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The sun captured by NASA’s SOHO solar telescope. © IMAGO/piemags/Nasa
A research team is conducting a thought experiment about “Hawking stars”: What if a star like our sun swallows a black hole?
Garching – The strangest thought experiments are sometimes carried out in research. One of them comes from the long-deceased physicist Stephen Hawking. In the 1970s he thought about stars that harbor a black hole in their center – the so-called “Hawking stars”. So far, such stars have not been detected – but the theory continues to inspire researchers to do new work.
A research team led by the Max Planck Institute for Astrophysics (MPA) in Garching has now modeled the evolution of “Hawking stars” and gained some interesting findings. The work was in the specialist magazine The Astrophysical Journal published. According to Hawking’s theory, there is a primordial black hole in a star named after him, a small black hole that was formed shortly after the Big Bang. Such a star could be captured during the formation of a new star.
Hawking Star has a primordial black hole at its center
“Scientists sometimes ask crazy questions to deepen their knowledge,” says Selma de Mink, director of the stellar research department at the MPA. “We don’t even know whether such primordial black holes even exist, but we can still do an interesting thought experiment.”
So if a newly formed star captures a black hole with the mass of an asteroid or a small moon, it would become a “Hawking star”. The black hole would only grow slowly because only a small amount of gas falls on the black hole due to the star’s escaping luminosity, according to Hawking’s theory. The MPA team has now modeled the evolution of such a star – with different initial masses for the black hole.
The astonishing result: If the mass of the black hole is small, its presence would not be noticed at all – the star would essentially be indistinguishable from a normal star.
The sun could be a “Hawking star” – you wouldn’t know it
“Stars that have a black hole at their center can live surprisingly long times,” says study leader Earl Patrick Bellinger. “Our sun could even have a black hole the size of the planet Mercury at its center without us noticing.”
The biggest difference between a normal star and a “Hawking star” is near the core: it becomes convective as the black hole attracts matter. However, on the surface you wouldn’t notice it. Only the relatively new field of astroseismology could prove this, says one notice of the MPA. Only in a later phase of the star’s evolution, when it becomes a red giant, could the black hole give rise to characteristic signatures.
However, according to the research team, further simulations are needed to find out what effects a central black hole has on different types of stars.
Neither “Hawking stars” nor primordial black holes have been detected yet
So far, neither “Hawking stars” nor primordial black holes have been detected. But the research team believes that the search for “Hawking stars” could lead to the detection of small black holes. “Although the Sun was used here only as an example, there are good reasons to believe that ‘Hawking stars’ are common in globular clusters and very faint dwarf galaxies,” says Matt Caplan, a 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 in dark matter.”
A research team recently discovered the oldest black hole known to date – but it is anything but small. Meanwhile, a well-known mathematician wants to show that Stephen Hawking was wrong about the “inner workings” of black holes.(tab)