If the Milky Way were the size of Los Angeles, the black hole TON 618 would be a golf ball. Yet that single golf ball weighs more than every star in the galaxy combined.
The Milky Way is a spiral galaxy spanning roughly 100,000 to 120,000 light-years in diameter. Light traveling at 186,000 miles per second takes over a hundred millennia to cross it.
TON 618 is the largest known black hole in the universe. Its event horizon—the sphere of darkness from which no light can escape—has a diameter of about 2,600 Astronomical Units (AU). Because one AU is the distance between the Earth and the Sun, TON 618 is large enough to swallow our solar system 30 times over.
However, translated into interstellar distances, TON 618 is only 0.04 light-years across.
Compared to the 100,000-light-year span of the Milky Way, TON 618 is 2.5 million times narrower.
Where TON 618 makes up for its tiny physical footprint is its extreme density.
The Milky Way is heavy. When you add up all of its gas, dust, planets, and the dark matter halo that surrounds it, the entire galaxy weighs about 1.5 trillion solar masses. However, if you strip away the dark matter and gas and only weigh the stars, the Milky Way’s stellar mass is estimated to be roughly 50 to 60 billion solar masses.
TON 618 has an estimated mass of 66 billion solar masses.
This single, ultra-dense sphere accounts for nearly 4.5% of the mass of our entire galaxy, dark matter included, crammed into a space that is physically infinitesimal by cosmic standards.