1. Stellar Physics Dr P.A. Hatherly Unit: 3/PH/S6

2. Topics to be Covered: n Properties of Stars –Distances, velocities, dimensions, masses, temperatures, luminosities. n Stellar Interiors –Pressures and temperatures, compositions, power sources. n Life-cycles of Stars –Star formation, evolution and death.

3. Resources Available n Recommended Texts: – –“Universe” (4 th or 5 th edition, W.J. Kaufmann) – –"The Physics of Stars" (2 nd edition, A.C. Phillips) n n IT – –CD-ROMS on Departmental PCs – Navigate via physicsnet at http://www.rdg.ac.uk/physicsnet/ –Unit Website Navigate via physicsnet at http://www.rdg.ac.uk/physicsnet/

4. Unit Structure n 14 Lectures/presentations –Weeks 4 and 8 for private study n 7 Workshops/discussion sessions –Friday Week 1 - no workshop n 2 assessed problem worksheets and 1 formal examination

5. Lecture Calendar

6. Assessment n Continuous Assessment –Selected problems set in weeks 3 and 7 n Posted on website on 14 th May and 11 th June –Answers returned in weeks 5 and 9 n To the General Office by 4pm, 28 th May and 25 th June –Results/feedback in weeks 6 and 10 n Results posted on website by Wednesday and problems discussed in Wednesday’s workshop –Contribution: 20%

7. Assessment n Formal Examination –1 1/2 hour paper next June –Contribution: 80%

8. Assumed Knowledge: n Classical Mechanics and Optics n Thermodynamics and Statistical Mechanics n Atomic and Molecular Physics n Ideas from Observational Astronomy –(useful, but not essential)

9. Distances of Stars p 1 AU d Stellar Parallax

10. Distances of Stars n The angle subtended, p, is simply given by: p = 1/d (with d in AU and p in radians) n Definition: – If a star gives a parallax of 1” (1 second of arc, arcsec = 1/3600°) then the distance to the star is 1 parsec (pc) –Hence, d (pc) = 1/p (arcsec)

11. Distances of Stars n Examples: –The first star to have its parallax measured was 61 Cygni. Its parallax was 0.33”. How far away is it? –d = 1/p = 1/0.33 = 3 pc –The nearest star, Proxima Centauri is at a distance of 1.3 pc. What is its parallax? –p = 1/d = 1/1.3 = 0.77”

12. Distances of Stars n Relationship to Other Units –1 pc = 2.06x10 5 AU –1AU = 1.5x10 8 km  1 pc = 3.086x10 13 km –Distance light travels in 1 year = 1 light year (ly) = 9.46x10 12 km  1 pc = 3.26 ly

13. Distances of Stars n Limitations of Parallax –Maximum distance from ground based observations, 50 pc –Maximum from space-based observations, 500 pc –Other methods required for greater distances n “Standard candles”

14. Velocities of Stars n Define: –Proper Motion: The angular velocity of a star tangential to the line of sight –Symbol,  ; Units, arcsec/year –Tangential Velocity: v t ; Units km/s –related to the proper motion by: v t = 4.74  d km/s (with d in pc)

15. Velocities of Stars n Define: –Radial Velocity: The velocity of the star along the line of sight. –Symbol, v r ; Units, km/s –Note a negative radial velocity means a star is approaching us

16. n Example: –Barnard’s Star (distance, 1.82 pc) –Proper motion = 10.32 arcsec/year –Tangential velocity = 89.1 km/s –Radial velocity = -111 km/s –Speed v s = (v r 2 + v t 2 ) 1/2 = 142.3 km/s –Angle to line of sight  = tan -1 (v t /v r ) = -38.75° vrvr vtvt Velocities of Stars vsvs 

17. n Measurement of Velocities –Proper motion - straightforward observation, maybe over many years, of the position of a star –Radial velocity - Use Doppler Effect Red shift - v r positive No shift - v r zero Blue shift - v r negative

18. Velocities of Stars n Example: –Barnard’s Star - 10.32 arcsec/year is easy to measure (= 0.6% angular diameter of full moon) –Doppler shift due to v r  /  = v r /c = -0.04%

19. Next Lecture... n Magnitudes and Luminosity n Stellar Spectra and Classification