Last year, astronomers finally unveiled the first images of the supermassive black hole at the center of our Milky Way. However, the black hole itself could not be seen, at least not directly: it is so dense that its gravitational pull prevents even light from escaping.
But the image of Sagittarius A, as our galaxy’s black hole is called, showed a halo of glowing gas around the object; an object whose mass is known to be millions of times that of the Sun. Recent discoveries like these, like many others, have amazed astronomers. “In recent years, everything we thought we knew about black holes has been questioned,” says Michela Mapelli, an astrophysicist and professor at the University of Padua in Italy.
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Everyone has heard of black holes, but few people realize how much these strange objects continue to annoy astronomers. Last year, astronomers were alerted to a black hole destroying and consuming a star that had wandered too close. Another has been described as the fastest-growing black hole ever observed, consuming the same mass of Earth every second, making it already 3 billion times more massive than our Sun.
This image shows the center of our galaxy. The black hole is known as Sagittarius A* or Sgr A*.NASA
cosmic minnows
Mapelli studies stellar black holes that form when a large, fast-burning star implodes. Compared to supermassive ones, these black holes are cosmic minnows. Astronomers expected such black holes to be between five and ten times the mass of the Sun, but in fact they have a much wider range of sizes. Several of up to 100 solar masses and a small one of only 2.6 have been discovered in recent years.
“We have discovered black hole properties and mass ranges that we could not have imagined prior to recent observations,” says Mapelli. He is particularly fascinated by binary black hole systems, in which two black holes orbit each other. This phenomenon can occur when two orbiting stars end their lives as black holes.
Representation of two black holes before collision and merger.Mark Myers ( (Mark Myers)
Again, there could be many other ways binary black holes form, something Mapelli is investigating in his DEMOBLACK project, funded by the European Research Council. “Seven years ago, most people were skeptical about the existence of double black holes,” he says. “Even theorists were not convinced of their existence.”
Now, Mapelli said, nearly a hundred have been discovered. They emit gravitational waves, perturbations in spacetime that can be detected with sophisticated detectors at the Laser Interferometry Observatory for Gravitational Waves (LIGO) in the United States and at the Virgo interferometer in Italy.
According to Mapelli, most astrophysicists doubted that two black holes could get close enough to merge, but then gravitational waves started sending signals of collisions between black holes. In 2019, a strange merger event occurred between sixty and eighty solar mass black holes.
Whether they formed directly from stars is unknown, as the assumption that star-born black holes have between five and ten solar masses has become unfounded. “Whether the maximum mass of a stellar black hole is only 60 solar masses or can reach 90 or even 300 solar masses is a big question mark,” explains Mapelli. “I feel guilty for this great uncertainty as I personally helped cause this situation.”
Graphic visualization of the black hole Gargantua by astrophysicist Kip Thorne in the Christopher Nolan film Interstellar (2014).Christopher Nolan
galactic monsters
The largest beasts are at the center of almost every galaxy. Almost all of them are active and contain hot gas inside that absorbs gravity. The mass of some of these black holes is up to 10 billion solar masses.
“Here are some real monsters,” says Professor Christopher Reynolds from the University of Cambridge (UK): “Their influence in a galaxy can be up to a hundred, even two hundred light years.” Stars and galaxies feel the gravitational pull of these black holes even at these astronomical distances, but the bursts of energy they produce by consuming matter can be felt even further away, 100,000 light-years or more away.
As part of the EU-funded DISKtoHALO project, Reynolds is studying how these supermassive black holes grow, sucking in hot gas and creating bursts of energy that are shot outward. “We know that these black holes create beams of energy that emit discharges outward,” he explains.
One thing astrophysicists have yet to figure out is why the gas at the core of some galaxies is so hot – between 10 and 100 million degrees Celsius – even though the systems are billions of years old and have therefore had plenty of time to cool down. The way black holes interact with their surroundings and with distant parts of their galaxies is an extremely complex puzzle. Computer models are of little help, since knowledge is required on relatively small scales and at the same time on gigantic scales measured in light-years.
Nine radio telescopes have captured this gigantic sinkhole at the center of the Centaurus-A galaxy. Capella Observatory
“We’re talking about something the size of a tennis ball regulating something the size of the Earth,” Reynolds compares. One way to study these supermassive black holes at the heart of galaxy clusters is to study the hot gases around them. It is impossible to see these gases through a telescope, but their energy can be observed with the X-rays they emit due to their high temperature.
Why hot gas doesn’t cool and form stars remains unclear. “You need a heat source to release energy in the center of the cluster, and the only ones that are powerful enough are black holes,” Reynolds muses. How exactly this heat source works is still a mystery to him and his colleagues. What is clear, however, is that supermassive black holes do not lead a quiet life. As Reynolds describes it, “These black holes aren’t even spherical; they rotate on themselves and form a disk full of instability”.
Despite new discoveries about these strange galactic creatures, the true nature of black holes remains unknown. Assumptions used in the past have been challenged. If anything is clear, it’s that black holes will continue to fascinate the brightest minds in astronomy.
The research mentioned in this article was funded by the European Research Council of the EU and the article was originally published in Horizon, the European Union’s research and innovation journal.
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