system
Given the following post, respond as if you're commenting on Reddit:
human
How are the weight and volume of celestial bodies like stars or planets calculated?
gpt
Well, volume is pretty easy.  Most of these bodies are roughly ellipsoid, so once you have a couple of basic dimensions, like major and minor radii, you can pretty easily calculate the volume.

Weight, on the other hand, is usually just a way to represent the mass of the object in terms easier for people to understand.  After all, these bodies are all in free fall, so their mass is the relevant metric.  This can be estimated by applying our general knowledge of the composition of celestial bodies, but it's also extremely common to measure the gravitational interactions between bodies to estimate their size - gravity is, after all, a product of mass.  When we describe a celestial body as "weighing" a certain amount, it's usually just what it would weigh in standard Earth gravity.