What is the Hertzsprung-Russell Diagram?

The Hertzsprung-Russell diagram, commonly referred to as the H-R diagram, is one of the most important diagrams in astronomy.

Using this diagram, you can trace the evolutionary path a star will follow, which itself depends on the mass of the star.

About the diagram and its axes

• Stellar brightness and colour are observable and measurable properties of stars.
• The colour of a star depends on its temperature.
• Observable stellar brightness is affected by distance — astronomers need to correct for this to determine the true brightness of a star.
• The H-R diagram is a plot of true stellar brightness against stellar temperature.
• By knowing a star’s true brightness and temperature, we can determine what stage of its life it is in.

Stellar evolution

• Stars form inside giant clouds of gas and dust called nebulae. When a star becomes hot enough to fuse hydrogen into helium in its core, it becomes a main sequence star.
• The long, diagonal band on the H-R diagram is called the main sequence.
• Stars, including the Sun, will spend most of their lives on the main sequence.
• Stars that are bigger than the Sun burn their hydrogen more quickly and spend less time on the main sequence compared to the Sun.
• The evolution of a star depends on the mass of the star.
• When a sun-like star runs out of hydrogen into its core, it will evolve into a red giant star whereas a more massive star becomes a red supergiant star.
• Red giant stars will fuse helium into carbon and oxygen, and once the helium runs out, they evolve into planetary nebulae, leaving behind a white dwarf star.
• Red supergiant stars are able to fuse heavier elements in their cores, but when they finally run out of fuel, they end their lives in a supernova leaving behind a neutron star or a black hole.

Common misconceptions

Students often have misconceptions related to the H-R diagram; here are a few we have come across:

• Students think that the diagram represents the movement of a star through space because educators talk about stars moving along the H-R diagram.
• Related to the above, students sometimes think the main sequence is a location in space that stars move to.
• The decreasing temperature scale on the x-axis is counterintuitive. Some students unintentionally assume hotter stars are red and bluer stars are cold, because they tend to look at the colours first rather than the temperature scale.
• Confusing the observed brightness of a star with its true brightness — students think that the brightest stars we see in the night sky are the biggest.