One thing I learned pretty quickly was that astronomers don’t only use a lot of weird units (like milliCrabs for example – more on this in another post) but they also use a lot of terms that don’t mean much at first hearing by the unenlightened. One such is ‘gravitational radius’… someone may say that something is ‘three gravitational radii from” an object.
All becomes clear when you understand that a synonym for gravitational radius is “Schwarzchild radius”. Any object whose radius is smaller than its Schwarzchild radius is a black hole. You too can become a black hole. If you weigh, say, 70kg then you feed that value into the expression (2GM)/c^2 to come up with the value 1.04*10^-25 metres. All you have to do then is cram your every atom into a sphere with a radius less than that and voila!… you’re a Schwarzchild black hole. If you’re spinning rapidly though, you’ll need to reach a different radius but when you do you’ll be a Kerr black hole. Such fun!
[TIDK = Things I Didn’t Know… until I started gathering the knowledge that in another lifetime may have made me a professional astronomer. This may become a long list…]