What happens if you put a black hole in the sun?

In a hypothetical scenario, small primordial black holes could be captured by forming stars. An international team, led by researchers at the Max Planck Institute for Astrophysics, has modeled the evolution of these so-called Hawking stars and found that they can have surprisingly long lives, resembling in many respects normal stars. The work is published in The Astrophysics Journal.

Asteroseismology could help identify such stars, which could in turn test the existence of primordial black holes and their role as a component of dark matter.

Let’s do a scientific exercise: if we assume that a large number of very small black holes were created just after the Big Bang (called primordial black holes), some of them could be captured during the formation of new stars . How would this affect the star over its lifetime?

“Scientists sometimes ask crazy questions to find out more,” says Selma de Mink, director of the stellar department at the Max Planck Institute for Astrophysics (MPA). “We don’t even know if such primordial black holes exist, but we can still do an interesting thought experiment.”

Primordial black holes are thought to have formed in the early universe with a wide range of masses, from the size of an asteroid to thousands of solar masses. They could constitute an important component of dark matter, in addition to being the seeds of the supermassive black holes at the centers of current galaxies.

With very low probability, a newly formed star could capture a black hole with the mass of an asteroid or a small moon, which would then occupy the center of the star. Such a star is called a “Hawking star”, named after Stephen Hawking, who first proposed the idea in a paper in the 1970s.

The black hole at the center of such a Hawking star would grow only slowly, because the influx of gas to fuel the black hole is hampered by outgoing brightness. An international team of scientists has now modeled the evolution of such a star with different initial masses for the black hole and with different accretion models for the stellar center. Their surprising result: when the mass of the black hole is small, the star is practically indistinguishable from a normal star.

“Stars with a black hole at their center can live surprisingly long,” says Earl Patrick Bellinger, MPA Postdoc and now assistant professor at Yale University, who led the study. “Our sun could even have a black hole as massive as the planet Mercury at its center without us realizing it.”

The main difference between such a Hawking star and a normal star would be near the core, which would become convective due to accretion onto the black hole. This would not modify the properties of the star on its surface and would escape current detection capabilities. However, it might be detectable in the relatively new field of asteroseismology, in which astronomers use acoustic oscillations to probe a star’s interior.

Also in their further evolution, in the red giant phase, the black hole could lead to characteristic signatures. With upcoming projects such as PLATO, such objects could be discovered. However, further simulations are needed to determine the implications of embedding a black hole in stars of varying masses and metallicities.

If primordial black holes did indeed form shortly after the Big Bang, the search for Hawking stars could be a way to find them.

“Even if the Sun is used as an exercise, there is good reason to believe that Hawking stars would be common in globular clusters and ultra-faint dwarf galaxies,” points out Professor Matt Caplan of Illinois State University, co-author of the study.

“This means that Hawking stars could be a tool to test both the existence of primordial black holes and their possible role as dark matter.”