The Hertzsprung-Russell Diagram

There are billions of stars in our galaxy. Each star has its story to tell. Each varies in terms of its mass, temperature, luminosity and size. When Astronomers started observing the stars, they found a particular pattern among them. The Hertzsprung-Russell diagram or the HR diagram is one of the most useful plot of the stellar astronomy. It is actually a scatter graph of the star’s luminosity verses its stellar classification.

Understanding the axes of the HR Diagram


The vertical axis of the diagram represents the luminosity axis. Astronomers use the concept of the absolute magnitude for explaining the luminosity. Absolute magnitude is the term which tells us that how bright an object would appear if it were 10 parsec away. The lower the magnitude, the brighter the star.

The horizontal axis is the spectral classification axis. From left to right, the temperature decreases. The O classification represents the blue supergiants while the M classification stars are the red stars. The color index is the difference in the magnitude of the stars when measured through two different colors.

Plotting the stars on the HR Diagram


The dots on the above graph represent individual stars. Note that the stars appear as groups. Let us explore them in detail

Most stars appear to be present in the Group A. These stars are the Main Sequence stars which produce their energy by the hydrogen fusion in the core. Our sun also lies in the group A. Other notable stars include alpha centuri and Sirius.

Once the star has finished producing energy in the core by hydrogen fusion. it exits the main sequence and enters Group B. The group B stars lie on the Asymptotic Giant Branch of the HR Diagram. These stars are the red giants with temperature about 6,000 K. These star’s radius can become as large as 1 A.U. Notable stars of this group include Aldebaran and Mira.

The stars in Group C are even more luminous than the giants. These are thesupergiants, the largest of stars with extremely high luminosities. A red supergiant such as Betelgeuse would extend beyond the orbit of Jupiter if it replaced the Sun in our solar system.

The stars in the Group D are known as the white dwarfs. These stars no doubt emit large amount of energy but are very faint which means that they are very small, hence the name white dwarfs. The popular examples of such stars includes Sirius B and Procyon B.

The Hertzsprung-Russell Diagram

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