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II-4. Stellar Size – Radius
(Main Ref.: Lecture notes;
Lec. 4
FK Sec.5-4, 17-6, Box 17-4)
Direct Measurement: Only Sun, planets, nebula, etc.
Note: Stars are ~ point source, even with telescope.
Solar radius:
Rsun = 696,000 km = 109 R
(where R = radius of the Earth = 6378 km.)
Most useful and most often used method: Use Stefan
Boltzman Law
F =  T4,
for blackbody
where F = energy flux = J/m2 –sec,
 = Stefan-Boltzman constant = 5.67 x 10-8 W m -2 K -4
Eqn (14)
Also, F = L / A
where A = area of the star = 4  R2
 F = L / 4  R2
From Eqns (14) and (15c), get:
L = 4  R2  T4,
• Relates L (luminosity = total power output), R (radius), and
T (temperature) of the star.
Divide Eqn(16) for star by that for Sun, and get
R / Rsun = (L / Lsun )1/2 ( Tsun / T )2,
)1/2 means Square root.
See class notes for derivation.
EX 18 Betelgeus: L = 60,000Lsun, T = 3500 K
What is its radius? Ans: R = 670 Rsun - Red Giant!
Note: Tsun = 5800 K.
See class notes, FK Box 17-4 Example 1.
EX 19 Sirius B - Fainter star of Sirius binary
system: L = 0.0025Lsun, T = 10,000 K
What is its radius? Ans: R = 0.017Rsun = 11832km = 1.86R
- white dwarf!
(R = radius of the Earth.)
See class notes, FK Box 17-4 Example 2.
How to get R from Observation?
Measure distance d and brightness b, then from
L = 4  d2 b
Finding Key Properties
of Nearby Stars
get L. Then, measure
Color  get T.
Then, from L, T, and
Eqn(17), get R!
See class notes for derivation
of Eqn(18).
Fig. II-25 () and class
notes for details..
Fig. II-25: How to find R
II-5. H-R Diagram
(Main Ref.: Lecture notes; FK Sec.5-9, 17-7, 17-8)
II-5a. Introduction
H-R Diagram means we plot A vs B, where
A = m, M, L.
B = T, CI (color index), spectral type (e.g., O2, A3, etc.)
Note the general location of Main Sequence (MS), Giants(G), White Dwarfs(WD),
Supergiants (SG). Note that black holes and neutron stars are outside.
See class notes and Fig. II-27 for the etails. (also FK Fig. 17-15(b))
II-5b. Different Objects and Radius (Main Ref.: Lecture notes;
FK Sec. 17-7)
Note location of various bright stars in H-R Diagram – see Fig. II-26 (also FK Fig.
Note that you can find radius of a star from the star’s location on the H-R Diagram
– see Fig. II-27 (also FK Fig. 17-15(b)).
See class notes for the details.
Fig. II-26: H-R. Diagram - I
Fig. II-27: H-R. Diagram -II
II-5c. Luminosity Class (Main
Ref.: Lecture notes; FK Sec. 17-8)
Ia: Luminous
Ib: Less Luminous
II: Bright Giants
III: Giants
IV: Subgiants
V: Main Sequence
See Fig. II-28 (also FK Fig.
17-18) for location of
various Luminosity class.
Fig. II-28: H-R. Diagram - III
• Classification of Stars:
Stars are classified by Spectral class (type), e.g., A1,
and Luminosity class, e.g., V.
EX 20: Vega ~ A0 V; Aldebaran ~ K5 III;
Rigel B8 Ia
See class notes and Fig. II-26 and 28 for the details.
Note: white dwarfs and neutron stars are outside.
II-5d. How to distinguish between stars of different
luminosity class but in the same spectral class?
(Main Ref.: Lecture notes; FK Sec. 17-8)
Best explained by example.
EX 21:
A. Rigel: B8 ( T= 13,400K); Ia (L= 58000 Lsun ), Supergiant (SG)
B. Algol: B8 ( T= 13,400K); V (L= 100 Lsun ), Main Sequence (MS)
Same color, same T, but different L. How to find
the difference?
Ans: Width of the spectral lines tells the difference  Lines such as H and H ,
strong for B8 stars, are narrow for supergiants (e.g. Rigel), but broad for main sequence
stars (e.g., Algol).
Why? Due to density difference, Doppler effect,etc.
See class notes and FK Sec.17-8 for the details.
By carefully examining a star’s spectral lines,
astronomers can determine whether that star is a
main-sequence star, giant, supergiant, or white dwarf
Fig. II-29: Line width
II-5e. How to find Radius R and Distance d from the H-R Diagram?
(Main Ref.: Lecture notes; FK Sec. 17-8)
▪(1) From observation, find spectral class
with line width   H-R Diagram
 get L and T  Eqn(17)  get R!
▪(2) From (1) get L. Then, measure m 
Eqn(6’)(*)  get b
L and b  Eqn(18)  get d!
(*) msun  m = 2.5 log ( b / bsun) Eqn(6’)
See class notes and Fig. II-30 for
the details.
Fig. II-30:Flow Diagram for finding R and d
• EX 22
Find distance d for a A0 Ib star with m = +10.
Ans: d = 10,000 pc.
Summary: m measured, Spectral class A0 and
luminosity class Ib (from width of line measured
)  H-R Diagram  get M.
Then, Eqn(8) gives d.
(See class notes for the details.)