The  Hertzsprung-Russell Diagram is a scatter plot of stars showing the relationship between the stars’ absolute magnitudes, otherwise known as their luminosity, versus their stellar classifications, otherwise known as their effective temperatures.

A simple definition of what the diagram represents is the plotting of a star’s brightness against its color.

Historical background

The diagram is called the Hertzsprung-Russell Diagram because it was created in 1910 by Ejnar Hertzsprung and Henry Norris Russell. Their diagram represents a major step towards an understanding of stellar evolution as the diagram plots the apparent magnitudes of stars against their color, usually for a cluster so that the stars are all at the same distance. It is said that this diagram can be seen as a color–magnitude diagram.

Utilization And Analysis

The majority of the stars that would be seen occupy the diagram region called the main sequence. When stars are found on the main sequence line, stars are are at the stage of their lives in which their cores are fusing hydrogen.

The next grouping of stars can be found on the horizontal branch of the diagram, which signifies that there is helium fusion occurring in the core and there is a shell surrounding said core that has hydrogen burning in it.

Utilized by scientists to measure distance from a galaxy or star cluster, the Hertzsprung-Russell Diagram can be helpful in determining how far these objects are from Earth. Scientists are able to measure distance by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances.

As the The SAO Encyclopedia of Astronomy explains, “there are 3 main regions, or evolutionary stages, of 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 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-Boltzmann law, means they also have 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.”

Role Played By Diagram In Stellar Physics Development

After they analyzed the diagram, scientists and astronomers speculated that the diagram might assist in demonstrating stellar evolution. This results from stars collapse from red giants to dwarf stars, then move down along the line of the main sequence during their lifetimes.

Before the introduction of the diagram, scientists thought stars radiated energy by converting gravitational energy into radiation through the Kelvin–Helmholtz mechanism. Therefore, scientists estimated that the age of the Sun was in the range of tens of millions of years old, which resulted in a conflict over the Solar System age. This was because at this time, there was clear evidence from the fossil records that Earth was far older than a few million years old.

This conflict was only resolved in the 1930s when nuclear fusion was identified as the source of stellar energy. However, getting to the understanding that nuclear fusion was the source of stellar energy took some time and some leaps of faith by some in the scientific community.

References and Future Reading

http://astronomy.swin.edu.au/cosmos/H/Hertzsprung-Russell+Diagram

https://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_hrintro.html

https://www.atnf.csiro.au/outreach/education/senior/cosmicengine/stars_hrdiagram.html