Emission & Absorption Spectra in Stars (AQA A Level Physics)

Emission & Absorption Spectra in Stars

• There are three types of light spectra:

  • Continuous emission spectra

  • Emission line spectra

  • Absorption line spectra

• Continuous spectrum: created when photons of all wavelengths are emitted

  • Appearance: a broad range of colours (depending on a star's temperature)

  • Produced by: hot, dense sources, such as the cores of stars

• Emission spectrum: created when photons are emitted by excited electrons in a hot gas

  • Appearance: discrete wavelengths represented by coloured lines on a black background

  • Produced by: hot, low-pressure gases, such as a nebula surrounding a star

• Absorption spectrum: created when photons are absorbed by electrons in a cool gas

  • Appearance: discrete wavelengths represented by dark lines on a continuous spectrum

  • Produced by: light passing through cool, low-pressure gases, such as the photosphere of a star

• Note: the lines in an absorption spectrum correspond to the same lines in the emission spectrum of the same element

The difference between continuous, emission & absorption spectra

An absorption spectrum is the combination of an emission spectrum on top of a continuous spectrum

Chemical Composition of a Star

• Stellar spectral lines are caused by the interactions between photons and the atoms present in gaseous layers of stars

• Photons produced by fusion reactions in a star’s core move towards the layers of gas in the outer atmosphere of the star

  • The photons produced in the core form a continuous spectrum

  • Photons are absorbed by the gas atoms, which excite and re-emit other photons of various frequencies in random directions

An absorption line will appear in a spectrum if an absorbing material is placed between a source and the observer

• Each gas produces a unique pattern of spectral lines due to the specific transition between the element’s energy levels

  • The presence of absorption lines in a star’s spectrum act as fingerprints

  • They can be used to determine the presence of a certain element within the star 

• The chemical composition of a star can be investigated even when extremely distant

  • If the element is present in the star, its characteristic pattern of spectral lines will appear as dark lines in the absorption line spectrum of the star

• The Sun is predominantly made up of hydrogen and helium gas

  • This can be verified by comparing the emission line spectra of hydrogen and helium with the absorption line spectrum of the Sun

The Hydrogen Spectrum

• Each element produces its own unique set of lines corresponding to specific energy level transitions

• The spectrum of hydrogen was the first to be studied in great detail

The full hydrogen spectrum

• In spectra of hydrogen:

  • The Lyman series converges on the ground state n = 1

  • The Balmer series converges on the second energy level n = 2

  • The Ritz-Paschen converges on the third energy level n = 3, and so on

• The Lyman series photons will have the most energy since they have the shortest wavelength

• The Pfund series photons will have the least energy since they have the longest wavelength

• Note: in this course, you only need to remember the Balmer series, the others are only mentioned here for context

Electron transitions in the hydrogen spectrum

• The discovery of these electron transitions has enabled astronomers to study the nature and chemical composition of objects in the Universe