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The Hertzsprung-Russell Diagram

At the beginning of the 20th century two astronomers, Ejnar Hertzsprung, and Norris Russell found (independently of each other) that if stars were plotted on a diagram with their luminosity on one axis, and their spectral class on the other, that stars formed three distinct groups. The largest group, the "Main Sequence" is where 90% of the stars are found. It propagates from upper left corner, down to the right corner. The group below the main sequence is the "White Dwarfs", which is a group of small, earth-sized stellar remnants. The third group, which is found above the main sequence is that of the giants. The diagram was named "The Hertzsprung-Russell Diagram", or the "H-R diagram" for short.
At the beginning these two astronomers, especially Russell thought that the main sequence showed how one star evolves during its' lifetime. Later studies have showed that he was incorrect. Stars appear on a specific place on the main sequence depending on their mass, which also determines when it will leave the main sequence.
Nevertheless, the H-R diagram is a significant tool for astronomers, when it comes to understanding stellar evolution.

The Diagram

As mentioned earlier the two axes of the H-R diagram are luminosity (usually plotted on the Y-axis) and spectral class (usually the X-axis). The luminosity axis is logarithmic since the brightness of stars can vary greatly. The axis that describes the spectral class depends on the surface temperature of the star. The higher the surface temperature, the closer the star is to the y-axis. The spectral classes are O, B, A, F, G, K, M. Sometimes the classes R, N, S are added to include very cool "stars", such as brown dwarfs. Each spectral type is divided into 10 degrees (0-9), and each star is given the designation of a roman number, ranging from I-V, where I is given to stars that are the most luminous, and V to the faintest. Hot stars usually have the spectral classes O and B, with surface temperatures of 10-40 000 K. Their cooler counterparts have the spectral classes K and M. The Sun, is located somewhere in the middle of the diagram: class G2V.
Stars that are located high on the y-axis are very luminous, and have a very large diameter. This is the realm of the giants on the H-R diagram. They are massive stars, but it is their great size (some have a radius more than 1 000 times the sun's, which is appr. 700 000 km!) that makes them luminous: The more surface area, the more light emitted. Thus, the stars located at the bottom are dwarfs. White dwarfs, that can be the size of the earth, have a special group on the H-R diagram. Dwarfs found in the lower right corner burn their hydrogen extremely slowly, and may thus live for billions of years longer than the stars found in the upper left, which may only live a few hundred thousand/million years.
Most stars are located on the "Main Sequence". Stars are believed to be on it when they are converting hydrogen to helium in their cores. The position of a star on the main sequence is a direct result of its' mass. Astronomers believe that when a star is on the main sequence it will not change its' position, unless there is a significant change in mass during the hydrogen burning phase of the star's life, which may happen in close binary starsystems with a heavy counterpart.
It should be mentioned that the more left a star is located on the main sequence, the rarer the type is. Thus, most stars are red dwarfs. O-type stars, for instance are only one in 3 million stars!
Planetary nebulae and protostars can also be plotted on the H-R diagram.

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Above: The famous Hertzsprung-Russell diagram. It features stellar brightness versus their surface temperature.

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