Hertzsprung-Russell Diagram Uses

H-R Diagrams can be used in a variety of stellar applications. A few of them are discussed below.

Because the horizontal axis is based upon stars' color, this can be extrapolated into a spectral classification. Stars that are in the same general region of the diagram share many similar properties. Therefore, based upon their position on the diagram, a star can be classified and its general properties are instantly known.

For example, if I were to tell you that the sun was a type G star, you would know that it had a temperature of around 6000 K, a color of yellow, and an absolute magnitude of around +5. In actuality, the sun has a temperature of about 5,800 K, is yellow, and has a magnitude of +4.85*. You would also have an idea of what its spectrum looked like.

*This is the sun's absolute magnitude, as opposed to its apparent magnitude. The Sun's apparent magnitude is -26.87. Apparent magnitude is a measure of how bright a star is from our vantage point here on Earth. The absolute magnitude is what the star's magnitude would be if it were viewed from a distance of 10 parsecs (approximately 33 light-years).

A second, very important, application of the H-R Diagram is the derivation of the distance to a star cluster or galaxy.

In order to accomplish this, accurate H-R Diagrams are used, where the color and absolute magnitudes are known. With this, you know where the main sequence is supposed to be. Then, an H-R Diagram is made of the cluster or galaxy to which the distance is in question. Graphing the color and apparent magnitude, you get a similar H-R Diagram.

However, it is shifted vertically due to the apparent vs. absolute magnitude. By appropriately shifting along the magnitude axis and lining up the main sequences, one can use the distance modulus (right) to determine the approximate distance to the object.