Types of Stars

Even if stars all appear the same to the naked eye, different star types exist that are mainly differentiated by the following parameters: temperature, mass, radius and luminosity. Some of these characteristics are not completely independent of others.

Stellar classifications

An important parameter of a star is its surface temperature. This influences its colour. The hottest stars with temperatures of up to 40,000 degrees Celsius are blue, while the coolest stars with temperatures of less than 3,000 degrees Celsius have a reddish-orange glow. Based on this colour spectrum, stars are classified into the stellar classifications O, B, A, F, G, K, M (see diagram below). For a detailed analysis, the starlight is fanned out into its components (e.g. with a prism), and the energy distribution of the individual colour components is examined. A star spectrum typically also contains fine black lines. These “gaps” occur when parts of the radiation are absorbed by certain chemical elements in the stellar atmosphere. Researchers can therefore also obtain information about the chemical composition of a star from the spectrum.

The luminosity classes

Stars differ not only in colour, but also in brightness. This reveals that even stars of the same spectral class (i.e. stars with a comparable temperature) differ greatly in terms of brightness. These variations in brightness can be explained by different stellar radii. The radius of a star is therefore another important characteristic for which the following luminosity classes have been introduced: supergiants, bright giants, giants, subgiants, dwarves. The supergiants, some of which have a radius a thousand times that of the Sun, are the brightest, while the dwarfs, which are perhaps the size of

The Hertzsprung-Russell diagram

The relationship between stellar classifications and luminosities is plotted in the Hertzsprung–Russell diagram. If this diagram is filled with a classified stellar population, the result is not a randomly distributed image, but the stars are preferentially grouped in certain areas of the diagram.

A diagonal band of stars from top left to bottom right is particularly prominent. These stars are designated as main-sequence stars. The Sun is also part of the main sequence. At the top right of the diagram are groups of giant stars, also known as giant branches, which emanate from the main sequence. At the bottom left there is an isolated group of so-called white dwarfs.

A star does not remain in the same place in the Hertzsprung-Russel diagram for its entire life, but travels through it in a very specific way. The branches and groups in the Hertzsprung-Russell diagram thus represent different phases of stellar evolution.