Stellar Spectral Types

Stars can be classified by their surface temperatures as determined from Wien's Displacement Law, but this poses practical difficulties for distant stars. Spectral characteristics offer a way to classify stars which gives information about temperature in a different way - particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. The standard classes are:

The commonly used mnemonic for the sequence of these classifications is "Oh Be A Fine Girl, Kiss Me".

O-Type Stars

Included are blue stars with surface temperatures of 20,000 to 35,000K. The thermal energy is so great at these temperatures that most surface hydrogen is completely ionized so hydrogen (HI) lines are weak. Unionized helium (HeI) is visible but the stronger spectra are of mulitple ionized atoms (HeII, CIII, NIII,OIII,SiV). Note the rise of the spectral curve toward the blue from the blackbody radiation curve.

The radiation rises in intensity toward the blue end of the spectrum. The blackbody peak will be in the ultraviolet. Note that the hydrogen spectrum is seen, but it is red-shifted from its laboratory wavelength of 656 nm.

O-Type Stars

The spectra of O-Type stars shows the presence of hydrogen and helium. At these temperatures most of the hydrogen is ionized, so the hydrogen lines are weak. Both HeI and HeII (singly ionized helium) are seen in the higher temperature examples.

The radiation from O5 stars is so intense that it can ionize hydrogen over a volume of space 1000 light years across. One example is the luminous H II region surrounding star cluster M16.

O-Type stars are very massive and evolve more rapidly than low-mass stars because they develop the necessary central pressures and temperatures for hydrogen fusion sooner. Because of their early development, the O-Type stars are already luminous in the huge hydrogen and helium clouds in which lower mass stars are forming. They light the stellar nurseries with ultraviolet light and cause the clouds to glow in some of the dramatic nebulae associated with the H II regions.

B-Type Stars

Blue-white stars with surface temperatures of about 15000K. The ionized helium (HeII) lines of the O-type stars have disappeared and neutral helium (HeI) lines are strongest at B2. The neutral hydrogen (HI) lines are getting stronger and singly ionized OII, SiII and Mg II are visible. A significant rise of the blackbody spectrum toward the blue is still evident.