COMP 521 · stellar radiation
A star’s temperature, written in light
A blackbody is an ideal absorber and thermal emitter. Its spectrum shows how radiation is distributed across wavelengths. Change its temperature. Watch the spectrum rise and its wavelength peak move. Then connect that peak to the root in Wien’s law.
Photosphere temperature
Ideal blackbody
Wavelength peak
Surface flux / 5000 K flux
Total energy per area per time
Predict → Play → Explain
Temperature: before doubling , predict how and the emitted surface flux change. Move the slider, then explain the two different scaling laws.
Normalization: compare 3000 K and 8000 K with absolute radiance, then normalize each peak. Which difference does normalization hide?
A stellar spectrum: a cool photosphere may peak in the infrared and still emit visible light. Use the broad curves to explain why the peak alone does not specify a star’s color.
Root selection: inspect and the positive root. Why does only one correspond to a finite peak wavelength?
The photosphere is the layer from which most of a star’s visible light escapes. These are illustrative photosphere temperatures, not measured stars. A star’s outer gas layers absorb some wavelengths, producing narrow dips called absorption lines. The temperature sweep compares models; it is not a stellar-evolution simulation. Surface flux is energy emitted per unit area per unit time. Luminosity is the whole star’s emitted energy per unit time. For a spherical blackbody star, , so radius also matters for luminosity.
Spectral radiance measures radiation in a direction per unit apparent emitting area, solid angle, and wavelength. Apparent area is the surface area viewed from that direction; solid angle measures a cone of directions. The plot uses centimeter–gram–second (CGS) units; an erg is a unit of energy, and sr means steradian, the unit of solid angle. The curve is a density per unit wavelength. A logarithmic wavelength axis changes its display, not its definition. A peak of cannot be converted into the peak of by substituting . Sources: Astropy blackbody model and NIST Wien constant.