The Hertzsprung - Russell (HR) Diagram (OCR A Level Physics)

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Katie M


Katie M



The Hertzsprung - Russell (HR) Diagram

  • Danish astronomer Ejnar Hertzsprung, and American astronomer Henry Noris Russell, independently plotted the luminosity of different stars against their temperature
    • Luminosity, relative to the Sun, on the y-axis, goes from dim (at the bottom) to bright (at the top)
    • Temperature, in degrees Kelvin, on the x-axis, goes from hot (on the left) to cool (on the right)


The Hertzsprung-Russell Diagram depicts the luminosity of stars against their temperature

  • Hertzsprung and Russel found that the stars clustered in distinct areas
  • Most stars are clustered in a band called the main sequence
    • For main sequence stars, luminosity increases with surface temperature
  • A smaller number of stars clustered above the main sequence in two areas, red giants, and red supergiants
    • These stars show an increase in luminosity at cooler temperatures
    • The only explanation for this is that these stars are much larger than main sequence stars
  • Below and to the left of the main sequence are the white dwarf stars
    • These stars are hot, but not very luminous
    • Therefore, they must be much smaller than main sequence stars
  • The Hertzsprung-Russell Diagram only shows stars that are in stable phases
    • Transitory phases happen quickly in relation to the lifetime of a star
    • Black holes cannot be seen since they emit no light

Worked example

Stars can be classified using the Hertzsprung-Russell (H-R) Diagram.


State the types of stars found in areas A, B, C and D
On the H-R diagram, plot the star with a surface temperature of 20 000 K and a luminosity 10 000 times greater than the Sun and label it Star X.

Part (a)

Step 1: Identify the main sequence on the HR diagram

    • The main sequence is the easiest to recognise as it is the long band diagonally central to the diagram where the majority of stars are found
    • The main sequence is region B

Step 2: Identify the white dwarf region on the HR diagram

    • White dwarf stars are hot, but not very luminous
    • Identify the area with a lower luminosity than the main sequence
    • The white dwarf region is area A

Step 3: Identify the red giant and red supergiant regions on the HR diagram

    • Red giants and super red giants have a greater luminosity than main sequence stars at a lower temperature
    • That means that they are bigger than main sequence stars
    • The bigger they are, the more luminous they are
    • So, the super red giants are more luminous than the red giants and will appear above them on the graph
    • The super red giant region is area C
    • The red giant region is area D

Part (b)

Step 1: List the known quantities

    • Surface temperature of Star X = 20 000 K
    • Luminosity of Star X = 10 000 times that of the Sun

Step 2: Use the graph to find the value for the luminosity of the Sun

    • Use a ruler and pencil to draw a line from the position of the sun to the luminosity axis (y-axis)
    • The Sun’s luminosity on this scale is 1 because the luminosities given are relative to the luminosity of the sun


Step 3: Calculate the luminosity of Star X

    • Star X is 10 000 times that of the Sun
    • The luminosity of the Sun is 1

10 000 × 1 = 10 000 or 104

Step 4: Plot the position of Star X on the HR diagram

    • Locate the surface temperature of Star X at 20 000 K
    • Locate the luminosity of Star X at 104


      • Plot the point and label it Star X


Exam Tip

You need to be able to identify the distinct areas of the Hertzsprung-Russell diagram out of context like in this exam question

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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