Lesson 8
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Luminosity Versus Spectral Type


* H-R Diagrams
* Luminosity Class
* Key Terms
* Review Questions
* Advanced Questions
* Online Help


H-R Diagrams

The Danish astronomer Einar Hertzsprung observed in 1905 that the luminosities of stars generally decrease with spectral type from O to M and that a few stars have much brighter luminosities than other stars of their spectral class. In 1913 the American astronomer Henry Norris plotted the absolute magnitude of stars against their spectral types. These graphs have been extremely useful for understanding the nature and evolution of stars. They are now called Hertzsprung-Russell, or H-R diagrams. Because absolute magnitude is related to the luminosity of a star, luminosity may be substituted for absolute magnitude. Similarly, temperature may be substituted for spectral type.

H-R Diagram with Hipparcos Data

The best H-R diagram ever made is based on data obtained from the Hipparcos orbiting observatory. The most significant feature of the H-R diagram is that stars do not fill it. There are a few areas of the H-R diagram that contain most of the stars and other regions in which few stars, if any, are located. The region that contains the most stars is called the main sequence, which runs diagonally from hot, luminous stars to cool, dim stars. Other populated areas are the giant region, which contains cool, luminous stars; the supergiant region, which contains the most luminous stars in the universe; and the white dwarf region, which contains hot, dim stars. The H-R diagram is just as important to astronomy as the periodical table is to chemistry and the chart of the nuclides is to nuclear physics.

H-R Diagram

The largest stars in the universe are the cool supergiants located in the upper left corner. Since their temperatures are low, they can only have high luminosities if they have large radii and therefore large surface areas to radiate light. White dwarfs are extremely hot, but they are not very luminous because they are small. In fact, white dwarfs are the smallest stars on the diagram.

Refer to your reading assignment for more on the H-R diagram.


Luminosity Class

The H-R diagram shows that stars can have differing luminosities even though they have the same surface temperature. The Sun, for example, has absolute magnitude +4.8 while G2 giants are 100 times brighter than the Sun at absolute magnitude 0. Furthermore bright G2 supergiants have an absolute magnitude of about -7.5, making them about 1000 times more luminous than G2 giants and 100,000 times more luminous than the Sun.

In the 1930's, W.W. Morgan and P.C. Keenan of the Yerkes Observatory of the University of Chicago developed a system of luminosity classes. Luminosity classes Ia and Ib are composed of supergiants; luminosity class II is composed of bright giants, a distinct upper tier to the giant region on the H-R diagram; and normal giants are assigned to luminosity class III. Luminosity class V includes all main sequence stars. A few stars fall between the main sequence and the giant region. These stars are sometimes called subgiants and are assigned to luminosity class IV.

Luminosity Classes

Astronomers commonly use a shorthand description that combines a star's spectral type and its luminosity class. For example, the Sun is said to be a G2 V star. The spectral type indicates the star's surface temperature and the luminosity class indicates its absolute brightness, which is related to its size. Thus, an astronomer knows immediately that any G2 V star is a main sequence star with a luminosity of about that of the Sun and a surface temperature of about 5800 Kelvins. Similarly, a description of Aldebaran as a K5 III star tells an astronomer that it is a red giant with a luminosity of about 100 times that of the Sun and a surface temperature of about 4000 Kelvins.

Refer to your reading assignment for more on the luminosity class system.


For this topic, study the true and false, fill in the blanks self-test, and review questions at the end of the Chapter(s) of your reading assignment. In addition, learn the key words and answer all questions that follow:


Key Terms (refer to your text for some these terms)

H-R diagram
main sequence
main sequence stars
bright giants
bright supergiants
white dwarfs
luminosity class
MK classification

Review Questions (refer to your text to answer some of these questions)

1. Sketch a Hertzsprung-Russell diagram and indicate the location of the main sequence stars, giant stars, supergiant stars, and white dwarfs.
2. Where on the H-R diagram are the largest stars? Explain.
3. Where on the H-R diagram are the smallest stars? Explain.
4. What is the purpose of the luminosity class?

Advanced Question (refer to your text to answer some of these questions)

Use the Stefan-Boltzmann law to show that bright main sequence stars are not only hotter, but bigger than dim main sequence stars.



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