Final answer:
The brightness of spectral lines depends mainly on the temperature of the emitting body, with more intense lines corresponding to higher temperatures. Pressure also affects the width of spectral lines, with high pressure causing broader lines.
Step-by-step explanation:
The brightness of spectral lines is predominantly determined by the temperature of the star or gas emitting the radiation. As the temperature rises, the speed and energy of atomic collisions in the gas increase, leading to more electrons occupying higher energy levels. This results in more intense spectral lines, as electrons have more energy transitions to undergo when emitting photons. Additionally, pressure can impact the width of spectral lines, as observed in stars of the same temperature but different pressures. Higher pressure causes broader lines due to interactions between particles affecting energy levels, while lower pressure leads to narrower lines.
Moreover, the absorption lines in a spectrum provide astronomers with crucial information about the temperature of the regions where the lines originate because the level where electrons start their upward jumps in a gas can serve as an indicator of the star's temperature.