The ends of the phases are connected together to form the so-called neutral- or star point. If all three resistors are the same size, the phase currents cancel each other out at the zero point and the neutral line can be omitted.
What you can see from the circuit:
In Star connection the line current is equal to the phase current: ILine = IPhase
What about the voltage? The relation between the Line voltage and the Phase voltage can be derived from the pointer diagram:
In star Connection the Line Voltage is √3 times of Phase Voltage: VLine = √3 VPhase