Started the photographic study of stellar spectra in 1885 the astronomer Edward Pickering at Harvard College Observatory and his colleague Annie J. Cannon concluded it. This research led to the discovery that the spectra of the star are arranged in a continuous sequence, depending on the intensity of certain absorption lines. The observations provide data of the ages of the different stars, as well as their degree of development.
The various stages in the sequence of the spectra, designated by the letters O, B, A, F, G, K and M, allow a full of all types of star rating. Subscripts 0 through 9 are used to indicate the inheritance model within each class.
Class O: Lines of helium, oxygen and nitrogen, in addition to the hydrogen. It very hot stars, and includes both showing bright line of hydrogen and helium Spectra showing dark lines of the same elements.
Class B: Lines of the helium reached it maximum intensity in the subsection B2 and pale progressively in subdivisions more high. The intensity of the lines of hydrogen increases steadily in all subdivisions. This group is represented by the star Epsilon Orionis.
Class A: This includes the so-called stars of hydrogen Spectra dominated by hydrogen absorption lines. A typical star of this group is Syrian.
Class F: In this group include the so-called lines H and K of the calcium and characteristics of hydrogen lines. A notable star in this category is Delta Aquilae.
Class G: It includes star with strong lines H and K of the calcium and less strong hydrogen lines. The spectra of many metals, especially of iron are also present. The Sun belongs to this group and that is why the stars G are called "solar-type stars".
Class K: Stars that have strong lines of calcium and others that indicate the presence of other metals. This group is typified by Arthur.
Class M; Spectra dominated by bands that indicate the presence of metallic oxides, especially those of titanium oxide. The violet end of the spectrum is less intense than the K stars. The star Betelgeuse is typical of this group.
Size and brightness of the stars
The biggest stars that are known are the canterell, with diameters about 400 times larger than the Sun, while the stars known as "white dwarfs" can have diameters of only one-hundredth of the Sun. However, giant stars tend to be diffuse and may have a mass just about 40 times larger than the Sun, while the white dwarfs are very dense despite its small size.
There may be star with one mass of 1,000 times larger than the Sun, and smaller-scale, balls of hot gas too small to trigger nuclear reactions. An object that could be such (a brown dwarf) was observed for the first time in 1987, and since then others have been detected.
The brightness of the stars is described in terms of magnitude. The brightest stars may be up to 1,000,000 times brighter than the Sun; White dwarfs are about 1,000 times less bright.
The classes established by Annie Jump Cannon are identified by colors:-Blue, as I Cephei star
-White-blue, like the star Spica
-White color, as the star Vega
-White-yellow color, as the Procion star
-Yellow color, as the Sun
-Orange, as Arcturus
-Red color, as the star Betelgeuse.
Often the stars are named using the reference to their size and their color: red giants, white dwarfs,...
Translation for educational purposes authorized by: Astronomía: Tierra, Sistema Solar y Universo