The Hertzsprung-Russell, or HR, diagram is the number one tool of astronomers. Basically, the HR diagram is graph that relates observable quantities of a star such as temperature, brightness and amount of light emitted . We can use this information to group stars into different categories, called spectral types, which we label by the letters OBAFGKM (As random as it seems, there is some logic behind the lettering. I'll explain in a later post). After years of observations, astronomers have discovered an age sequence that is often drawn over the HR diagram (the line in the graph above). After a star is born, it begins on the main sequence at a moderate brightness and temperature. Over time, the star evolves into a giant or supergiant star and changes its position on the HR diagram, heading towards the upper right hand corner where temperature and brightness are high. When the star dies, it plummets leftward and down on the HR diagram, getting colder and dimmer.
The beauty of the HR diagram is this: Astronomers can use telescopes to measure the brightness, amount of light emitted, and temperature of a star. They then place it on the HR diagram and viola we now know its rough age and spectral type! We can then compare it to stars of the same type and age and learn so much more about the star, just by looking at one graph. Such a wonderful tool!
