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.
(a)
State the types of stars found in areas A, B, C and D
(b)
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.
Answer:
(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
(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
Examiner Tip
You need to be able to identify the distinct areas of the Hertzsprung-Russell diagram out of context like in this exam question
