Black holes: the mysterious “giants” that control our galaxies | Sciences

Last year, astronomers were finally able to reveal the first images of the supermassive black hole at the center of our Milky Way galaxy. However, the black hole itself cannot be seen, at least not directly: it is so dense that its gravity prevents even light from escaping.

But the image of Sagittarius A, as our galaxy’s black hole is known, revealed a halo of glowing gas around the object; An object that we know has a mass of one million times that of the sun. Recent discoveries like these, like many other things, have astonished astronomers. “Over the past few years, everything we thought we knew about black holes has been called into question,” he says. Michaela Mapellian astrophysicist and professor at the University of Padua, Italy.

Everyone has heard of black holes, but few people realize how much these strange objects continue to infuriate astronomers. Last year, astronomers learned that a black hole is destroying and swallowing a star wandering too close. Another black hole has been described as the fastest growing of all, devouring the equivalent mass of Earth every second, and is thus already 3 billion times more massive than our Sun.

cosmic minnows

Mapelli studies stellar black holes, which form when a massive, rapidly burning star explodes. Compared to supermassive holes, these black holes are small cosmic holes. Astronomers expected these black holes to be five to ten times the mass of the Sun, but in fact they have a much wider range of sizes. In recent years, several masses of up to 100 solar masses and as small as only 2.6 have been discovered.

“We have discovered properties and mass ranges of black holes that we could not have imagined before recent observations,” says Mapelli. He is particularly fascinated by binary black hole systems, where two black holes orbit each other. This phenomenon can occur when two stars orbiting each other end up as black holes.

Again, there could be many other ways for binary black holes to form, something Mapelli is considering in his project. demoblackFunded by the European Research Council. “Seven years ago, most people were skeptical about the existence of binary black holes,” he says. “Even theorists were not convinced they existed.”

Now, nearly a hundred have been discovered, Mapelli said. They’re spewing out gravitational waves, perturbations in space-time that can be picked up by sophisticated detectors at the Laser Interferometer Gravitational-Wave Observatory (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 the black holes. In 2019, a strange merger occurred between black holes weighing sixty and eighty solar masses.

Whether they formed directly from stars is unknown, as the assumption that star-born black holes are between five and ten solar masses has become unfounded. “Whether the maximum mass of a stellar black hole is only 60 solar masses or goes as high as 90, or even 300, is a huge question mark,” Mapelli explains. “I feel guilty for this great uncertainty, because I personally contributed to causing this situation.”

Galaxy monsters

The largest monsters are found in the center of almost all galaxies. Almost all of them are active and have hot gas inside that absorbs gravity. Some of these black holes have up to 10 billion times the mass of the Sun.

“These are some real monsters,” he says Professor Christopher Reynolds From the University of Cambridge (UK): “Its impact on the galaxy can extend to a hundred, or even two hundred light-years.” Stars and galaxies feel the gravitational pull of these black holes even at such astronomical distances, but the bursts of energy they generate by consuming matter can be felt far away, 100,000 light-years or more away.

under the project DISKtoHALOFunded by the European Union, Reynolds is investigating how these supermassive black holes grow, sucking in hot gas and generating bursts of energy that are released outward. “We know that these black holes produce jets of energy that send the discharges outward,” he explains.

One thing that astrophysicists have not yet been able to figure out is why the gas at the cores 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 the more distant parts of their galaxies is a very complex puzzle. Computer models are of little help because knowledge is needed on relatively small scales and, at the same time, on gigantic scales measured in light years.

“We’re talking about something the size of a tennis ball organizing something the size of the Earth,” Reynolds compares. One way to study these supermassive black holes at the center of galactic clusters is to examine the hot gases in their vicinity. It is impossible to see these gases through a telescope, but their energy can be observed by the X-rays they emit, due to their high temperature.

Why the hot gas doesn’t cool down and form stars is anyone’s guess. “You need a heat source to release energy in the middle of the mass, and the only ones that are powerful enough are black holes,” says Reynolds. How exactly this heat source works remains a mystery to him and his colleagues. However, it is clear that supermassive black holes do not lead quiet lives. Reynolds describes it: “These black holes are not even spherical. They spin in on themselves and form a disk full of instabilities.”

Despite new discoveries about these strange galactic creatures, the true nature of black holes is still unknown. Assumptions used in the past have been questioned. If anything is clear, it is that black holes will continue to intrigue the brightest minds in astronomy.

search you are referring to This article It was funded through the European Research Council of the European Union and the article was originally published in horizonResearch and innovation journal of the European Union.

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