The H-R Diagram and the Properties of Stars

Stars are characterized by bulk properties such as brightness and color. By examining brightness and color for a large number of stars it is possible to compare the Sun to other stars and to determine how stars live their lives. We have already discussed how to measure the luminosity of star (the photometry lab) and how we can determine its spectral type (the atomic spectra lab).

Let us now see how these properties relate to the evolution of stars. During the first decade of the twentieth century, Ejnar Hertzsprung and Henry Norris Russell each independently decided to make a graph plotting stellar luminosities on one axis and spectral types on the other. This has come to be known as the H-R Diagram. From it we can determine a number of patterns in the properties of stars. Stellar luminosities, in terms of the Sun's luminosity, are on the vertical axis.   They can also be shown as absolute magnitudes (right axis). The stellar luminosities span a wide range so notice that each increment up the left side of the graph increases by 100. Note also that spectral types are on the horizontal axis and are directly correlated to the surface temperatures of the different types of stars. The temperature decreases from left to right. Recall from the Atomic Spectra lab: O stars are hottest, M stars coolest.

Each dot on the diagram represents the spectral type and luminosity of a single star.

Image provided by Richard Powell. Click here for more information

Patterns in the H-R Diagram

The two fundamental factors that determine a star's position on the H-R diagram are its mass and evolutionary state.

Most stars fall somewhere along the main sequence, that prominent streak that runs from the upper left to the lower right. It is fundamentally a sequence of mass: mass decreases from upper left to lower right.  Our Sun right now is a main sequence star. The common trait of main-sequence stars is that they are fusing hydrogen into helium in their cores. Because stars spend most of their lives fusing hydrogen, most stars will fall along the main sequence.

The stars in the extreme upper right are called supergiants because they are very large in addition to being very bright.  These are stars near the end of their lives.

Just below the supergiants and above the main sequence are the giants, which are smaller and less luminous than the supergiants. Our Sun will become a red giant after it has used up enough hydrogen in its core.

The stars in the very lower left are small and appear white because of the their high temperatures; they are called white dwarfs. This is the end point of our Sun's life in another 5 billion years or so.

The Sun's Position on the H-R Diagram

The Sun's temperature is about 6000 K. Use the table on the last page of the Atomic Spectra lab to estimate the spectral type of the Sun. Then determine its position on the H-R diagram.

The Evolution of Stars

The patterns in the H-R diagram are related to how different stars live their lives. It is through careful study of the H-R diagram that astronomers have learned how stars evolve. The next page describes the fusion reactions that power stars.  Once we understand those, we'll look at how stars are born, how they live and how they die.

The following link can be used for the problems associated with this page: INTERACTIVE HR DIAGRAM