Astronomy

The majority of the light emitted by a star comes out of its dense interior and thus shows a characteristic black body or continuous spectrum of energy distribution. Superimposed upon this continuous spectrum are the absorption lines due to absorption at specific wavelengths by atoms in the cooler atmosphere immediately above the stellar photosphere. The specific pattern of absorption features is related to the chemical composition of the absorbing material. However, because most stars are chemically similar, the more important factor in determining which absorption features are actually present in a spectrum is the photospheric temperature of a star.

The system of spectral classification was designed by Annie Jump Cannon at Harvard University. On the basis of the number and pattern of absorption features, she classified stellar spectra into classes A (simplest looking), B (next simplest), and so on, up to type W (most complex). Years later when the newly developed understanding of atomic physics was applied to interpretation of stellar spectra, scientists realized that temperature was the key factor that determined at what wavelengths a chemical element present in a stellar atmosphere would absorb light (see Figure 1).

Figure 1

Illustration of spectral types. Some of the elements responsible for the absorption features are marked (Ca calcium; He heliem, Mg magnesium, Na sodium, and TiO for titanium oxide).

When ordered in terms of decreasing photospheric temperature, the surviving spectral types form a sequence O B A F G K M. For historical reasons, O and B stars are often referred to as early‐type stars, and K and M stars are known as late‐type stars. With refinement of the system of spectral classification, each spectral type was further broken down in tenths of classes, for example A0, A1,…, A9, F0, and so on. The spectral types directly correlate with temperature, as shown in Table 1 and Figure 2.

Figure 2

Relative strength of absorption lines as a function of temperature and spectral type.

In addition to the OBAFGKM types, astronomers still use spectral types W, R, N, and S. Type W stars are variants of type O stars, the central stars of planetary nebulae, which are very hot and often show emission lines. R and N stars have the same temperatures as K and M stars, respectively, but show extremely strong carbon and carbon molecule absorption features (these are also called carbon stars). S stars, also similar to M stars in temperature, have a peculiar chemistry and show absorption by zirconium oxide (ZrO) and lanthanum oxide (LaO). Recently spectral types L and T have been proposed for the very coolest and faintest stars that have been discovered. Type L stars have temperatures between 1,300 and 2,000K; their spectra show absorption by iron hydride (FeH) and chromium hydride (CrH) molecules. Even cooler objects, temperatures between 700 and 1,300K and that show strong methane absorption, would be termed T spectral types. These very coolest objects may not be true stars in the sense of being able to generate energy by central thermonuclear reactions; they have also been termed brown dwarfs.

A primary advantage of classifying stars by spectral type is that with even a modest amount of experience, a person can easily recognize a star's spectral type from a photographic spectrum or a digital spectrum that shows quantitatively the energy distribution with wavelength.