The photographic study of the star spectra was initiated in 1885 by astronomer Edward Pickering at the Harvard College observatory and concluded by his colleague Annie J. Cannon.
This research led to the discovery that the spectra of the stars They are arranged in a continuous sequence, depending on the intensity of certain absorption lines. The observations provide data on the ages of the different stars, as well as their degrees of development.
The various stages in the sequence of the spectra, designated with the letters O, B, A, F, G, K and M, allow a complete classification of all types of stars. Subscripts 0 through 9 are used to indicate the sequences in the model within each class.
Class O: Helium, oxygen and nitrogen lines, in addition to those of hydrogen. It comprises very hot stars, and includes both those that show bright line spectra of hydrogen and helium and those that show dark lines of the same elements.
Class B: Helium lines reach maximum intensity in subdivision B2 and progressively pale in higher subdivisions. The intensity of the hydrogen lines increases steadily in all subdivisions. This group is represented by the star Epsilon Orionis.
A class: It includes the so-called hydrogen stars with spectra dominated by the hydrogen absorption lines. A typical star of this group is Sirius, which appears in the previous photo.
Class F: In this group, the so-called calcium H and K lines and the hydrogen characteristic lines stand out. A notable star of class F is Delta Aquilae.
G class: It comprises stars with strong calcium H and K lines and less strong hydrogen lines. The spectra of many metals are also present, especially that of iron. The Sun belongs to this group and therefore the G stars are called "solar type stars".
K class: Stars that have strong calcium lines and others that indicate the presence of other metals. This group is typified by Arturo.
M class; Spectra dominated by bands that indicate the presence of metal oxides, especially those of titanium oxide. The violet end of the spectrum is less intense than that of the stars K. The star Betelgeuse is typical of this group.
Stars size and brightness
The largest known stars are the supergiants, 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 usually they are diffuse and can have a mass barely 40 times greater than that of the Sun, while white dwarfs are very dense despite their small size.
There may be stars with a mass 1,000 times greater than that of the Sun and, on a smaller scale, hot gas balls too small to trigger nuclear reactions. An object that could be of this type (a brown dwarf) was first observed in 1987, and since then others have been detected.
The brightness of the stars is described in terms of magnitude. The brightest stars can 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 They are identified with colors:
- Blue color, like the star I Cephei
- White-blue color, like the Spica star
- White color, like the star Vega
- White-yellow color, as Proción
- Yellow color, like the Sun
- Orange color, like Arcturus
- Red color, like the star Betelgeuse.
Often the stars are named using the reference to their size and color: white dwarfs, red giants, ...
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