system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If Nebulas are giant clouds of dust and gas, how do Nebulas keep their shape?

For instance on the Horsehead Nebula, every picture I've seen over time looks the exact same and appears the nebula has not changed or altered shape.
gpt
They don't. They are constantly expanding and changing shape, it is just that they are so huge, that it takes forever for the changes to be noticeable.

The Horsehead nebula is slightly different now than it was 20 years ago, and in a way that it is just barely visibly notable.