As a clump of interstellar gas contracts to become a main-sequence star, its changing position on the H-R diagram tells us .

how its outward appearance is changing

Explanation:

H-R diagram is a fundamental tool used to classify stars. The position of stars on the H-R diagram is dependent on its luminosity and temperature.  

The position of the star is represented by its outward appearance specially the color. The hottest and brightest the star, the more it lies between the red and blue color range.  

The cooler stars lie on the right side of the H-R diagram while the hot stars lie on the left side of the H-R diagram.  

a electromagnetic waves

the duration of a photographic flash is related to an rc time constant, which is 0.100 μs for a certain camera.