. So how could you possibly tell if what you think is a black hole is really a boson star?

Black holes can cast a shadow because they capture photons. Boson stars, at least as they appeared in the simulation, were missing the surface and event horizon of black holes, so they had no shadow. There is also a difference between the accretion discs of black holes and boson stars.

“In the standard picture, accretion is driven by a plasma instability known as the magneto-rotational instability,” Olivares said. “This instability is active at all radii for black holes, but due to the properties of the space-time, it stops operating very close of the center of some boson stars. This produces a 'hole' in the accretion flow, which may look similar to a black hole shadow in radio-images.”

There is now speculation that our active galactic nucleus (AGN), Sagittarius A* (Sag A*) could be a boson star instead of a supermassive black hole. Any evidence of that could radically change how we look at the evolution of galaxies. Olivares believes that, if boson stars are ever proven to be galactic nuclei, we would have to find out exactly how the bosons accumulated in the center of the galaxy to begin with, and then look further out into the universe to find out where else they could exist and how they came to exist there. However, a boson star would actually have to be detected by a telescope to give observers on Earth that kind of evidence. That could be problematic.

Actually detecting a boson star would mean radio-imaging one and then measuring the mass and size of the dark region surrounded by its ghostly halo. Black holes can already be detected by certain size parameters they fall into. How stars and gas move around black holes also gives away their mass, which is why there is an estimated mass for Sgr A* and possible supermassive black holes in M87 — the galaxy in which a black hole (or at least its accretion disc) was imaged, with that image going on to break the internet. Existing technology could use these factors to make out whether future images of Sgr A* and anything in M87 look more like boson stars or black holes.

Anything that initially appears like a black hole but whose dark region turns out to be too small could end up being a boson star, but it would have to be extensively looked into.

“Unfortunately, due to the combination of their size and distance, it will be very difficult to image other possible supermassive black holes any time soon,” Olivares said. “We could also be limited by observing conditions. Future developments in the EHT, as adding additional stations on Earth or in space, or moving to higher observing frequencies, could help increasing resolution and the possibilities to distinguish between the two objects.”