Brightness, angular size, spectrum, line width, redshift, and sky position come from calibrated instruments and always carry uncertainty.
ILLUSTRATIVEStructure & light
Light fades outward; the fitted curve turns pixels into a radius and concentration.
Σ
Equation family
Related tools for this lesson
Mass, age, metallicity, star-formation rate, and halo structure follow only after an equation, calibration, geometry, or population model is chosen.
The center animation is illustrative, not a simulation or observation. Sliders expose trends while suppressing geometry, noise, degeneracy, and selection bias.