Common envelope

So imagine you have two things, let's say two balloons. One big, one small. If you blow up the big balloon and try to fit the small balloon inside of it, you might find that it can't fit. But if you push the small balloon inside the big balloon and let go, they will both be inside the big balloon together!

Now imagine that the big balloon is a star that is getting old and puffy, and the small balloon is another star that is orbiting around it. As the big star gets bigger, it might start to pull the small star closer to it, until they are so close together that they are almost touching.

At this point, the atmosphere of the big star might start to eat away at the small star, kind of like how your stomach can digest your food. This is called a common envelope, because the two stars share an atmosphere. The small star might even end up spiraling inside the big star, like how the small balloon is inside the big one in our earlier example.

Eventually, the small star might get engulfed completely by the big star and they will merge together to become one star. It's kind of like if you took the small balloon out of the big balloon and they turned into one big balloon that's a mix of both of them. This is called a common envelope event, and it's one way that stars can change and evolve over time.