From here on Earth, the Milky Way looks quiet. Yet at its center is Sagittarius A*, a black hole millions of times more massive than the Sun. It’s been quiet for a long time, but astronomers think the silence won’t last forever.
New research suggests that this sleeping giant will reawaken when our galaxy collides with a smaller neighbor called the Large Magellanic Cloud, possibly in about two million years. The event should cover the black hole with new gas and turn it into a bright galactic nucleus, although experts say the Earth is unlikely to face a doomsday scenario.
A sleeping giant at the heart of the galaxy
Astronomers rank Sagittarius A* as the supermassive black hole, which means it packs about four million times the mass of the Sun into the tiny space at the center of the Milky Way. NASA describes it as the nearest known black hole, which is more than twenty-five thousand years away from the constellation of Sagittarius.
The Institute of Astrophysics of Andalusia notes that Sagittarius A* is unusually bright and transforms matter hundreds of times less efficiently than most supermassive black holes. In 2020, the Nobel Prize in Physics honored Roger Penrose, Reinhard Genzel and Andrea Ghez for the work that helped prove that this dark source is indeed the supermassive black hole at the center of our galaxy.
To see how such giants form galaxies, astronomers also look far away. Using the James Webb Space Telescope, a team that included Aaron Romanowsky studied a distant system called The Sparkler, a small galaxy about three percent the size of the Milky Way and surrounded by small globular clusters that resemble a miniature version of our halo.
A future collision could feed the black hole
The Large Magellanic Cloud is a small galaxy that now orbits the Milky Way at a distance of two hundred thousand light years from Earth. Statistics based on its motion suggest that it is losing orbital momentum and will orbit to merge with our galaxy in about two billion years.
A 2019 study using EAGLE cosmic simulations found that when galaxies like the Milky Way merge with partners like the Large Magellanic Cloud, their central black holes can expand several times as gas falls in. Similar research has shown that the halo of stars and the general structure of the galaxy are also reshaped during this type of merger.
During the active phase, the falling material forms a hot disk around the black hole and heats up to millions of degrees. Astronomer Nathalie Degenaar of the University of Amsterdam explains that this matter can shine everywhere, and recent observations by NASA’s Imaging X ray Polarimetry Explorer have revealed a small burst from Sagittarius A* almost two hundred years ago.
The meaning of this resurrection in the world
Talking about a reborn black hole might sound like a doomsday movie, but the numbers paint a sobering picture. Professor Carlos Frenk of Durham University emphasizes that the active galactic nucleus produced by the future merger with the Large Magellanic Cloud is not expected to be powerful enough to cause a serious threat to life on Earth.
Joseph Michail at the Harvard and Smithsonian Center for Astrophysics notes that twenty-six thousand light years may not sound like much on a map of the universe, but it’s still a big difference. Our planet also has many protective layers, including the atmosphere, gravity, and even the gaseous atmosphere of the Milky Way, which should absorb most of the radiation from the galaxy.
Thus, most future observers may see a spectacular sky rather than an immediate disaster, a reminder that the galaxies are not standing in the background but living things that have existed for billions of years.
A major study on this upcoming conflict has been published in Monthly notices of Royal Astronomical Society.
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