What is a Hertzsprung-Russell Diagram?
The Hertzsprung-Russell diagram is a tool that astronomers and astrophysicists use to classify different types of stars. It is sometimes called the Color-Magnitude Diagram (CMD). The Hertzsprung-Russell diagram primarily plots color/heat of a star against its absolute magnitude/luminosity. Color and heat are the x-axis. They share an axis because color and heat correlate very well with each other. Up to about 3500 K surface temperature, stars are red, up to about 4500 K they are orange, up to about 6000 K they are yellow, up to around 9000 K they are white, and beyond that are blue. Some stars, such as Wolf-Rayet stars, have surface temperatures up to 25,000 K. Absolute magnitude/luminosity are the y axis of the Hertzsprung-Russell diagram. Again, like with the x axis, the y axis can share two variables because the two are correlated. Absolute magnitude is measured using a standard luminosity scale, while the luminosity side is measured in terms of solar units. The solar units side is