continuous spectrum

A continuous spectrum is the familiar, unbroken rainbow — every colour blending smoothly into the next, with no bright lines added and no dark lines missing. It is what you get from a glowing solid, a glowing liquid, or a very dense glowing gas: a light bulb's filament, hot lava, molten iron, or the deep interior of a star.

Why dense things glow this way comes down to crowding. In a dense, hot source the atoms are packed so tightly and jostle so constantly that their tidy individual energy levels smear together into one continuous blur. Instead of a few special colours, the source can emit light at essentially every wavelength, with the brightness following the smooth blackbody curve set by temperature. Cooler dense bodies glow dull red; hotter ones shift toward white and blue.

In a stellar spectrum the continuous part is the underlying smooth glow — the continuum — produced by the dense gas deep inside the star. The dark absorption lines we read are scratched onto this continuum by the thinner atmosphere above. The continuum itself is not wasted information: its overall shape and where it peaks reveal the star's temperature, even before a single line is identified.