AST 1002: Outline for Topic H
[Sections Taught by Prof. H.L. Cohen]
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Last updated April 21, 2003
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Be sure to check if you can omit any Study Guide Questions
Topic H: Stellar Aging (Structure and Evolution of the Stars)
- Stellar Structure: Computing a Stellar Model
- Know what is meant by a stellar model (i.e., what do models describe or tell about stars?)
- What two fundamental properties of the star are chosen to make a model? (Ans. mass & chemical composition)
- Know what is meant by hydrostatic (gravitational) equilibrium
–Know what are the inward and outward forces
–In what types of stars is radiation pressure important for hydrostatic equilibrium? (Ans. O and B stars)
- Three methods of energy transport — know name of each method
–In what medium (solid, liquid, gas or vacuum) does each method work best?
–Which methods are important in most stars? Hence, which methods are not important?- H (Hydrogen) Burning (Ref. Table 15)
- Know the difference between chemical burning and (thermo)nuclear burning
- What is the approximate solar lifetimes for fossil fuel burning, gravitational contraction, and for H burning?
- See Table 14, Col. 8 in printed Study Guide
- Note: You only need to know the approximate times in the last column of Table 14, not the details.
- What do physicists usually call (thermo)nuclear burning? [Ans. (thermo)nuclear fusion]
- Which energy production method most likely dominates in Sun at present time?
(Ans. hydrogen fusion or burning)- What is the approximate age of Sun? (Ans. About 5 billion years)
- Be familiar with the basic H burning equation (top of Table 15):
–Know the reactants and products (viz., 4 H atoms form 1 He atom + energy)
–How does the mass of one He atom compare to the mass of four H atoms?
–What does mass defect mean?
–Where does the energy come from in H fusion reactions?
–What is the approximate minimum temperature needed for stars to burn hydrogen?
(Ans. About 5–10 million Kelvins)- Know the names of the two basic procedures that accomplish H burning in stars (See Table 15)
–Which H burning procedure produces more energy at "low" temperatures?
–Which H burning procedure produces more energy at "high" temperatures?
–What is the approximate crossover temperature between "low" and "high" above?
–Which H burning procedure produces most of Sun's energy now?- What are the approximate H burning lifetimes of stars?
Ref. See printed Study Guide Table 17, Column 4. (This is the star's "Lifetime on Main Sequence"):
- Early (i.e., O and B) main sequence stars;
- G type main sequence stars;
- Late (i.e., K and M) main sequence stars
- A Solar Model (Ref. Figures 17 and 18)
- Understand how Sun's thermostat works to regulate gravitational (hydrostatic) equilibrium, temperature, pressure, etc.
(See Figure 23 on Web)- Approximate X, Y and Z for core and surface of Sun at:
(1) zero age, (2) now, and (3) at end of Sun's H burning life
(Note: X, Y and Z = relative amounts of hydrogen, helium and metals.
For example, we might have X = 70%, Y = 30%, Z = 2%)
- Hence, know Sun's present chemical composition and how it has changed
- Rational (reasoning) that leads to above values of X, Y, Z (See Study Guide Slides #18-19)
- Location of Sun's radiation and convection zones (Fig. 18)
- Know which part of Sun contains most of Sun's
(1) mass, (2) energy production by H burning, (3) and luminosity (see Study Guide Fig. 17 & 18)- What is meant by solar granulation? What does granulation demonstrate?
- Families and Evolutionary Sequences of Stellar Models
- What does a family of H burning stellar models represent? (Ans. A sequence of stellar models with the same chemical compositions and basic stellar structure equations but with different masses)
- What does an evolutionary sequence of stellar models represent?
(Ans. A sequence of stellar models of the same star but at different stages of its life)- What specific feature of the HR Diagram do H burning families of stars reproduce? (Ans. The Main Sequence)
- What is the significance or meaning of the main sequence? (See Study Guide Slide #23)
- Where is the location of H burning stars in the HR Diagram?
- What principle factor determines where H burning stars fall on the main sequence? (Ans. mass)
- Why are main sequence stars typically more common than other types of stars?
- Evolution of The Sun: A One Solar Mass Star (Ref. Table 16 and Figure 19)
- What is the meaning of an evolutionary track in the HR Diagram?
- What is the difference between real and evolutionary ages of a star?
- Know each step (briefly) or stage in evolution of Sun
(Study Guide Table 16, steps 1–16, last column)- Know location in HR Diagram of each principle stage in the Sun's evolution (Ref. Fig. 19)
- What is the approximate lifetime (years) for each stage
(Ref. See Table 16, 3rd column)- Definitions or terms to know and understand:
–Zero Age or a Zero Age Main Sequence Star and Zero Age Main Sequence (ZAMS)
–Planetary Nebula and Planetary Nebula Star
–White Dwarf
–Black Dwarf- Can you guess how do young (pre-main sequence) giants differ from old (post-main sequence) giants?
(Hint: See Study Guide PowerPoint Slides #40–42 in Topic H, Part 4)- Know the Sun's principle energy source (gravitational contraction, H burning, He burning, etc.) for each stage of evolution
- Know the structure of a
–Pre-Main Sequence Giant
–Main Sequence Star
–Post-Main Sequence Giant
–Planetary Nebula Star
–White Dwarf
Note: The next two items refer to Table 17 in the Study Guide
- Know how stellar evolutionary time scales differ between
–Main sequence life (H-burning)
–Pre-main sequence gravitational contraction and
–Post-main sequence expansion to a giant or supergiant- Know how stellar evolutionary time scales differ between stars of different masses on the main sequence
- Tentative List of Study Guide Questions To Omit for Topic H
- Omit #3 through #6, #8, #36 through #39, #42 through #45
* 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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