Star Colours
The bluest stars are the hottest and the reddest are the coolest — the opposite of the red and blue on a tap.
What’s going on
A star’s colour tells us how hot its surface is. Red stars are about 3,000 kelvin; blue stars can be hotter than 30,000 kelvin; the Sun sits in between, near 6,000. (Kelvin is like Celsius, but starts at absolute zero.) In Orion you can compare orange-red Betelgeuse with blue-white Rigel. Astronomers use colour to take the temperature of stars they could never visit.
Myth
Red means hot and blue means cold.
Fact
For stars, as for flames, blue is hotter than red.
Learn more
NASA APOD, “Orion’s Star Colors” (2 Dec 1996); OpenStax, Astronomy 2e, §17.2 “Colors of Stars”.
Look up tonight
Find Orion on a clear evening between about December and March, and compare orange Betelgeuse with blue-white Rigel.
Play Myth or Fact? Read out either the sentence on the card or the myth above. Can everyone tell which is true?
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