A "Star Tracker"? I've never heard of that. Is it software or a device?
A device. You put it on top of a tripod, and it is kinda like a wedge whose angle must be set to an angle corresponding to your latitude. A motor on the wedge then drives the camera/telescope mounted to it at 1 rev per 24 hours, thereby "despinning" the Earth. Or to put it another way, it compensates for the rotation of the Earth when you take long exposures, so you don't get star trails. So you can expose for very long times.
Canon 5D3 , 70-200mm f/2.8 L IS II , 24-105mm f/4 L IS , 50mm f/1.4 , 85mm f/1.8 , EF 2x III
Olympus OM2-SP , 50mm f/1.2 , 55mm f/1.2 , 50mm f/1.4 Silvernose , 135mm f/2.8 , 28mm f/2.8