Section 1 of 5Axes and Layout
The diagram is a plot that sorts stars by how much energy they emit and how hot their surfaces are.
Vertical axis
- Luminosity is plotted upwards, usually in solar units where the Sun has a luminosity of 1, so brighter stars sit higher.
Horizontal axis
- Surface temperature in kelvin is plotted along the bottom, but it runs backwards, with the hottest stars on the left.
Colour link
- Stars on the left are blue or white, and stars on the right are red, because hotter surfaces emit bluer light.
Scales
- Both axes usually span huge ranges, so the scales often increase by factors of ten rather than by equal steps.
Section 2 of 5Regions of the Diagram
Most stars fall into a few clear groups.
Regions of the Diagram
Main sequence
- A band running from top left to bottom right containing most stars, including the Sun, during the stable fusion stage of their lives.
Red giants
- Found at the upper rightcool surfaces but high luminosity because the stars are very large.
Red supergiants
- Found above the red giants, being even larger and more luminous while still having cool, red surfaces.
White dwarfs
- Found at the lower lefthot surfaces but low luminosity because the stars are very small.
| Group | Temperature | Luminosity |
|---|---|---|
| Main sequence | Wide range | Wide range, rising with temperature |
| Red giant | Low | High |
| White dwarf | High | Low |
Section 3 of 5Life Cycles on the Diagram
A star moves across the diagram as its size and temperature change.
1
A new star joins the main sequence once fusion of hydrogen in its core begins and it settles into a stable state.
2
When the core hydrogen runs low, a star similar to the Sun swells and cools, moving to the red giant region at the upper right.
3
After the red giant stage the outer layers are lost and the core shrinks and heats, so the star moves to the white dwarf region at the lower left.
4
A much more massive star leaves the main sequence for the red supergiant region instead, before ending as a neutron star or black hole.
Section 4 of 5Reading Values from the Diagram
Plotting a star
- Find the temperature on the reversed horizontal axis, find the luminosity on the vertical axis, and mark where the two meet.
Using solar units
- A star 10 000 times as luminous as the Sun has a luminosity of 10 000, which is 10⁴ on a logarithmic scale.
Comparing stars
- The hotter and more luminous of two stars is the one nearer the top left;
- the cooler and fainter lies nearer the bottom right.
Estimating size
- For equal temperature, the more luminous star must be larger, which is why giants lie well above the main sequence.
Section 5 of 5Exam Checklist & Tips
5.2.5 – Key Points for the Exam
- Hertzsprung-Russell diagram A graph of star luminosity against surface temperature, with temperature decreasing to the right.
- Main sequence The band of stars that are fusing hydrogen in their cores.
- Red giant A large, cool, luminous star near the end of the life of a Sun-like star.
- White dwarf A small, hot, faint star that is the final visible stage of a Sun-like star.
- Solar luminosity A unit equal to the luminosity of the Sun, used to scale the vertical axis.
Common Mistakes
- Plotting temperature increasing from left to right instead of reversed.
- Placing white dwarfs at the top because they are white and hot; they are faint, so they lie near the bottom.
- Saying all stars lie on the main sequence; giants and white dwarfs lie outside it.
- Mixing up the Sun's positionit is a main sequence star with luminosity 1, not a giant.