Final answer:
The brightness of specific spectral lines is influenced by factors like the star's temperature, pressure, gas density in the photosphere, and the Doppler effect due to stellar motion, which causes line broadening and can indicate rotational speed.
Step-by-step explanation:
The difference in the brightness of specific spectral lines can be accounted for by several factors, one of which is the temperature of the stars. Stars with different temperatures emit spectra with different intensities of light. Moreover, other factors like star pressure, gas density in the star's photosphere, and the motion of the star due to the Doppler effect also have significant impacts on spectral line characteristics. For example, a giant star with a very-low-pressure photosphere exhibits very narrow spectral lines, while a smaller star with a higher-pressure photosphere presents much broader spectral lines. This difference in line width, also known as line broadening, can provide information about the physical conditions within the star as well as its rotation speed.