Orion is prominent in the winter sky. The star Aldebaran on one shoulder is clearly red to the naked eye. It is a red giant, a star that is running out of hydrogen fuel. It begins helium burning which causes the star to expand. As it expands, it cools. This cooling is relative - it still has a surface temperature of several thousand degrees. But the colour of the light emitted is a clue. Red photons have a lower frequency than the yellow ones from the Sun and thus carry less energy. Hence the red giant has a cooler surface. It is so bright because the expansion has vastly increased the size of the surface and light can only escape through the surface. The bigger the surface, the more light emitted at once.
