What is the range of the mothership core's "Photon Overcharge"?
According to Starcraft wiki and Liquipedia the weapon has range 13, which is the same distance as a Siege Tank, targets air and ground and deals 20 damage.
The range of the Nexus means that no unit can easily attack while photon overcharge is active (60 seconds). Even tanks that have the same range, have 2 units less vision so need a spotter - this is also true of the Nexus - it has a range of 13 but sight of 11, according to http://wiki.teamliquid.net/starcraft2/Nexus