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
Kirchhoff's Laws are:
A hot solid, liquid or gas, under high pressure, gives off a continuous
A hot gas under low pressure produces a bright-line or emission line
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:
Each atom or molecule exhibits a different pattern of lines (rather like
a fingerprint or DNA signature).