The HR Diagram |
In the early 1900s, Danish astronomer Ejnar Hertzsprung discovered that in addition to "normal" stars such as our Sun, some stars are "giants" and others are "dwarfs." In 1911 he plotted the apparent magnitudes of stars in several star clusters against their spectral classification. Although he did not know the distances or absolute magnitudes of these stars, he assumed that stars in the same cluster were all at the same approximate distance. Hertzsprung was quick to notice that the stars did not fall randomly across the chart, but rather formed patterns.
In 1913, American astronomer Henry Norris Russell independently plotted the absolute magnitudes of nearby stars against their spectral classifications. He, too, quickly noted the pattern. Since then, all plots of stellar absolute magnitude (or a related quantity, luminosity) versus spectral class (or temperature, which is directly related to spectral class) are called Hertzsprung-Russell Diagrams, or HR Diagrams for short.
The HR Diagram is one of the most important tools in astronomy. By building the diagram using stars whose characteristics are well-known (i.e., the nearby stars), the patterns in the diagram became obvious. Stars do not form with random characteristics. These characteristics are related to spectral class, which can be determined simply by analyzing the star's light, without having to know the distance. Thus when astronomers determine the star's spectral type, and they can then look horizontally on the chart to read the star's absolute magnitude, M. Apparent magnitude, m, is just how bright the star appears to astronomers on Earth. So given M from the spectral type and m from observation, astronomers can apply a simple equation to find the star's distance, as discussed in section 12.8. Simply, knowing the the star's brightness falls off as the inverse square of its distance from us, we can compare its observed brightness to its known true brightness (absolute magnitude) to work backward and find the distance.
Please note that astronomers also have to consider whether the star is a Main Sequence star, a giant or supergiant, or a dwarf star. Without this information, which can be inferred from a detailed analysis of the star's spectral lines, the luminosity and absolute magnitude estimates would be off, and hence the distance estimate would be incorrect. (This relationship between spectral lines and luminosity is discussed in section 12.7 of your text. While two stars may have the same color, temperature and spectral class, the width of absorption lines in their spectra may vary. Astronomers have determined that the more luminous a star is, the narrower its spectral lines. In this way, they have further classified Main Sequence and giant stars into 5 luminosity classes, designated by Roman Numerals I-V. The least luminous are the Main Sequence stars, which are in luminosity class V. The most luminous stars are luminosity class I, which is further divided into Ia and Ib. Dwarf stars do not receive a luminosity class rating.)
Without the HR diagram (or equivalent knowledge), astronomers would have very limited knowledge of stars too far away for their distances to be calculated directly (i.e., those few stars for which we can measure parallax). Without the distance, astronomers would be able to determine only the temperature and color of stars, as well as gain some knowledge of their chemical composition through their spectral lines. In addition, astronomers can determine the relative motion of a star in space by observation. With a knowledge of distance, which can be inferred from the HR diagram, many more parameters can be deduced, including absolute magnitude, luminosity, mass, radius and age.
Why not try your hand at plotting stars on the HR diagram in our activity (or Thought Question #4 on page 381 of your text).
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