Astronomy
Weighing the Invisible: How Astronomers Measure Supermassive Black Hole Masses
Quick fact
The supermassive black hole at the center of our Milky Way, Sagittarius A, has a mass of about 4 million Suns, yet it would fit within the orbit of Mercury.
Why this is interesting
You can't see a black hole, but you can still weigh it. How do astronomers put a number on something that swallows light itself?