Black holes have basic properties we might call “popular”, and we also understand that their formation requires some of the most destructive conditions in the universe. However, when it comes to visualizing their size, we must tread carefully. Why? Because their scale has the potential to reach truly chilling values, to such an extreme that the black hole at the center of our galaxy becomes insignificant in comparison. Do you want to know which is the largest black hole in the universe…?
To reach that answer, it's best to start from the opposite side and explore the smallest ones. The so-called primordial black holes are fascinating: It is believed that the ideal conditions for their formation occurred immediately after the Big Bang, and their mass would be equivalent to a trillion kilograms… with the size of a proton. Why do we say “would be”? Because their current state is hypothetical, and so far we haven't observed any. The task is extraordinarily difficult: A primordial black hole with the mass of Earth would be as large as a coin.
Visualizing the largest black hole in the universe
(Note from the editor: Subtitles in English, for now)
The next leap in scale takes us into the territory of stellar black holes. The smallest detected to date, GW170817 (although some believe that designation is premature), has “just” 2.7 solar masses and a diameter of 16 kilometers. The star V723 Mon has a diameter of 30 million kilometers, but it is bound to the whims of its companion, a black hole dubbed “Unicorn” with 3 solar masses and 17.2 kilometers in diameter. The number of stellar black holes continues to grow until reaching a ceiling of 150 solar masses, but there is a kind of void that forces us to jump to the category of supermassive black holes, with millions of solar masses.
Science is having difficulties explaining the existence of these monsters: The natural process is too slow, and the universe is not old enough, therefore… something else happened. The best hypothesis within our reach is that of the quasi-star, a type of massive star (in comparison, the largest star in the universe is relatively small), which does not rely on nuclear fusion for energy, but on material falling into a black hole at its center. It is estimated that the approximate lifespan of a quasi-star was about 7 million years, and that they served as “seeds” for the current universal titans.
Supermassive, Ultramassive
Sagittarius A* is the supermassive black hole at the center of the Milky Way, but contrary to what many believe, it doesn't really “dominate” our galaxy, but rather represents a tiny percentage of its mass. In fact, it is a dwarf compared to black holes like the one at the center of the galaxy Cygnus A, with 2.5 billion solar masses and 14.7 billion kilometers wide. The black hole in Messier 87 (you know, the one in the photo) has 6.5 billion solar masses, and would occupy the entire Solar System if placed in the Sun's position. However… … there is a higher classification level: The ultramassive black hole. These black holes are the engines of quasars, with tens of billions of solar masses. The black hole at the center of the galaxy OJ 287 exceeds 18 billion solar masses, and has a black hole 40 times the mass of Sagittarius A* orbiting… but even so, it is not the largest black hole in the universe. As of this writing, that title belongs to TON 618: 66 billion solar masses, eleven Solar Systems in diameter, and light takes a week to reach the singularity once it crosses the event horizon. If you feel small now, believe me, you are not alone.
