Simplified - structure of an atom. If you want to know what's going on in the life cycle of stars, the structure of an atom is important.
When a massive star collapses after the fuel runs out, we need to have at least a fuzzy idea about atoms. The diagram showed a simplified structure, but not the scale. The electrons seem to be almost cozying up to the nucleus.
Various analogies try to help us out. One sporty person (American) suggests,
Imagine the atom is the size of a football stadium. In that gigantic stadium the nucleus would only be the size of a small marble sitting on the 50-yard line
!
Another one says,
My favorite comparison here is that if an atom were the size of the Earth, then the nucleus would be the size of a football stadium (whatever game you mean when you say ‘football’).
Not being very sporty, I rather like this one:
If an atom were blown up to the size of a cathedral, the nucleus would be no larger than a bee buzzing about in the center, while the electrons would be ‘orbiting’ near the outermost edge.