Lesson Objectives:- The H-R Diagram
- Classification of stars
- The main sequence of the H-R Diagram
- Giants, supergiants, and white dwarfs
A Hertzsprung-Russell diagram, usually referred to simply as an H-R diagram, plots stars on a graph based on their luminosities and spectral types.
On an H-R diagram, the horizontal axis represents stellar surface temperature, which corresponds to spectral type, while the vertical axis represents stellar luminosity. Stars in the upper left are hot and luminous while stars in the lower right are cool and dim.
Most stars, including our Sun, fall along the main sequence of the H-R diagram. In the upper-right, the supergiants and giants are very large and bright. Remember, if two stars are the same temperature, but one is 100 times bigger than the other, the larger star will shine much more brightly because it has a larger surface area for radiating light. You can see in the diagram that no matter how bright the giants and supergiants are, their color corresponds to their temperature.
Finally, we have in the lower-left corner the white dwarfs, which appear white because of their high temperatures but are not very luminous because of their small size.
Stars are classified by their spectral type, O, B, A, F, G, K, or M to tell us their color and surface temperature. To fully classify a star, however, we use both spectral type and luminosity class.
Luminosity class is based on a star's luminosity but also tells us about the star's size, with class 1 stars having the highest luminosity and largest radii, with radii and luminosity decreasing down to the class 5 stars. Since our Sun is on the main-sequence, its complete classification is G2 5. The G2 refers to its spectral type and the 5 refers to its luminosity class, indicating that it is a main sequence star.
Note that white dwarfs fall outside this classification system and are usually designated as "wd".
Most stars fall along the main sequence of the H-R diagram. All stars along the main sequence are fusing hydrogen into helium in their cores. For a main sequence star, mass is the most important property since it is the key factor in determining how fast a star is fusing hydrogen.
For example, as you can see in the diagram, we have circled in red where a star would fall on the main sequence if it has a mass ten times that of our Sun. With only ten times the mass, it is 10,000 times more luminous, even though it is not much larger than the Sun. That tells you that to shine that much brighter with a similar surface area, the star must be fusing hydrogen at a rate that is 10,000 times faster than our Sun's rate of hydrogen consumption.
Furthermore, since we now know that on the main sequence, a star's luminosity, surface temperature, and mass are all related, if we know a star's spectral type, then we can infer its mass and its luminosity. We can also predict that a higher mass star will have a shorter lifetime because it is consuming its fuel at a much faster rate.
Giants and supergiants are to the upper-right of the main sequence on the H-R Diagram. The fact that their luminosity is so high while their temperatures are relatively low tells us that they must be extremely large. As a matter of fact, the largest of the supergiants have a radius about 1,000 times that of our Sun's radius.
Meanwhile, white dwarfs are relatively tiny, with a typical white dwarf being no larger in size than Earth.
What separates these types of stars from the main sequence stars is their energy production. The giants and supergiants are stars that have run out of hydrogen fuel and are fusing heavier elements.
When giants and supergiants eventually run out of fuel altogether, they lose their outer layers and all that is left is a dead core in which all nuclear fusion has stopped. They are hot but they do not have an energy source and are only radiating leftover heat into space. Despite being smaller than Earth, they usually have a mass similar to that of the Sun, meaning they are made of matter compressed to an extremely high density.