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What is the Hertzsprung-Russell Diagram?

The H-R diagram is one of the most important diagrams in astronomy, enabling the evolutionary path a star will follow to be traced

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 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.
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Physics, Astronomy, and Space: Teaching Secondary Science

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