We talked about how L T are two of the most important properties used to describe stars. L versus T. Saw that most stars lie along the main sequence. He in core. He it sits in a stationary place on the main sequence. Reading the diagram this means that the star sits there with constant L and T. Main sequence is not some track along which stars move.
Stars are essentially stationary while on it.
Track is due to many different masses of stars lying at different places along the main sequence. Somewhat suprisingly high mass stars live for less time than low mass stars. Even though they have more fuel they burn it faster. This diagram is very fundamental and we will use it throughout the rest of this class. On tests you will need to know how to reason out problems using the HR diagram.