how does density affect pressure?

Answer 1

Pressure is to gases as density is to solids.

A scuba tank can hold gas pressurized to several atmospheres. A trash compactor can hold a large amount of trash compacted to a dense tight box.

Molecular movement is determined by temperature; the faster the movement of the molecules the hotter the item is. When density increases the item is compacted into a narrower space. If you increase the density of a gas you do this by forcing more gas into a strongly contained space, which would increase the pressure.

If you have a block of aero-gell you have a substance that is around 90% empty air, it is the least dense substance ever made by man. Styrofoam is a similar but more dense substance. If you put both under extreme pressure you can easily compact them to a denser form.

Inside a star the temperature are so high that the matter is converted to a plasma and the pressure or density is so high at the core that the process of fusion takes place. A hydrogen bomb uses an atomic bomb to create a super pressure wave that puts the hydrogen under enough pressure for fusion to happen.