Jupiter2U School
Chapter 11

11. The Hertzsprung–Russell Diagram

High School

One remarkable chart organizes almost every star in the universe by just two properties, revealing the whole story of stellar life at a glance.

At a glance
Core ideaPlotting luminosity vs temperature sorts nearly every star into place.
Key termMain sequence — the diagonal band holding ~90% of stars.
You can…Classify a star by its spectral type (O B A F G K M).
Watch outSame temperature ≠ same spot — size (giant vs dwarf) matters too.
01 · Theory

Plotting luminosity against temperature

The Hertzsprung–Russell (H–R) diagram plots each star's luminosity (brightness, vertical axis) against its surface temperature or spectral type (horizontal axis, plotted backwards — hot on the left, cool on the right). Stars are classified into spectral types, hottest to coolest, remembered by the mnemonic: "Oh Be A Fine Girl/Guy, Kiss Me" — O B A F G K M.

Roughly 90% of stars, including the Sun (a G-type star), fall along a diagonal band called the main sequence, running from hot-bright (upper left) to cool-dim (lower right). Off that band: red giants and supergiants occupy the cool-but-bright upper right (huge surface area compensates for lower temperature), while white dwarfs sit hot-but-dim in the lower left (small, hot, faint).

02 · Explanation

Reading a star's life story from its position

A star's position on the H–R diagram is a snapshot of its current stage of life. A star spends most of its life on the main sequence, slowly moving along it as it ages. When core hydrogen runs out, a star swings off the main sequence up into the giant branch, and eventually collapses down toward the white dwarf corner (for Sun-like stars) — you can trace an entire stellar biography as a path across the chart.

Key ideaBecause luminosity depends on both temperature and size, two stars can share the same temperature yet sit in wildly different places on the diagram — a cool red giant is bright because it is enormous, while a cool red dwarf is dim because it is tiny.
Luminosity → ← Temperature (hot to cool) Main sequence Giants White dwarfs O B A F G K M
Most stars fall on the diagonal main sequence; giants sit upper right, white dwarfs sit lower left.
03 · Practical

Worked example — classifying a star

A star has a surface temperature of about 3 500 K and a luminosity roughly 500 times the Sun's. Where does it sit on the H–R diagram, and what is it?

Solution
  1. 3 500 K is a cool temperature — spectral type M, the coolest common class (the Sun is about 5 800 K, type G).
  2. A main-sequence M star would be very dim (a red dwarf), far less luminous than the Sun — but this star is 500 times brighter than the Sun.
  3. Cool but extremely luminous means the star must be physically enormous — placing it in the upper right of the diagram.

Answer: this is a red giant (or supergiant) — a cool, aging, hugely swollen star, not a small, dim main-sequence red dwarf.

04 · Q&A

Test your understanding

What are the two axes of the H–R diagram?

Luminosity (brightness) on the vertical axis, and surface temperature (or spectral type) on the horizontal axis, conventionally plotted with hot on the left and cool on the right.

What spectral type is the Sun, and where does it sit on the H–R diagram?

The Sun is a type G star, sitting comfortably in the middle of the main sequence — neither the hottest nor coolest, neither the brightest nor dimmest of stars.

Why are white dwarfs found in the lower-left of the diagram?

White dwarfs are very hot (placing them to the left) but extremely small — roughly Earth-sized — so despite their high temperature, their tiny surface area makes their total luminosity very low (placing them near the bottom).

What is the O B A F G K M sequence, and what does it order?

It's the spectral classification of stars from hottest to coolest: O (hottest, blue) through B, A, F, G, K, to M (coolest, red). The mnemonic "Oh Be A Fine Girl/Guy, Kiss Me" helps remember the letter order.

Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

Luminosity axisTemperature axisMain sequenceGiantsSupergiantsWhite dwarfsThe H-R Diagram
Infographic

The key facts, visualised

Main sequence
Diagonal band where stars fuse hydrogen most of their lives
Giants
Cool but very luminous; upper right of the diagram
White dwarfs
Hot but faint; lower left of the diagram
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1A star has surface temperature ~3,500 K and luminosity ~500 times the Sun. Where does it sit and what is it?

  1. Low temperature places it on the cool (right) side
  2. High luminosity places it near the top
  3. Cool but very bright means it is large: a red giant

Example 2A star is hot (~10,000 K) but very faint. What kind of star is it?

  1. Hot places it on the left
  2. Faint places it near the bottom
  3. Hot yet dim means small
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1What two properties do the axes of the H-R diagram show?
Luminosity (vertical) and surface temperature (horizontal).
2What is the main sequence?
The diagonal band where stars spend most of their lives fusing hydrogen.
3Where do red giants lie on the diagram?
Upper right: cool but very luminous.
4Where do white dwarfs lie?
Lower left: hot but faint.
5Why is a cool, very luminous star necessarily large?
To be bright despite a cool surface it must have a huge radiating area.
6Does a star's position on the H-R diagram change over its life?
Yes, it moves off the main sequence as it evolves into a giant and later a remnant.