25.2 Stellar Radii

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Main Content

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Part 1: Black Bodies — What Are They?

Before we can understand the laws about stars, we need to understand what a black body is.

A black body is a perfect absorber and a perfect emitter of all electromagnetic radiation (light, heat, etc.). It:

• Absorbs all radiation that hits it — it reflects nothing and transmits nothing.
• Emits all radiation equally well in every direction (this is called being a diffused emitter).

Stars are treated as black bodies. This is because stars absorb radiation that falls on them and emit radiation equally in all directions. This makes stars the best real-world example of a black body.

The radiation a black body emits has a special pattern — it produces a characteristic curve on a graph of intensity vs. wavelength. The shape of this curve depends only on the temperature of the body.

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Part 2: The Intensity–Wavelength Graph

When scientists plot the intensity (brightness) of radiation from a star against the wavelength of that radiation, they get a curved graph. Here is what the graph looks like and what it tells us:

• Each temperature produces its own curve.
• Every curve has a peak — a point where the intensity is highest. The wavelength at this peak is called λ_max (said: "lambda max").
• As the temperature increases, the peak of the curve shifts to shorter wavelengths (moves to the left on the graph).
• As the temperature increases, the height of the peak also increases — meaning the star emits more intense radiation overall.

What this means in practice:

• Hotter stars emit most of their light at shorter wavelengths — they appear blue or white.
• Cooler stars emit most of their light at longer wavelengths — they appear red or orange.

Here is a rough guide to star colours and temperatures: