The Hertzsprung–Russell diagram Edexcel International A Level Physics revision
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In plain words
Plot every star on one chart, with how bright it truly is up the side and how hot it is along the bottom, and the stars don't scatter at random. Most fall along a single diagonal band, with a few clumps off to the sides. That chart tells the life story of a star.
5 things to know
- The Hertzsprung–Russell diagram plots luminosity (up the side) against surface temperature (along the bottom). The temperature scale runs backwards: hot on the left, cool on the right.
- The main sequence is a diagonal band from the top left (hot, bright stars) to the bottom right (cool, dim stars). Stars spend most of their lives on it, fusing hydrogen into helium in their cores.
- Red giants are at the top right: cool but very luminous, so they must be very large. White dwarfs are at the bottom left: hot but dim, so they must be very small.
- The life of a star like the Sun: a cloud of gas and dust collapses under gravity and heats up until fusion starts; it joins the main sequence; when the hydrogen in its core runs out it swells into a red giant; finally it throws off its outer layers and the core is left as a white dwarf, which slowly cools.
- Stars much more massive than the Sun are hotter and brighter, use up their hydrogen much faster, and end far more violently.
Tips and tricks
- Remember that the temperature axis runs backwards: the hottest stars are on the left.
- "Cool but bright" means big, and "hot but dim" means small, because the power radiated depends on surface area as well as temperature.
It lands in your notebook with its questions as flashcards.
The Hertzsprung–Russell diagram: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is plotted on the two axes of a Hertzsprung–Russell diagram?
Temperature runs backwards along the bottom.
Where are white dwarfs on the diagram?
They are hot but dim.
Where are red giants on the diagram?
They are cool but very luminous.
What is happening in the core of a main sequence star?
This is what keeps it shining steadily for most of its life.
How does a main sequence star much more massive than the Sun compare with the Sun?
It burns through its hydrogen far faster.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
The Hertzsprung–Russell diagram
Edexcel International A Level Physics WPH · 8 marks · papermunch.org
Name ______________________________ Date ______________
Describe where main sequence stars, red giants and white dwarfs are found on a Hertzsprung–Russell diagram.[3]
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Main sequence stars lie on a diagonal band from the top left (hot and luminous) to the bottom right (cool and dim). Red giants are at the top right (cool but luminous). White dwarfs are at the bottom left (hot but dim).
Describe the path of a star like the Sun on the diagram during its life.[3]
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It joins the main sequence and stays there for most of its life. When the hydrogen in its core runs out, it moves up and to the right to become a red giant. It then loses its outer layers and moves to the bottom left as a white dwarf.
Explain how a red giant can be more luminous than a hotter main sequence star.[2]
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The power radiated depends on surface area as well as temperature. A red giant is so large that its enormous surface area more than makes up for its lower temperature.
Answers: The Hertzsprung–Russell diagram
- 1. Main sequence stars lie on a diagonal band from the top left (hot and luminous) to the bottom right (cool and dim). Red giants are at the top right (cool but luminous). White dwarfs are at the bottom left (hot but dim).
- 2. It joins the main sequence and stays there for most of its life. When the hydrogen in its core runs out, it moves up and to the right to become a red giant. It then loses its outer layers and moves to the bottom left as a white dwarf.
- 3. The power radiated depends on surface area as well as temperature. A red giant is so large that its enormous surface area more than makes up for its lower temperature.