Event though stars might look very similar to our eyes, there exist diverse types of them. Stars differ mainly in the properties such as temperature, mass, radius and luminosity. Some of these characteristics are not independent from others. A luminous star, for example, is often also very massive.
The spectral classes:
A major property of a star is its surface temperature which influences the color of the star. The hottest stars with temperatures of over 40’000 °C have a blue color whereas the coldest representatives with temperatures as low as 3’000 °C shine orange-reddish. Based on this color spectrum, stars are assigned to the spectral classes: O, B, A, F, G,K, M. For a proper analysis the star light is broken up into its components (for example with a prism) to examine the energy distribution in the color spectrum. A stellar spectrum also contains fine dark lines. These ‘gaps’ are produced when the corresponding parts of the radiation is absorbed by certain elements in the stellar atmosphere. Therefore, the spectrum also includes information about the chemical composition of the stars atmosphere.
The luminosity classes:
Stars do not only differ in their color, but also in their luminosity. One can find stars of the same spectral class (and therefore stars with similar temperatures) that vary drastically in their luminosity. Such brightness variations are explained by different star radii. Therefore, the radius is another very important attribute of a star. The luminosity has its own classification scheme: supergiants, bright giants, giants, subgiants, dwarfs. In this scheme supergiants with sometimes thousandfold solar radii are the brightest stars, whereas small dwarfs (in the size of the earth) are the faintest ones.
The Hertzsprung-Russel diagram:
In the Hertzsprung-Russel diagram the spectral classes are plotted against the luminosity classes. When filling the diagram with a classified star population, one does not get a random distribution, but will recognize a clear clustering of the stars in certain regions of the plot.
The most prominent structure in the diagram is a diagonal band spanning from the top left to the bottom right. Stars within this band are called main sequence stars. Also our sun is residing on the main sequence. In the top right one finds groups of giant stars, also called giant branches. An isolated group of white dwarfs exists in the bottom left of the plot.
A star won’t remain on the same spot in the Hertzsprung-Russel diagram for its entire life, but will wander around in the graph following a very specific path. Therefore the branches and groups in the Hertzsprung-Russel diagram represent different phases in the evolution of a star.
Die Plejaden erscheinen blau und haben eine sehr hohe Temperatur.
Das Winterdreieck erscheint weiss und fällt in die Mitte des Temperaturspektrums.
Betelgeuse im Sternbild Orion erscheint rötlich und hat eine tiefe Temperatur.
Die schwarzen Linien im Sonnenspektrum wurden 1841 von Joseph Frauenhofer entdeckt. Er ordnete den Linien jeweils einen Buchstaben zu.
Später erkannte man, dass jede Linie von einem chemischen Element in der Sternatmosphäre erzeugt wird: