INSpiREzine Stars! | Page 18

The Hertzsprung - Russell (HR) Diagram is a graph that plots a star's color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or 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.

There are 3 main regions (or evolutionary stages) on the HR diagram:

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 into helium in their cores. Main sequence stars have a 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. 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.