Stellar Spectral Classes (AQA A Level Physics): Revision Note

Stellar spectral classes

• The spectral classes used today were categorised by the astronomer Annie Jump Cannon

• She reordered the original alphabetical system into seven temperature classes:

O B A F G K M

• The table shows how the spectra of stars can be categorised based on their:

  • intrinsic colour

  • temperature

  • prominent absorption lines

• The intrinsic colour of a star is related to its peak emission wavelength which is attributed to its temperature, as described by Wien's law

• The relationship between temperature and absorption spectra is related to the effect of the energy on the state of the atoms or molecules present in the atmospheres of stars

• At low temperatures:

  • There may not be enough energy to excite atoms or break molecular bonds

  • This results in the TiO and neutral atoms, as seen in classes K and M

• At higher temperatures:

  • Atoms have too much energy to form molecules

  • As a result, ionisation can take place, as seen in classes F and G

• At the hottest temperatures: 

  • Hydrogen and helium are found to be in higher abundance in the atmospheres of the hottest stars

  • This means that their spectral lines start to dominate, as seen in classes O, B and A

The Balmer series of hydrogen

• The absorption and emission spectra of hydrogen and helium are of particular importance to astronomers due to their abundance in the universe

Spectra of different spectral classes

• There are many series of spectra, but the most important is the Balmer series, which involves

  Electron transitions either to or from the second energy level $\left(n=2\right)$

• The Balmer series is of great importance because the wavelengths of photons created are in the visible spectrum

• The prominence of Balmer lines in a star's atmosphere varies depending on the surface temperature, as shown in the table: