Physics
Black Hole
Quick fact
The supermassive black hole at the center of our Milky Way galaxy, Sagittarius A, has a mass about 4 million times that of the Sun, yet its event horizon is only about 12 million kilometers across—smaller than Mercury's orbit.
Why this is interesting
Imagine an object so dense that its gravity pulls in everything around it, even light itself. How can something that emits no light be seen, and what happens to matter that falls in?