Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram (or HR Diagram) is a way to relate all stars to each other. It is by no means an evolutionary diagram of stars. The Sun will never become a star like Rigel or Betelgeuse in the above image, for example. What we use it for is to help astronomers relate different stars by four properties: their temperature, their brightness, their mass, and their radius.

The diagram was independently created by Ejnar Hertzsprung and Henry Norris Russell when they plotted stars on a graph comparing the luminosity or brightness of the star to its temperature. When looking at the graph, a pattern appears.

NASA/GSFC/High Energy Astrophysics Science Archive Research Center

.Some things to point out about the HR Diagram:

1. Supergiants are in the upper right. This means that they are relatively cool (3000 K), but very bright due to a large radius (Luminosity goes as the radius of the star squared), but has a much smaller mass than stars on the main sequence at the same luminosity. These are stars that started out in the upper left on the main sequence and are nearing the end of their life cycles. A great example of a supergiant is the red star Betelgeuse in Orion. Rigel, on the other side of the diagram, is a blue supergiant, but has a smaller radius than Betelgeuse.
2. Giants are smaller in radius than supergiants, but still are relatively luminous. These are also stars nearing the end of their lives, but unlike the supergiants, these stars started out in the middle of the main sequence. The Sun will up end as a giant when it reaches the end of its main sequence phase.
3. The main sequence or dwarf phase is the stage of a star's life that it spends the majority of its life on. As shown above, these stars come in a range of temperatures, masses, radii, and luminosities. Sirius and the Sun are main sequence stars, but will end up at completely different locations on the HR Diagram after they leave the main sequence. Rigel will become a red supergiant, Sirius will become a less luminous supergiant (not shown), and as mentioned above, the Sun will become a giant star.
4. White dwarf stars are the final stages for main sequence stars that are average in size and temperature (stars around the Sun's size and temperature). Generally, these stars have masses no bigger than 1.4 solar masses (the mass of the Sun), and we will discuss more about these stars later.
5. It is unclear what happens to really low-mass main sequence stars as the red dwarfs that we see have not yet evolved off the main sequence. They probably are very similar to giants and white dwarfs as they age, but we have yet to discover an aged red dwarf.

On the x-axis of the HR Diagram, there are a variety of measurements that can be made for a star, all of them related to the surface temperature of the star. Notice that the temperature increases from right to left instead of left to right on most graphs. The temperature of the star is related the spectral type of the star, which is dependent on the material found in the star (seen in its spectrum). The temperature is also related to something called the color of the star, which is the difference between the stars magnitude in two different filters. The most common filters used are U (ultraviolet), B (blue), V (visible - green/yellow), R (red), and I (infrared). Magnitudes will be discussed more in detail later.

On the y-axis of the HR Diagram, there are a variety of measurements that relate the brightness of the star. The most common is the luminosity which is the energy output of the star. As mentioned above, the luminosity is related to the square of the star's radius as well as the temperature of the star. For stars with larger radii and/or temperatures, the brighter the star will be. The luminosity is also related to something called absolute magnitude which should not be confused with apparent magnitude. The absolute magnitude is how bright a star would be if the star was 10 parsecs from the observer. It is based on making Vega an absolute magnitude (M) of 0.0. The Sun has an absolute magnitude of 4.83. The apparent magnitude (m) is just how bright the star looks based on its actual distance from the Sun. Stars are assigned luminosity classes based on where they lie on the HR Diagram.