Why are standard candles not perfect?

Standard candles are not perfect because they can get tired or dirty, just like your favorite toy.

Imagine you have a flashlight that always shines the same brightness, that's like a standard candle. Scientists use it to measure how far away stars and galaxies are. But sometimes, the flashlight gets covered in dust, or maybe it starts to flicker. That means the light doesn’t shine as bright as it used to. If scientists think the flashlight is still shining at full power when it’s not, they might get the wrong answer about how far away something is.

1. They can change over time, like a toy that gets old and doesn't work as well.
2. They can be blocked or covered up, like when you put your hand in front of a flashlight, making it dimmer.

So even though standard candles are really helpful, they’re not perfect because sometimes they don’t do exactly what we expect them to do.