AST 1002: Outline for Topic E

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

Site Map • Prof. Cohen • Department of Astronomy • University of Florida

Last updated January 1, 2003

Home Page

Course Information

Study Guide

Chaisson Textbook

Other Resources

Alachua Astron. Club

Undergrad. AstroPhy. Soc.

---

Examples

Figures

Other

Outlines

PowerPt

Tables

---

Topic E: Measuring Stars (Fundamentals of the Stellar Population)

• Parallaxes and Stellar Distances (Ref. Fig. 8)
  • Difference between extrinsic and intrinsic properties of stars
  • Definitions of all terms on back of Fig. 8 in Study Guide including:
    annual trigonometric parallax, arc minute, arc second, parsec, and light year
  • Understand concept of parallax, units measured in
  • Know how to calculate distance from parallax and vice versa using d(pc) = 1/p(")
  • Know the approximate conversion factors for parsecs to astronomical units & to light years
    (See second page of Study Guide Fig. 8)
  • Be sure to see Topic E questions, #7–12
  • Know how parallaxes measured (read & study Fig. 9 in Study Guide)
  • Know the approximate distance that the trig parallax method will give a "reliable result" (see Fig. 9b)
  • Know name of star with largest known parallax. (Also know approx. value of its parallax — Ref. Table 9, Fig. 9b)
  • Can you think of any current/future techniques that could improve trig parallaxes?
• Proper Motions of Stars (Ref. Fig. 9 & 10)
  • Definition of proper motion and how different from parallax
    (Hence, what causes proper motion?)
  • Know what units of measurement usually used for proper motion
  • Know what factors influence the size of proper motions
    –Know which factor usually affects the size of proper motions most
    –Know how proper motions help to decide if stars are probably near or far
    
  • What is different about the proper motions of many stars in the Ursa Major region of sky
    (compared to the usual proper motions of most other stars)?
    –What do proper motions tell us about this group of stars in Ursa Major?
    –What is this group of stars in the Ursa Major region called?
    
  • Know name of star with largest known proper motion (Also know approx. value of its proper motion — Ref. Table 9)
  • Be able to distinguish between radial, tangential and space velocity (Ref. Fig. 10)
  • Know what two quantities are needed to find the space velocity (motion) of a star
  • Know how the radial and tangential components of space velocity change as a star approaches and then recedes from the solar system (Ref. Fig. 10)
• The Doppler Effect
  • What types of sources show Doppler phenomenon?
  • Which source (sound or radiation) produces the most noticeable Doppler effect (Why?)
  • What is a Doppler shift and what is its "cause"
    –What factor influences the size of a Doppler shift (i.e., how large it is)?
    –What factor influences the sign (positive or negative) of a Doppler shift?
    –Implications of negative, positive and zero Doppler shifts
       (What does the sign of the Doppler shift tell you?)
  • Definition of radial velocity
    –What do negative, positive and zero radial velocities imply or tell us about a star's motion?
       (This is similar to above questions about Doppler shifts)
    –Usual units of measurement used for radial velocity by astronomers
    
  • Know how radial velocities determined (i.e., know material in Fig. 11), including
    –What is a comparison spectrum & how used
    –How Doppler shifts measured
    –What is the relation between the Doppler shift and radial velocity? (See Fig. 11)
       (You will not have to do calculations using Doppler formula on last line of Fig. 11)
  • How does the distance of a star influence its Doppler shift? Why?
  • What types of spectra are most useful for measuring Doppler shifts (Ref. Topic E, #23)
  • Astronomical uses of the Doppler shift
  • Know some example instruments that use the Doppler principle
• Magnitudes of Stars and the Magnitude Scale (Ref. Table 10)
  • Know how magnitude scale works (what do algebraically larger magnitudes mean?)
  • Know definition of magnitude scale (i.e., what does a 5 magnitude difference imply?)
    (You do not to know brightness ratios for magnitude differences not multiples of five)
  • Know how to convert magnitude difference of 5 to brightness ratios (and vice versa)
  • Know the apparent magnitude of Polaris and the naked eye limit (ref. Table 1)
  • Omit material on magnitude conversions (unless difference is five magnitudes)
    (Omit Study Guide Example 5 and Topic E Questions #34–39)
• Absolute Magnitudes of Stars
  • Definition of apparent magnitude and absolute magnitude
  • Inverse Square Law of Light Propagation (Ref. Fig. 12)
    –How used to find brightness changes
    –Under what conditions would the inverse square law of light propagation be invalid?
    –See E Questions #47–51, 53–54 for problems on inverse square law
  • Know basic steps used to find absolute magnitude from apparent magnitude & distance
    1. Move star to 10 pc (i.e., makes use of definition of absolute magnitude)
    2. Determine brightness change (i.e., use "Inverse Square Law of Light Propagation")
    3. Convert brightness change to a magnitude change
  • However, you will not have to calculate absolute magnitude from apparent magnitude and distance or vice versa
    (i.e., omit Examples 6A–6D in Study Guide)
    –Hence, omit Topic E Questions #43–45, 57–62, 76
  • Know definitions of luminosity and bolometric
  • Hence, know the difference between a visual and a bolometric magnitude or luminosity
  • You will not have to calculate luminosity from absolute magnitude (or vice versa) unless the magnitude difference is five
• Spectral Classification of Stars (Ref. Table 11 incl. notes at bottom of page)
  • Know who were Annie J. Cannon, Henry Draper and Henry Norris Russell, and what they did
  • Know the seven basic main classes (in correct order), viz., O, B, . . ., M
  • How each main spectral class subdivided into numerical subclasses (see Table 11, Note #2)
  • Know the basic (primary) significance of the spectral sequence
    (i.e., What principle stellar characteristic causes the major differences in stellar absorption line spectra?)
  • Also know the two secondary stellar characteristics that cause stellar spectra to differ
  • Know the astronomical definition of early and late
  • Know the astronomical definitions or meanings of metal, molecule, neutral atom, ion, ionization state, ionization
    (See Notes #3–7 at bottom of Table 11)
  • Know meaning of color index (Note 9); you can omit most questions on color index (e.g., #70–71)
  • How do astronomers denote neutral atoms and ions using Roman numeral notation?
  • What directly observable stellar characteristic defines each spectral type?
    (Ans. The absorption or dark lines in a stellar spectrum)
  • Know the key spectral line features of each main class (Columns 3–7 of Table 11)
  • Know how the number of spectral lines between early and late stars differ (Table 11, Note #10) and why they differ
• The Sun: Know Basic Properties (Values)
  • Mean distance from Earth (in AU's, miles, kilometers)
  • Absolute magnitude
  • Spectral class, surface temperature (in Kelvins), color and color index
• Tentative List of Study Guide Questions To Omit for Topic E
  • Do #1 through 33, 40 through 42, 46 through 51, 53 through 54, 63 through 69, 72 through 75, 77 through 80, 83 through 90
  • You can omit other Topic E questions

---

* 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.

[Return to Top]

---

Recommended web browser to view this site is Netscape Navigator 4.0 (or above) with video display supporting at least 256 colors.
High color (16 bit) or true color (24 bit) display is preferred.

---

---

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