The Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram (HR diagram) is the most important plot in stellar astronomy. It graphs stars by temperature (or colour) against luminosity (or absolute magnitude) — and reveals that stars are not randomly scattered but fall into distinct groups that trace their life stories.
Reading the diagram
- X-axis: Surface temperature, running right to left (hot → cool, or equivalently: blue → red). Typical range: 40,000 K (O-type) to 2,500 K (M-type).
- Y-axis: Luminosity relative to the Sun ($L_\odot$), on a logarithmic scale. Range: $10^{-4} L_\odot$ to $10^6 L_\odot$.
The main sequence
About 90% of all stars lie on a diagonal band running from hot, luminous blue stars (upper left) to cool, dim red stars (lower right). This is the main sequence — stars that are fusing hydrogen into helium in their cores. Where a star sits on the main sequence is determined almost entirely by its mass:
- O/B stars (upper left): Massive ($10\text{–}100 M_\odot$), hot, luminous, short-lived (millions of years)
- G stars (middle): Sun-like ($\sim 1 M_\odot$), ~10 billion year lifetimes
- M stars (lower right): Low mass ($0.1\text{–}0.5 M_\odot$), cool, dim, lifetimes exceeding the age of the universe
The Sun is a G2V star — G2 spectral class, V (Roman numeral 5) for main sequence. Perfectly average.
Off the main sequence
Red giants (upper right): Cool but very luminous — therefore enormous. Stars evolve here after exhausting hydrogen in their cores. The Sun will become a red giant in ~5 billion years, expanding to engulf Mercury and Venus.
White dwarfs (lower left): Hot but very dim — therefore tiny (roughly Earth-sized). The exposed cores of dead low-mass stars, slowly cooling over billions of years.
Supergiants (top): The most luminous stars of all, both hot (blue supergiants) and cool (red supergiants like Betelgeuse). Massive, short-lived, and destined to explode as supernovae.
Spectral classification
The mnemonic: Oh Be A Fine Girl/Guy, Kiss Me — O, B, A, F, G, K, M (from hottest to coolest).
| Class | Temperature | Colour | Example |
|---|---|---|---|
| O | 30,000–50,000 K | Blue | 10 Lacertae |
| B | 10,000–30,000 K | Blue-white | Rigel |
| A | 7,500–10,000 K | White | Sirius, Vega |
| F | 6,000–7,500 K | Yellow-white | Canopus |
| G | 5,200–6,000 K | Yellow | Sun, Alpha Centauri A |
| K | 3,700–5,200 K | Orange | Arcturus |
| M | 2,400–3,700 K | Red | Betelgeuse, Proxima Centauri |
Stellar evolution as a journey
A star’s life is a path across the HR diagram. A Sun-like star: 1. Contracts from a gas cloud (moves right to left as it heats up) 2. Joins the main sequence and stays for ~10 billion years 3. Becomes a red giant (moves right and up) 4. Sheds its outer layers as a planetary nebula 5. Becomes a white dwarf (moves down and left) 6. Slowly cools (moves right along the bottom)
The HR diagram isn’t just a snapshot — it’s the map of stellar destiny.