* Light travels at a finite speed: Light, which is how we see objects in space, travels incredibly fast, but it's not instantaneous. It takes time for light to reach us from distant objects.
* Stars are incredibly far away: Even the closest stars are light-years away. A light-year is the distance light travels in one year. This means the light we see from those stars left them years ago.
* Constellations are collections of stars: Constellations are groupings of stars that appear close together from our perspective on Earth. However, these stars are not actually close to each other in space. They can be vastly different distances away from us, meaning the light from each star has traveled for different amounts of time.
Example:
Let's say you are looking at the constellation Orion. The star Betelgeuse in Orion is about 640 light-years away. This means the light you see from Betelgeuse today left the star 640 years ago. You are literally seeing Betelgeuse as it looked 640 years in the past.
In summary: When you look at the stars, you're not seeing them as they are right now, but as they were in the past, depending on how far away they are. This is why astronomers often say we are "looking back in time" when we study the night sky.
