Stars spend most of their lives on the main sequence, where hydrogen fusion in the core balances gravitational compression.
The Hertzsprung-Russell (HR) diagram
The HR diagram plots luminosity (brightness, y-axis) against surface temperature (x-axis, decreasing left to right).
Stars on the main sequence form a diagonal band from hot, bright stars (top left) to cool, dim stars (bottom right).
How mass determines properties
| Property | Low-mass star | High-mass star |
|---|---|---|
| Surface temperature | Lower (cooler) | Higher (hotter) |
| Colour | Red/orange | Blue/white |
| Luminosity | Dim | Very bright |
| Main sequence lifetime | Very long (billions of years) | Short (millions of years) |
| Fuel consumption | Slow | Very fast |
A more massive star fuses hydrogen faster, so it is brighter but runs out of fuel sooner.
Colour and temperature
| Star colour | Approximate surface temperature |
|---|---|
| Red | 3000 K |
| Orange | 4500 K |
| Yellow (like the Sun) | 5800 K |
| White | 8000 K |
| Blue | 20 000 K+ |
Other regions of the HR diagram
- Red giants / supergiants: Top right. Large, cool, very bright. Stars that have left the main sequence.
- White dwarfs: Bottom left. Small, hot, dim. Remnants of low-mass stars.
Common errors and how to correct them
- Saying red stars are hotter than blue stars. Blue stars are the hottest; red stars are the coolest.
- Thinking bigger stars live longer. Massive stars have shorter lifetimes because they burn fuel much faster.
How to apply this in an exam
Place the star on the HR diagram based on its temperature and luminosity. Describe how mass affects colour, temperature, luminosity and lifetime.
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