AST 1002: Outline for Topic F

[Sections Taught by Prof. H.L. Cohen]

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Last updated January 1, 2003

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Topic F: The HR Diagram (Sorting Out Stellar Characteristics

• Hertzsprung-Russell Diagram Basics (Ref. Fig. 13)
  • Know alternate names for this diagram (e.g., "Color-Magnitude Diagram")
  • Know usual labels for each axis (i.e., absolute magnitude and spectral class)
  • Also know alternate labels for both the vertical axis and horizontal axis (e.g., horizontal = temperature)
  • Know names & locations of each region in diagram (e.g., main sequence, giants, etc.)
  • Know Roman numeral designations (luminosity classes) for each region (see Fig. 13)
  • Know importance of the H-R Diagram in stellar astronomy
• Star Frequency in Each Region of the HR Diagram
  • Where are the brightest appearing stars found in HR Diagram (Fig. 14)?
    –What types of stars are typically the brightest appearing stars on sky?
    –What types of stars are missing from the brightest appearing stars on sky?
    
  • Where are the nearest stars to the Sun in HR Diagram (Study Guide Fig. 15)?
    –What types of stars are nearest the Sun? Not near the Sun?
       (See Textbook, pp. 290–293 including the figures for additional help on above topics.)
  • What types of stars in our Galaxy are probably most numerous? Least numerous?
  • How common (very, moderate, rare, etc.) are "solar type" stars (i.e., G2 V stars) ?
  • Know which luminosity class (I, II, etc.) contains most stars
  • The Main Sequence
    –Which two stellar parameters does the main sequence show a correlation between?
    –Know the frequency of each main sequence spectral class
       (i.e., what types of main sequence stars are most numerous? Least numerous?)
• Stellar Radii
  • Which two stellar parameters determine stellar luminosity?
  • Know what the horizontal axis of the HR Diagram represents
    –What is a line of constant temperature?
    –What do lines of constant temperature tell about stars?
  • Know what the vertical axis of the HR Diagram represents
    –What is a line of constant luminosity?
    –What do lines of constant luminosity tell about stars?
  • What is a line of constant radius?
  • Where would a star's new position lie in HR Diagram if
    –Radius changes but temperature does not?
    –Temperature changes but radius does not?
    –Both radius and temperature change?
  • Why it is difficult to subjectively (qualitatively) determine radii of main sequence stars?
  • Where in the HR Diagram are stars that are smallest? Largest?
  • Know LRT relation in its mathematical form (L = R²T⁴)
    –What does the radius (R) represent or symbolize? (Ans. Surface Area)
    –What "law" does the temperature (T) represent or symbolize? (Ans. Stefan-Boltzmann Law
    
  • Know how the LRT relation can be used to determine (i.e., calculate) either L, R or T if you are given the other two
  • You will not have to use the LRT relation in any calculations but, should, in principle, know what you would have to use the relation
  • Know (qualitatively) how the sizes (radii) of stars compare to Sun for
    –Early and late main sequence stars (Ref. Table 13)
    –Giants and supergiants
    –White dwarfs
    –Figure 13b may help you visualize some of this material
• Properties of Main Sequence Stars (Ref. Table 13, Columns 1–7; Fig. 13b)
  • Is the main sequence a line of constant radius?
  • If not, is the main sequence slope larger or smaller than a line of constant radius?
  • Know the main sequence trend for temperature, radius and luminosity
    (That is, know how each changes as one "climbs" or goes up the main sequence)
  • See Table 13 for example main sequence properties
  • The last two columns of Table 13 (mass and density) are part of Topic G
    (Omit for Exam #4 but include for Exam #5.)
  • Table 9 gives specific example stellar properties for a small selection of stars
• HR Diagram and Stellar Radii
  • See Figure 13b (not in printed Study Guide) for a visual summary of some HR Diagram properties
  • Be sure to see questions you should be able to answer following the HR Diagram in Figure 13b.
• Star Clusters (OMIT except for third bulleted item—shown in bold)
  • Types of star groups (binaries, multiples, etc.)
  • Basic characteristics of both Open Clusters and Globular Clusters (Ref. Table 12)
  • Be able to recognize open clusters and globular clusters by their appearance
    (For example, see textbook figures 11-22 and 11-23 on pp. 294-295)
    
  • Why is apparent rather than absolute magnitude OK to use in star cluster HR Diagrams?
  • Why is color-index rather than spectral class often used in star cluster HR Diagrams?
  • Be able to describe and recognize HR Diagrams of star clusters (Ref. Figure 21; see also F, Questions #59, 63)
• Tentative List of Study Guide Questions To Omit for Topic F
  • Do numbers 1 through 42, 54
  • Also do #62. (You will need to use the answer from #61, i.e., distance = 400 pc)
  • Omit numbers 43 through 53, 55 through 58, 61 and 66
  • Also omit question relating to star clusters. This includes numbers 59, 60, 63 through 70.

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* This list is reasonably complete but students remain responsible for all material presented in class. Tables, figures, examples, etc., listed here refer to Study Guide items.

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For comments and suggestions contact Howard L. Cohen at cohen@astro.ufl.edu About this site (department disclaimer and recommended web browser) © Copyright 1999-2003 Howard L. Cohen