Science Puzzle
Which Star Is the Hottest?
On a clear winter night you can see two very different stars in the constellation of Orion. Betelgeuse, at his shoulder, glows orange-red. Rigel, at his foot, shines a brilliant blue-white. Our own Sun is somewhere in between: yellowish white.
Which of the three has the hottest surface?
The Answer
Rigel. Its surface is about 12,000 °C, more than twice as hot as the Sun's 5,500 °C. Red Betelgeuse is the coolest of the three, at about 3,300 °C.
Anything hot enough glows, and the colour of the glow depends on the temperature. Heat a poker in a fire and it first glows a dull red, then orange, then yellow. Get something hotter still and it shines white, and the very hottest things glow bluish white. As the temperature rises, more of the light comes out at the blue end of the spectrum.
Stars follow exactly the same rule, so astronomers can read a star's surface temperature from its colour. That's the opposite of the everyday code we use on taps, where red means hot and blue means cold. In the sky, red stars are the cool ones.
Betelgeuse may be cool, but it's enormous: if it sat where the Sun is, it would swallow the orbits of Mercury, Venus, Earth and Mars. Its huge glowing surface makes it one of the brightest stars in the sky.
The principle: Colour and temperature. Hot objects glow, and the hotter they are the more their light shifts from red through yellow and white to blue, so a star's colour shows its surface temperature.