Hertzprung-Russell Diagram

The Hertzprung-Russell (HR) Diagram is one of the most important tools in the study of stellar evolution. Developed indenpendently in the early 1900s by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity, or the color of stars against their absolute magnitude.

Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR Diagram as it evolves. This reveals the true power of the HR diagram - astronomers can know a star’s internal structure and svolutionary stage simply by determining its position in the diagram. (Pic Credits: AZO Quantum)

The Hertzsprung-Russel Diagram shows a group of stars in various stages of their evolution. By far the most prominent feature is the main sequence, which runs from the upper left (hot luminous stars) to the bottom right (cool, faint stars) of the diagram. The giant branch is also well populated and there are many white dwarfs. Also plotted are the Morgan-Keenan luminosity classes that distinguish between stars of the same temperature but different luminosity:

1. The main sequence stretching from the upper left (hot luminous stars) to the bottom right (cool, faint stars) dominates the HR diagram. It is here that stars spend about 90% of their lives burning hydrogen and helium in their cores. Main sequence stars have a Morgan-Keenan luminosity class labelled V.
2. Red Giant and Supergiant stars (luminosity classes I through III) occupy the region above the main sequence. They have low surface temperatures and high luminosities which, according to the Stefan-Boltzman law, means they also have a large radii. Stars enter this evolutionary stage once they have exhausted the hydrogen fuel in their cores and have started to burn helium and other heavier elements.
3. White Dwarf stars (luminosity class D) are the final evolutionary stage of low to intermediate mass stars, and are found in the bottom left of the HR diagram. These stars are very hot but have low luminosities due to their small size.

Our Sun is found on the main sequence with a luminosity of 1 and a temperature of around 5,400 Kelvin.

Astronomers generally use the HR diagram to either summarize the evolution of stars, or to investigate the properties of a collection of stars. In particular, by plotting a HR diagram for either a globular or open cluster of stars, astronomers can estimate the age of the cluster from where stars appear to turnoff the main sequence.