Download Stellar Physics - University of Reading

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”
(4th or 5th edition, W.J. Kaufmann)
– "The Physics of Stars"
(2nd 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 Mon Tue Wed Thu Fri Sat Week
28
29
30
1
1
2
3
1
4
5
6
7
8
9
10
2
11
12
13
14
15 16
17
3
18
19
20
21
22 23
24
4
25
26
27
28
29 30
31
5
1
2
3
4
5
6
7
6
8
9
10
11
12 13
14
7
15
16
17
18
19 20
21
8
22
23
24
25
26 27
28
9
29
30
10
1
2
3
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 14th May and 11th June
– Answers returned in weeks 5 and 9

To the General Office by 4pm, 28th May and 25th 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, p, is simply given
by:
p = 1/d (with d 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 = 2.06x105 AU
– 1AU = 1.5x108 km
\1 pc = 3.086x1013 km
– Distance light travels in 1 year = 1 light
year (ly) = 9.46x1012 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: vt ; Units km/s
– related to the proper motion by:
vt = 4.74md km/s (with d in pc)
Velocities of Stars

Define:
– Radial Velocity: The velocity of the star
along the line of sight.
– Symbol, vr ; Units, km/s
– Note a negative radial velocity means a
star is approaching us
Velocities of Stars
vt

Example:
vs
q
vr
– 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 vs = (vr2 + vt2)1/2 = 142.3 km/s
– Angle to line of sight
q = tan-1(vt /vr ) = -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 - vr positive
No shift - vr zero
Blue shift - vr 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 vr
Dn/n = vr /c = -0.04%
Next Lecture...
Magnitudes and Luminosity
 Stellar Spectra and Classification
