Hertzsprung-Russell Diagrams (Edexcel IGCSE Physics (Modular))
Revision Note
Written by: Ashika
Reviewed by: Caroline Carroll
Hertzsprung-Russell diagrams
The properties of stars can be classified using the Hertzsprung-Russell (HR) diagram
This is a plot of luminosity on the y-axis and temperature on the x-axis
Usually, it is given in solar units, where the luminosity of the Sun = 1, so
For stars which are brighter than the Sun, luminosity > 1
For stars which are dimmer than the Sun, luminosity < 1
Surface temperature is measured in kelvin (K) and is plotted backwards from hottest to coolest
It can also be displayed as a colour where
The hottest stars are blue
The coolest stars are red
The Hertzsprung-Russell diagram
The Hertzsprung-Russell (HR) diagram is a way of displaying the properties of stars and representing their life cycles
The key areas of the H-R diagram are:
The brightest stars (high luminosity) are found near the top
The dimmest stars (low luminosity) are found near the bottom
The hottest stars (high temperature) are found towards the left
The coolest stars (low temperature) are found towards the right
The life cycle of a star can be shown on a Hertzsprung-Russell diagram
The main features of the Hertzsprung-Russell diagram are:
Most stars are found to lie on the main sequence. This is the band of stars going from top left to bottom right
Below the main sequence (and slightly to the left) are the white dwarfs
Above the main sequence on the right-hand side are the red giants
Directly above the red giants are the red supergiants
This means that
The hottest, brightest stars are the largest main sequence stars, also called supergiant stars
The coolest, brightest stars are red supergiants
The hottest, dimmest stars are white dwarfs
the coolest, dimmest stars are the smallest main sequence stars, also called red dwarfs
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)
A = white dwarf stars
B = main sequence stars
C = red supergiant stars
D = red giant stars
(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 Tips and Tricks
Make sure you remember the key components of this diagram to be able to draw it from memory, more specifically which way around the axis goes - the x-axis is the opposite way round to what you might be used to!
If you forget how where the different types of stars are found on the HR diagram, try and think about it logically:
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
White dwarf stars are hot, but very small, so they are not very luminous, so you need to identify the area with a lower luminosity than the main sequence
Red giants and supergiants have clues in their names - they are giant, so they are very luminous and they are red, so they tend to be on the cooler end of the temperature scale
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