The Hertzsprung-Russell Diagram Explained

## The Hertzsprung-Russell Diagram

Developed independently by Ejnar Hertzsprung (1911) and Henry Norris Russell (1913), the H-R diagram plots stars by their surface temperature (or spectral type) against their luminosity (or absolute magnitude). This single chart reveals stellar structure, evolution, and classification.

### Reading the Diagram

- **X-axis**: Surface temperature (decreasing left to right: 40,000K → 2,500K) or spectral type (O B A F G K M)
- **Y-axis**: Luminosity (increasing upward: 0.0001 L_sun → 1,000,000 L_sun) or absolute magnitude (+15 → -10)

### The Main Sequence

About 90% of all stars fall on the **main sequence**, a diagonal band from hot, luminous O-type stars (upper left) to cool, dim M-type stars (lower right). Main-sequence stars are fusing hydrogen into helium in their cores.

| Spectral Type | Temperature | Mass (M_sun) | Luminosity (L_sun) | Main-Sequence Lifetime |
|---------------|-------------|-------------|--------------------|-----------------------|
| O5 | 42,000 K | 60 | 790,000 | 1 million years |
| B0 | 30,000 K | 18 | 20,000 | 11 million years |
| A0 | 9,800 K | 2.1 | 54 | 1 billion years |
| F0 | 7,200 K | 1.6 | 6.5 | 3 billion years |
| G2 (Sun) | 5,800 K | 1.0 | 1.0 | 10 billion years |
| K0 | 5,300 K | 0.8 | 0.4 | 17 billion years |
| M0 | 3,800 K | 0.5 | 0.08 | 56 billion years |
| M8 | 2,500 K | 0.08 | 0.0001 | Trillions of years |

### Giants and Supergiants

Stars that have exhausted core hydrogen expand and cool, moving to the **upper right** of the diagram:

- **Red giants** (luminosity class III): 10-100 L_sun, 3,500-5,000 K. Examples: Arcturus, Aldebaran
- **Supergiants** (I-II): 10,000-500,000 L_sun. Examples: Betelgeuse (red), Rigel (blue)

### White Dwarfs

Beneath the main sequence in the **lower left** sit white dwarfs — stellar remnants with masses around 0.6 M_sun compressed into Earth-sized volumes. Surface temperatures of 8,000-40,000 K but luminosities of only 0.001-0.0001 L_sun. Example: Sirius B.

### Evolutionary Tracks

A star's path across the H-R diagram traces its life:
1. Pre-main-sequence: contracts rightward from the Hayashi track
2. Main sequence: billions of years at a fixed point
3. Red giant branch: moves right and up
4. Horizontal branch or red supergiant: helium fusion
5. Planetary nebula → white dwarf (low mass) or supernova → neutron star/black hole (high mass)

The H-R diagram is thus both a snapshot of stellar diversity and a roadmap of stellar evolution.