system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know how big neutron stars are?

I've heard they are around 20 km in diameter. But how on earth do we know this when they are so far away? Is it just a guess based on their mass?
gpt
We don't. 

It's a big open question right now. 

We know how massive they are, we can get that from observations of orbiting binaries.

To get their radius, we have to interpret other observations, or use theory. For example, we know what neutron stars are made of, so we can predict how 'squishy' the neutrons and protons in their core are. Knowing this, we can calculate the radius of a neutron star of a given mass. But we don't know *exactly* what they're made of, and we don't know *exactly* how squishy that stuff is. So we're modeling them. At present, we know they're about 11-13 km in radius or so, but hopefully we'll know it to within 10% once the NICER mission gives us some results.