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