The spectrum of an object is the variation in the intensity of its radiation at different wavelengths.
Objects with different temperatures and compositions emit different types of spectra. By observing an object's spectrum, then, astronomers can deduce its temperature, composition and physical conditions, among other things.
Kirchhoff's Laws are:
A hot solid, liquid or gas, under high pressure, gives off a continuous spectrum.
A hot gas under low pressure produces a bright-line or emission line spectrum.
A dark line or absorption line spectrum is seen when a source of a continuous spectrum is viewed behind a cool gas under pressure.
The wavelength of the emission or absorption lines depends on what atoms are molecules are found in the object under study.
What atoms or molecules exist depend on:
temperature
chemical composition.
Each atom or molecule exhibits a different pattern of lines (rather like a fingerprint or DNA signature).
