What does it mean when a star appears blue in colour?

A star's colour is one of the simplest and most reliable indicators of its surface temperature, governed by Wien's Displacement Law. According to this law, as an object's temperature rises, the wavelength at which it radiates most strongly shifts toward the shorter, bluer end of the spectrum. So when a star glows blue, it tells us its surface is exceptionally hot.

• Our own sun, with a surface temperature of roughly 5,500°C, appears yellowish-white
• Blue stars run considerably hotter, with surface temperatures often between 10,000°C and 30,000°C or more
• Red stars sit at the opposite end, typically below 3,500°C, while orange and yellow stars occupy the middle range
• Astronomers rely on this colour-temperature relationship to classify stars and estimate their evolutionary stage