9.2.8 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

Relationship between colour and temperature of stars

The colour of a star correlates to its temperature

• 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

Spectral classes in terms of the prominence of their Balmer lines

• 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 

• 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: