Let's do a scientific experiment: If we assume that a large number of very small black holes (so-called primordial black holes) were created after the Big Bang, then some of these black holes may be trapped during the formation of new stars. What effect would this have on the lifespan of a star?

In one hypothetical scenario, small primordial black holes could be captured by newly formed stars. An international team of researchers, led by researchers at the Max Planck Institute for Astrophysics, has now modeled the evolution of these so-called "Hawking stars" and found that they can be surprisingly long-lived and resemble ordinary stars in many ways. Asteroidometry can help identify such stars, thereby testing the existence of primordial black holes and their role as components of dark matter.

Selmade Mink, director of the Stellar Department at the Max Planck Institute for Astrophysics (MPA), said: "Scientists sometimes ask crazy questions in order to learn more. We don't even know if such primordial black holes exist, but we can still do an interesting thought experiment."

Primordial black holes should have formed in the early universe, with masses ranging from as small as an asteroid to thousands of solar masses. They may be an important component of dark matter, or they may be the seeds of the supermassive black holes at the centers of today's galaxies.

An image of a small black hole placed at the center of the sun in a thought experiment. Image credit: ©MPA, background image: Wikimedia/Creative Commons.

In a very small probability, a newly formed star might capture a black hole with a mass equivalent to that of an asteroid or moonlet, which would then occupy the center of the star. Such stars are called "Hawking stars", named after Stephen Hawking, who first proposed the idea in a paper in the 1970s. A black hole at the center of such a Hawking star would only grow slowly because the outflowing luminosity blocks the inflow of gas that powers the black hole.

Now, an international team of scientists has simulated the evolution of such a star using different initial masses of the black hole and different accretion models at the star's center. They came up with a surprising result: when the black hole is less massive, the star is essentially indistinguishable from an ordinary star.

Earl Patrick Bellinger, an MPA postdoc who led the study and is now an assistant professor at Yale University, said: "Stars with black holes in their centers have surprisingly long lifespans. There may even be a black hole as big as Mercury in the center of our sun, and we are not aware of it."

These two images show the radial evolution of a star with a mass similar to that of the Sun without (left) and with (right) a black hole with an initial mass similar to that of an asteroid. The solid black line represents the photospheric radius, and the vertical dashed line represents the current age of the Sun. The red area shows where hydrogen is converted in nuclear fusion to helium, which provides most of the Sun's luminosity before the black hole begins to grow significantly (black area; at lower ages, the black hole is too small to be seen in this image). Black holes drive convection currents that mix the innermost parts of stars. Note the different scales of the y-axis.

The main difference between such Hawking stars and ordinary stars is that near the core, the core will become convective due to the accretion of the black hole. It does not change the properties of the star's surface and cannot evade current detection capabilities. However, it can be detected using the relatively new field of asteroseismology, where astronomers are using acoustic oscillations to probe the interiors of stars.

In addition, during the later stages of stellar evolution, known as the red giant stage, black holes may produce characteristic signals. With the development of projects such as PLATO, such objects may be discovered. However, further simulations will be needed to determine the effects of placing a black hole in stars of different masses and metallicities.

"If primordial black holes were indeed formed shortly after the Big Bang, then looking for Hawking stars may be one way to find them." Professor Matt Caplan of Illinois State University, a co-author of the study, pointed out: "Although the sun is used for practice, we have good reason to believe that Hawking stars are common in globular clusters and ultra-faint dwarf galaxies. This means that Hawking stars can be a tool to test whether primordial black holes exist and their possible role as dark matter."

Compiled source: ScitechDaily