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

• 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

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