Black Body Radiation
A star is made of superheated plasma, not a neat laboratory surface — and yet astrophysicists routinely treat stars as if they were a simple, idealised object with a name that sounds almost like a contradiction: a black body. This modelling choice is what lets a star's temperature be read straight off its spectrum.
What a black body is
A black body is an idealised object that absorbs all electromagnetic radiation falling on it, reflecting none, and — because it must also be a perfect emitter to remain in thermal equilibrium — radiates energy across a continuous range of wavelengths. The crucial property of a black body is that the radiation it emits depends on only one thing: its temperature. It doesn't depend on what the object is made of, its size, or its shape — only on how hot it is.
No real object is a perfect black body, but a star is an extremely good approximation to one. A star is opaque, dense, and radiates from deep within a thick outer layer of gas, so despite its complicated internal structure and chemical make-up, the light leaving its surface behaves almost exactly like ideal black-body radiation. This is the single modelling assumption that underlies everything else in this section of the course: treat a star as a black body radiator.
The shape of the black-body spectrum
If you plot the intensity of radiation emitted by a black body against wavelength, the resulting curve has a very distinctive and consistent shape:
- At very short wavelengths, the intensity is close to zero.
- As wavelength increases, intensity rises, reaching a single, well-defined peak at some particular wavelength.
- Beyond the peak, intensity falls away again, but more gradually than it rose — tailing off slowly toward zero at long wavelengths.
Every black body produces a curve of this same general shape, but the details — how tall the peak is, and where along the wavelength axis it sits — depend entirely on the object's temperature:
- A hotter black body produces a curve with a taller peak (it radiates more total power at every wavelength) and a peak that is shifted toward shorter wavelengths.
- A cooler black body produces a curve with a lower, broader peak, shifted toward longer wavelengths.
This is exactly the behaviour familiar from everyday heated objects: a metal bar heated in a flame first glows a dull red, then orange, then yellow-white as it gets hotter — its emission peak is moving to shorter wavelengths as the temperature climbs, eventually shifting enough of its output into the visible range for the colour to become obvious to the eye.
A star's black-body spectrum is completely characterised by a single number: its surface temperature. Everything covered in the rest of this section — Wien's law and Stefan's law — is really about extracting that one number, and the star's total power output, from features of this curve.
Where this leads next
This lesson sets up the black-body picture in outline, without yet putting any numbers on it. The next two lessons build directly on this idea quantitatively: one gives a precise relationship between a black body's temperature and the wavelength at which its emission curve peaks, and the other gives a precise relationship between a black body's temperature and the total power it radiates. Together, these turn the qualitative curve-shape description above into tools that let a star's temperature, size and power output be calculated from its observed spectrum.
Key takeaways
- A black body is an idealised perfect absorber and perfect emitter of radiation, whose emission spectrum depends only on its temperature — not on composition, size or shape.
- A star is treated as a black body radiator throughout this topic, despite being made of plasma rather than a simple solid surface, because this approximation matches observed stellar spectra well.
- The black-body emission curve (intensity against wavelength) rises from near-zero, reaches a single peak, then falls away gradually at longer wavelengths.
- Hotter black bodies have taller peaks (more total power radiated) and peaks shifted to shorter wavelengths than cooler black bodies.
- This qualitative curve shape is the foundation for Wien's displacement law and the Stefan–Boltzmann law, covered in the next two lessons.