Stellar Evolution

Stars come in different colours

• Look at the night sky carefully and you can pick out different colours: Betelgeuse in Orion looks distinctly orange-red; Rigel in the same constellation is a brilliant blue-white; the Sun is yellow; many faint stars look a quiet red
• A star's colour is set by the temperature of its surface. Hot objects glow blue-white; cooler objects glow red. This is exactly the same physics as a piece of metal heated in a furnace: at moderate heat it glows red, at higher heat it turns orange, then yellow, then white-hot, then blue-white

Colour ↔ surface temperature

Hotter star → bluer colour

Cooler star → redder colour

• Be careful: in everyday life we associate red with hot (red-hot iron, red flames) and blue with cold (cold water taps, ice). For stars it is the other way around: blue is the hottest, red is the coolest. The flame analogy is closer to the truth than the tap one, since a Bunsen burner's blue cone is hotter than its yellow tip

What sets a star's temperature?

• A star's surface temperature depends on its mass and the stage of its life it is in
• Massive young stars on the main sequence are blue-white and very hot
• Lighter main-sequence stars (like the Sun) are yellow
• Aging stars that have swelled into red giants or red supergiants are cool and red, even though they may be very large and very bright
• As a star expands, its surface cools; as a star contracts, its surface heats up. So when the Sun eventually becomes a red giant, its outer layers swell out and cool to red; when it then collapses to a white dwarf, those layers contract again and heat to white-hot