Stellar Physics

Dr P.A. Hatherly Unit: 3/PH/S6

Topics to be Covered:

   Properties of Stars – Distances, velocities, dimensions, masses, temperatures, luminosities.

Stellar Interiors – Pressures and temperatures, compositions, power sources.

Life-cycles of Stars – Star formation, evolution and death.

Resources Available

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

Unit Structure

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

Lecture Calendar

April May June July Sun 4 11 18 25 1 8 15 22 29 Mon 28 Tue 29 Wed Thu 30 5 12 19 26 2 9 16 23 30 6 13 20 27 3 10 17 24 7 14 21 28 4 11 18 25 1 8 15 22 29 5 12 19 26 Fri 2 9 16 23 30 6 13 20 27 Sat 3 10 17 24 31 7 14 21 28 Week 1 2 3 1 1 2 3 4 5 6 7 8 9 10 10 Key: Public Holidays Stellar Physics 10-11, D Stellar Physics 9-10 D, workshop 10-11 131 Stellar Physics 9-10 D Stellar Physics 11-12 D, workshop 12-1 131 Private Study Release Assessment 1 on 14th May Return Assessment 1 on 28th May Release Assessment 2 on 11th June Return Assessment 2 on 25th June

Assessment

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

Assessment

 Formal Examination – – 1 1/2 hour paper next June Contribution: 80%

Assumed Knowledge:

    Classical Mechanics and Optics Thermodynamics and Statistical Mechanics Atomic and Molecular Physics Ideas from Observational Astronomy – (useful, but not essential)

Distances of Stars

Stellar Parallax

d p

1 AU

Distances of Stars

  The angle subtended, by: p = 1/ d (with d p, is simply given in AU and p in radians) 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)

Distances of Stars

 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”

Distances of Stars

 Relationship to Other Units – – – 1 pc = 2.06x10

5 1AU = 1.5x10

8 \ AU 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

Distances of Stars

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

Velocities of Stars

 Define: – – Proper Motion: The angular velocity of a star tangential to the line of sight Symbol, m ; Units, arcsec/year – – Tangential Velocity: v t ; Units km/s related to the proper motion by: v t = 4.74

m d km/s (with d in pc)

Velocities of Stars

 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

Velocities of Stars

v t v s q  Example: v r – – 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 q = tan -1 ( v t / v r ) = -38.75°

Velocities of Stars

 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

Velocities of Stars

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

Next Lecture...

  Magnitudes and Luminosity Stellar Spectra and Classification