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

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Katie M

Expertise

Physics

## 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.

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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