2.7 - Star Spectra

Continuous, emission and absorption spectra

When light is passed through a prism, it is refracted and spread out into a spectrum.

Continuous spectrum

A hot, dense object emits radiation across an unbroken range of frequencies with no gaps.

Emission spectrum

• A hot gas emits radiation at specific frequencies only.
• This produces a pattern of bright lines called an emission spectrum.
• Each element produces a unique emission spectrum.

Absorption spectrum

• When a continuous spectrum passes through a cooler gas, the gas absorbs specific frequencies.
• This leaves dark lines called an absorption spectrum.
• An element's absorption lines fall at the same frequencies as its emission lines.

Using spectra to find a star's composition

1. A star emits a continuous spectrum of radiation from its hot interior.
2. As this light passes through the cooler gases in the star's atmosphere, those gases absorb their characteristic frequencies.
3. The light reaching Earth shows an absorption spectrum, with dark lines matching the elements present in the star's atmosphere.
4. By comparing these absorption lines with the known spectra of elements tested in a laboratory, astronomers can identify what elements the star contains.