(e) different atoms have different spectral lines which can be used to…

The idea

Every element has its own unique arrangement of electron energy levels, so the set of photon energies it can emit or absorb is unique. This produces a characteristic pattern of spectral lines, often called a spectral fingerprint. Astronomers record the spectrum of a star, identify the wavelengths of the lines present, and match them against laboratory spectra of known elements. For example, strong hydrogen lines and helium lines in a stellar spectrum show those elements are present in the star's outer layers. The same principle applies to absorption lines formed when cooler gas in the star's atmosphere absorbs specific wavelengths from the continuous spectrum beneath. Comparing line patterns therefore reveals composition without visiting the star.

What earns the marks

• States that each element has a unique set of discrete energy levels.
• Links the unique levels to a unique pattern of emitted or absorbed wavelengths.
• Describes matching observed stellar line wavelengths with laboratory reference spectra.
• Applies the method to identify elements present in a star's outer layers.
• Recognises that absorption lines arise when cooler gas absorbs specific wavelengths from a continuous spectrum.
• Explains that the technique works remotely because light carries information about the source.

Common mistakes and how to put them right

• Thinking all stars show identical spectra.

  Put it right: Line patterns differ because stellar composition and temperature differ, which is exactly what makes identification possible.

• Confusing emission and absorption lines.

  Put it right: Emission lines are bright on dark, absorption lines are dark on a continuous spectrum, but both can identify elements.

• Assuming a single line is enough to identify an element.

  Put it right: Identification relies on matching the whole pattern of lines, since some lines may be weak or blended.

• Believing the lines come from the star's core.

  Put it right: The observed lines form in the cooler outer layers where atoms can absorb or emit at characteristic wavelengths.

Exam tips

• Refer to a spectral fingerprint or unique line pattern, and state that it is compared with laboratory spectra.
• Mention both emission and absorption spectra where relevant, and say which appearance matches each.
• Use a named example such as hydrogen or helium lines to make the identification argument concrete.