What will a forming star look like in HR diagram?
Temperature changes relatively simple
Starts out large and relatively cool   Must be on red side of diagram
It heats up as it contracts     Must towards the blue
Luminosity more complicated because it depends on T and R
Not much energy to start with     Luminosity must start out low
Collapse releases grav. energy     Luminosity will rise
Fusion begins, releases more energy   Luminosity at a peak
Collapse slows, only have fusion now   Luminosity declines
Finally stabilizes on the main sequence