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2.4.6Forces, Space and Radioactivity

Hertzsprung-Russell Diagram

5 min read 9 parts Lesson 26 of 41 1 quiz

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Section 1 of 9What an HR diagram shows

A Hertzsprung-Russell (HR) diagram is a scatter graph in which each star is plotted as a point according to its luminosity and its surface temperature. Plotting thousands of stars in this way reveals clear patterns, which show how stars are related and how they change over their lives.

Vertical axis
The vertical axis shows luminosity, usually compared with the Sun (1 = the Sun’s luminosity), on a logarithmic scale,
  • so that each step up is a factor of 10 or 100 greater.
  • It runs from about 10−4 to 106 times that of the Sun.
Horizontal axis
The horizontal axis shows surface temperature,
  • from about 40 000 K to 3000 K, and, unusually, it runs from high temperature on the left to low temperature on the right.
  • The colour of the star therefore goes from blue on the left to red on the right.

Section 2 of 9The main regions

1Main sequence
Main sequence a broad diagonal band running from the top left (hot, luminous, massive stars) to the bottom right (cool, faint, low-mass stars).
  • Most stars, including the Sun, lie on it.
  • The Sun is near the middle at about 5800 K and a luminosity of 1.
2Red giants
Red giants upper right, above the main sequence. They are cool (so red) but very luminous because they are so large.
3Supergiants
Supergiants along the very top, extremely luminous; the blue supergiants are to the left and the red supergiants to the right.
4White dwarfs
White dwarfs bottom left. They are hot (so white or blue-white) but have low luminosity because they are tiny.

Section 3 of 9Reading the diagram

Information can be read from the position of a star:

Left
Further left means a hotter surface and a bluer colour; further right means a cooler surface and a redder colour.
Higher means a greater luminosity.
  • For two stars at the same temperature, the higher one must be larger, as it has more surface area to radiate from.
    • (the same vertical line)
  • So red giants are bigger than red dwarfs.
On the main sequence, hotter and more luminous also means more massive, and a shorter lifetime.

Lines of equal radius run diagonally across the diagram, from the top left to bottom right of each star category, with the largest stars at the top right and the smallest at the bottom left.

Section 4 of 9Tracking the life of a star

As a star evolves, its temperature and luminosity change, so it moves across the HR diagram.

1Protostar
A protostar appears on the right (cool) and moves towards the main sequence as it heats up.
2Main sequence
During its long life, the star stays on the main sequence.
3Red giant
As hydrogen runs out the star expands and cools, so it moves up and to the right, onto the red giant branch (a red supergiant if it is massive).
4White dwarf
A Sun-like star later ejects its outer layers and its hot core appears at the lower left as a white dwarf,
  • which fades by moving down and to the right as it cools.

Linkneutron stars and black holes are too faint to be plotted.

Section 5 of 9Why the diagram is useful

The HR diagram lets astronomers estimate the luminosity of a star from its temperature or colour (if it is on the main sequence).

Why the diagram is useful
If the apparent brightness is measured too, the inverse square law then gives the distance to the star.
It also shows that stars are not randomly distributed: most are on the main sequence because this is the longest stage of life,
  • and relatively few are giants or dwarfs because those stages are short-lived or faint.

Section 6 of 9Common exam tasks

Typical questions ask you to label regions of a blank diagram, to say where a star with given properties would be plotted, or to describe how the position of the Sun changes over its life.

Common exam tasks
Practise reading the axes carefully: temperature increases to the left, and luminosity increases upwards on a logarithmic scale,
  • so a gridline step up means a factor of ten or more, not an addition of a fixed number.
A star far above and to the right of the main sequence is a giant; one far below and to the left is a white dwarf.
When asked to compare sizes, use the fact that at the same temperature the more luminous star is the bigger one.

Section 7 of 9Worked examples

  1. Star A is at 25 000 K with luminosity 104 times the Sun’s, on the main sequence band. Star B is at 3500 K with a luminosity of 104 times the Sun’s. Describe where each lies on the HR diagram and what type of star B is.

  2. Describe the path of the Sun on an HR diagram from now until it becomes a white dwarf.

  3. Two stars are both at 6000 K. Star C has luminosity 100 times that of star D. Which is larger, and by roughly what factor in radius?

Section 8 of 9Key terms

TermMeaning
HR diagramPlot of stellar luminosity against surface temperature.
Main sequenceBand of stable hydrogen-fusing stars from top left to bottom right.
Red giantCool, luminous star at upper right of the diagram.
White dwarfHot, faint star at lower left.
SupergiantExtremely luminous star at the top of the diagram.

Section 9 of 9Exam technique

1
Reading the temperature axis in the usual way. It runs from hot on the left to cool on the right.
2
Placing red giants at the bottom right. They are at the top right.
3
Thinking that red stars are all faint. Red giants are very luminous.
4
Saying the main sequence is a line of stars of the same age. It shows stars at their stable hydrogen-fusing stage at different masses.
5
Plotting white dwarfs in the top left. They are hot but have a low luminosity, so they sit in the bottom left.
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