Star Color, Temperature & Luminosity Explained
Why are hot stars blue and cool stars red? Learn Wien’s law and the Stefan–Boltzmann luminosity relation with a visual model and worked examples.
The idea behind it
Temperature strongly affects emitted power per unit area. A large, cool star can be more luminous than a small, hot star because it has more radiating surface. Wien’s law relates temperature to the wavelength of peak emission.
The relationship to remember
L / L☉ = 1² × (5,772 / 5772)⁴ = 1. Wien peak = 2,897,772 / 5,772 = 502.04 nm.
Make your own discovery
Keep the radius fixed and double the temperature. Luminosity increases sixteenfold.
Open the Star Color & Luminosity Calculator ↗Key formulas
| Quantity | Formula | Note or example |
|---|---|---|
| Luminosity (Stefan–Boltzmann) | L / L☉ = (R / R☉)² × (T / 5772 K)⁴ | Radius and temperature relative to the Sun |
| Wien’s displacement law | λ_peak = 2,897,772 nm·K ÷ T | Hotter stars peak at shorter wavelengths |
Worked examples
Each row uses one of this lab’s presets and the same model as the interactive tool.
| Example | Luminosity | Peak wavelength | Apparent color |
|---|---|---|---|
| Sun-like | 1 Sun | 502 nm | Yellow-white |
| Cool dwarf | 0.00292 Suns | 966 nm | Orange-red |
| Hot star | 1,140 Suns | 193 nm | Blue-white |
| Sirius A | 25.72 Suns | 292 nm | White |
| Red giant | 216.32 Suns | 828 nm | Orange-red |
What this model leaves out
Stars are approximated as blackbodies. Display colors are illustrative, not calibrated spectra. Radius is shown with a compressed visual scale.
Check your understanding
Are blue stars hotter than red stars?
Yes. A hotter blackbody emits relatively more short-wavelength light, shifting its appearance toward blue-white.
Does a brighter-looking star emit more light?
Not necessarily. Apparent brightness also depends on distance and any intervening dust.
What color is the Sun?
The Sun emits most strongly near 500 nm, but its light mixes all visible colors and looks white from space. It appears yellow or orange near the horizon because the atmosphere scatters blue light out of the beam.
Continue with the formula reference or test yourself in space practice.