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Stars


Hertzspung-Russell (HR) Diagram

The Hertzspung-Russell diagram was introduced in the 1910s to plot a point representing a star with a certain values of luminosity and surface temperature as shown in Figure 08-03a.

It soon became apparent that the HR diagram is not randomly populated, but that stars preferentially fall into certain regions. The majority of them occupy a strip called main sequence as indicated in Figure 08-03a. This just reflects the fact that all the stars spend most of their time burning off hydrogen fuel with a constant luminosity and surface temperature. There are variable stars, which change their brightness, color, spectrum and other characteristics in the order of hours to few hundred days. They appear as a transient phenomenon in the HR diagram. The evolutionary track of an individual star with a given mass can be traced in the HR diagram as shown in Figure 08-04, 08-05a, and 08-05b. The age of the globular clusters can be estimated from the branch-off point in the HR diagram.

The HR diagram comes with many variations since luminosity and temperature are related to other quantities. For example,
HR Diagram HR Centennial Commemoration luminosity is related to the Absolution Magnitude; and since different temperature of the stars generate different set of absorption lines, this scale can be translated into spectral types such as O, B, A, F G, K, and M as shown in Figure 08-03a. Each spectral type is further subdivided into numerals, e.g., G2 for the Sun, which is located in the middle of the main sequence. Sometimes the horizontal axis is labelled by the colour index B - V or mB - mV, i.e., magnitude in blue - magnitude in visual (yellow). It is a directly measurable quantity from photometer with colour filters.

Figure 08-03a HR Diagram
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Figure 08-03b HR Diagram, Centennial Commemoration




As a centennial commemoration of the HR diagram, the July 2011 issue of Scientific American has offered a free HR diagram (Figure 08-03b) in the form of poster with numerous names of various kinds of stars, and explanations for the groupings.

HR Pre-Main-Sequence Figure 08-04 is another version of the HR diagram. It shows the progression of mass along the main sequence, and the pre-main-sequence evolutionary track for different masses from 0.5 to 15 Msun. The mass of a star determines all its properties in the HR diagram. The observed upper limit for stellar mass is about 60 Msun, the star becomes unstable beyond this limit. The heavy star will be an O type located at the upper left corner of the main sequence. The observed lower limit is about 0.05 Msun occupying a position down in the lower right corner of the main sequence as M type stars. Protostar below this limit is not able to ignite hydrogen burning and becomes a brown dwarf. The contraction time to the main sequence is plotted as contours in range from 104 to 107 years. The pre-main-sequence stars begin their life as interstellar clouds (with a size of several light years), which collapse under the influence of gravity to a stage called T Tauri stars before settling down onto the main sequence (see Figure 08-07).

Figure 08-04 HR Pre-Main-Sequence
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