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Spectral Classification and the H-R Diagram

Terms you will need

  • spectral classificationSpectral classification is a way to sort stars based on the patterns of light they emit, which tells us about their temperature and chemical makeup.
  • hertzsprung-russell diagramThe Hertzsprung-Russell diagram is a graph that shows the relationship between a star's temperature and its brightness, helping astronomers understand the life stages of stars.
  • blackbody radiationBlackbody radiation is the light emitted by an object that absorbs all light that hits it, and the type of light it emits depends only on its temperature.
  • wien displacement lawThe Wien displacement law states that the wavelength at which an object emits the most light shifts to shorter wavelengths as the object's temperature increases.
  • stefan-boltzmann lawThe Stefan-Boltzmann law says that the total energy a hot object emits increases with the fourth power of its temperature, meaning hotter objects emit much more energy.
  • apparent magnitudeApparent magnitude is a measure of how bright a star appears in the sky from Earth, with lower numbers meaning brighter stars.

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Spectral classification and the Hertzsprung-Russell (H-R) diagram are essential tools in understanding the properties and evolution of stars. By plotting a star's luminosity against its surface temperature or spectral class, the H-R diagram reveals patterns that help astronomers classify stars and study their life cycles. This diagram was independently developed by Henry Norris Russell and Ejnar Hertzsprung in the early 20th century and has since become one of the most important tools in astronomy. The H-R diagram shows that most stars fall along a narrow band called the main sequence, where they spend the majority of their lives fusing hydrogen into helium. Other regions of the diagram, such as the area for giants and supergiants or the lower-left corner for white dwarfs, represent different stages in a star's life. Understanding these classifications and patterns helps astronomers determine a star's age, mass, and future evolution.

Figures (6)

Figure 18.12 Height versus Weight. The plot of the heights and weights of a representative group of human beings. Most points lie along a “main sequence” representing most people, but there are a few exceptions.
Figure 18.13 Hertzsprung (1873–1967) and Russell (1877–1957). (a) Ejnar Hertzsprung and (b) Henry Norris Russell independently discovered the relationship between the luminosity and surface temperature of stars that is summarized in what is now called the H–R diagram.
Figure 18.14 H–R Diagram for a Selected Sample of Stars. In such diagrams, luminosity is plotted along the vertical axis. Along the horizontal axis, we can plot either temperature or spectral type (also sometimes called spectral class). Several of the brightest stars are identified by name. Most stars fall on the main sequence.
Figure 18.15 Schematic H–R Diagram for Many Stars. Ninety percent of all stars on such a diagram fall along a narrow band called the main sequence. A minority of stars are found in the upper right; they are both cool (and hence red) and bright, and must be giants. Some stars fall in the lower left of the diagram; they are both hot and dim, and must be white dwarfs.
Figure 18.16 The Sun and a Supergiant. Here you see how small the Sun looks in comparison to one of the largest known stars: VY Canis Majoris, a supergiant.
Figure 18.17 Two Views of Sirius and Its White Dwarf Companion. (a) The (visible light) image, taken with the Hubble Space Telescope, shows bright Sirius A, and, below it and off to its left, faint Sirius B. (b) This image of the Sirius star system was taken with the Chandra X-Ray Telescope. Now, the bright object is the white dwarf companion, Sirius B. Sirius A is the faint object above it; what we are seeing from Sirius is probably not actually X-ray radiation but rather ultraviolet light that has leaked into the detector. Note that the ultraviolet intensities of these two objects are completely reversed from the situation in visible light because Sirius B is hotter and emits more higher-frequency radiation. (credit a: modification of work by NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute), M. Barstow and M. Burleigh (University of Leicester) and J.B. Holberg (University of Arizona); credit b: modification of work by NASA/SAO/CXC)

Spectral classification sorts stars by their surface temperature and chemical features, using letters like O, B, A, F, G, K, and M. Most stars fall on the main sequence, a diagonal band showing stars fusing hydrogen in their cores. Giants and supergiants appear above the main sequence, being large and cool but very bright.

White dwarfs are small, hot, and dim, found in the lower-left. The H-R diagram reveals patterns that show how stars evolve over time.

Key Points

  • Spectral classification is the classification of stars based on their spectral characteristics, primarily summarizing the ionization state and giving an objective measure of the photosphere's temperature.
  • The Hertzsprung-Russell diagram is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities and their stellar classifications or effective temperatures.

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