What are extragalactic standard candles?

Extragalactic standard candles are tools astronomers use to measure how far away galaxies are, like using a known flashlight to tell how far away a dark room is.

Imagine you're playing hide and seek, and your friend has a flashlight that always shines the same brightness. If they’re close, the light seems bright; if they’re far away, it looks dimmer. By knowing how bright the flashlight actually is, you can guess how far they are from you, even in the dark.

In space, standard candles are like those flashlights. They are objects that have a known brightness, for example, supernovas, which are exploding stars. When astronomers see one of these explosions in a distant galaxy, they know how bright it should be. If it looks dimmer, they know the galaxy is farther away.

This helps scientists map out the universe and understand how it's expanding, kind of like knowing how far your friends are hiding to figure out how big the room is!