In 1802, William Hyde Wollaston noted that the spectrum of sunlight did not appear to be a continuous band of colors, but rather had a series of dark lines superimposed on it. Wollaston attributed the lines to natural boundaries between colors. Joseph von Fraunhofer made a more careful set of observations of the solar spectrum in 1814 and found some 600 dark lines, and he specifically measured the wavelength of 324 of them. Many of the Fraunhofer lines in the solar spectrum retain the notations he created to designate them. In 1864, Sir William Huggins matched some of these dark lines in spectra from other stars with terrestrial substances, demonstrating that stars are made of the same materials of everyday material rather than exotic substances. This paved the way for modern spectroscopy.
Standard Stellar Types (O, B, A, F, G, K and M)
While the differences in spectra might seem to indicate different chemical compositions, in almost all instances, it actually reflects different surface temperatures. With some exceptions (e.g. the R, N, and S stellar types discussed below), material on the surface of stars is "primitive": there is no significant chemical or nuclear processing of the gaseous outer envelope of a star once it has formed. Fusion at the core of the star results in fundamental compositional changes, but material does not generally mix between the visible surface of the star and its core.
Ordered from highest temperature to lowest, the seven main stellar types are O, B, A, F, G, K, and M. Astronomers use one of several mnemonics to remember the order of the classification scheme. O, B, and A type stars are often referred to as early spectral types, while cool stars (G, K, and M) are known as late type stars. The nomenclature is rooted in long-obsolete ideas about stellar evolution, but the terminology remains. The spectral characteristics of these types are summarized below:
Type Color Surf. temp. Characteristics
O Blue > 25,000 K Singly ionized helium lines either in emission or
absorption. Strong ultraviolet continuum.
B Blue 11,000 - 25,000 Neutral helium lines in absorption
A Blue 7,500 - 11,000 Hydrogen lines at maximum strength for A0 stars,
F Blue/White 6,000 - 7,000 Metallic lines become noticeable.
G White/Yellow 5,000 - 6,000 Solar-type spectra. Absorption lines of neutral
metallic atoms and ions grow in strength.
K Orange/Red 3,500 - 5,000 Metallic lines dominate. Weak blue continuum.
M Red < 3,500 Molecular bands of titanium oxide noticeable.
Within each of these seven broad categories, Canon assigned subclasses numbered 0 to 9. A star midway through the range between F0 and G0 would be an F5 type star. The Sun is a G2 type star.
The Harvard scheme specifies only the surface temperature and some spectral features of the star. A more precise classification would also include the luminosity of the star. The standard scheme used for this is called the Yerkes classification (or MMK, based on the initials of the authors William W. Morgan, Philip C. Keenan, and Edith Kellman). This scheme measures the shape and nature of certain spectral lines to measure surface gravities of stars. The gravitational acceleration on the surface of a giant star is much lower than for a dwarf star (since g = G M / R2 and the radius of a giant star is much larger than a dwarf). Given the lower gravity, gas pressures and densities are much lower in giant stars than in dwarfs. These differences manifest themselves in different spectral line shapes which can be measured.
The Yerkes scheme uses six luminosity classes:
- Ia - Most luminous supergiants
- Ib - Less luminous supergiants
- II - Luminous giants
- III - Normal giants
- IV - Subgiants
- V - Main sequence stars (dwarfs)
Thus the Sun would be more fully specified as a G2V type star.
R and N type stars
A number of giant stars appear to be K or M type stars, but also show significant excess spectral features of carbon compounds. They are often referred to as "carbon stars" and many astronomers collectively refer to them as C type stars. The most common spectral features are from C2, CN, and CH. The abundance of carbon to oxygen in these stars is four to five times higher than in normal stars. The presence of these carbon compounds will tend to absorb the blue portion of the spectrum, giving R and N type giants a distinctive red colour. R stars are those with hotter surfaces which otherwise more closely resemble K type stars. S type stars have cooler surfaces and more closely resemble M stars.
S type stars
S type stars have photospheres with enhanced abundances of s-process elements. These are isotopes of elements which have been formed from the capture of a free neutron (changing the isotope of the element) followed by a beta decay (a neutron decays into a proton and an electron, thus changing the element to one with a higher atomic number and an isotope with one less neutron). The s-process is one of the mechanisms by which elements with atomic numbers higher than 56 (Iron) can be made. The s stands for slow. By way of contrast, its partner r-process (for rapid) takes place when there are a sufficient supply of free neutrons for additional neutrons to be acquired in the atomic nucleus before the captured neutron has a chance to beta decay.
Instead of (or in addition to) the usual lines of titanium, scandium, and vanadium oxides characteristic of M type giants, S type stars show heavier elements such as zirconium, yttrium, and barium. A significant fraction of all S type stars are variable.
W type stars
Recently, a new type of star was introduced. See Wolf-Rayet stars
for a complete description.