Star brightness and colour
Why stars differ in brightness and colour, what magnitude means and how a star’s colour reveals its temperature.
Brightness: magnitude
Astronomers measure brightness on the magnitude scale inherited from Hipparchus: smaller numbers are brighter. Each step is about 2.5 times, so 5 magnitudes is exactly 100 times. Sirius is −1.46, Polaris about 2, and from a dark site you can see stars to magnitude 6. A city sky may stop at 3.
How bright a star looks depends on two things: how luminous it really is and how far away. Deneb looks fainter than Vega but is about 2,600 light years away; in reality it is tens of thousands of times more luminous than the Sun.
Colour: temperature
Look at Orion: Betelgeuse (top left) glows orange-red, Rigel (bottom right) blue-white. Colour shows surface temperature:
- red: about 3,000 °C (Betelgeuse, Antares)
- orange: 4,000 °C (Arcturus, Aldebaran)
- yellow: 5,500 °C (the Sun, Capella)
- white: 9,000 °C (Vega, Sirius)
- blue-white: 12,000 °C and more (Rigel, Spica)
Colours are subtle to the eye; binoculars slightly out of focus make them easier to see. Compare them in the bright stars list.