What is implied in several of the answers is that you should think of ground in real devices (not schematics or simulations) as a circuit, not a node. Each piece of wire, each connection, each chunk of metal is one or more components in this circuit. If you are careful, and use a Kelvin setup (4 point resistance) you can measure the values in this circuit. They are usually measured in small fractions of an ohm, but when amps are flowing and you are worried about millivolts those small fractions are important.
Star connections simplify the circuit, making it easier to understand and deal with. Each leg of the star is just a series connection of impedances. They aren't absolutely necessary but it is easy to overlook something or make other mistakes in other configurations. When measuring resistances (or impedances) in a non star configuration remember that you are measuring two terminals of a black box which has unknown internal configuration and very possibly other terminals connected to other points in your ground circuit.