Hertzsprung-Russell Diagram
Higher Tier
- 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
- The luminosity of a star is defined as
The total amount of light energy emitted by the star
- Luminosity is a measure of a star's brightness
- 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
Colour & Temperature of Stars
The temperature of stars can be classified according to their colour
- 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 Hertzsprung-Russell Diagram
The Hertzsprung-Russell (HR) diagram is a way of displaying the properties of stars and representing their life cycles
- 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.
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:
Exam Tip
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 go - the x-axis is the opposite way round to what you might be used to!
If you forget how where the different types of star 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
