Stars - Colour and Spectrum

Dr Andrew Conway

The Hertzsprung Russell (HR) Diagram

HR Diagram

Source: Richard Powell CC-BY SA 2.5

Coloured dots of light

Kelvin temperature scale

Typical temperatures

Example Temperature
Cosmic background 3 K
Pluto 44 K
Mars 210 K
Earth 288 K
Venus 737 K
Proxima Centauri 3000 K
The Sun 5800 K
Rigel 11,000 K
Solar corona 2,000,000 K

The electromagnetic spectrum

Electromagnetic spectrum

Source: P. Wiena CC-BY SA 3.0

Temperature and colour

Fundamental physics tells us that objects change colour as they heat up, e.g.

Spectrum and temperature

Wien's law

Source: Darth Kule Public Domain


Wien's law

Wien's law states that temperature in K is inversely proportional to peak wavelength.

For example:

But, this is just peak emission - the Sun also emits in the infrared and Rigel (11,000 K) emits in the visible too.

Emission examples

Example Temperature Emission
Cosmic background 3 K Microwaves
Pluto 44 K Infrared
Mars 210 K Infrared
Earth 288 K Infrared
Venus 737 K Infrared
Proxima Centauri 3000 K Infrared/visible
The Sun 5800 K Visible
Rigel 11,000 K Ultraviolet/visible
Solar corona 2,000,000 K X-rays

Real data

Real star spectrum

Source: Michael Richmond CC-BY-NC-SA 2.0

Interactive HR diagram