ASTRONOMY 373
INTRODUCTION TO
ASTRONOMY –
Stars, Galaxies, & Universe
Spring 2015
Sachiko Tsuruta
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Lec 1
I. INTRODUCTION
FK (= Freedman, Geller & Kaufmann 10th Edition) Ch. 1)
II. INTRODUCTION TO CLASSICAL
ASTRONOMY
II-1. Stellar Distance and Stellar Motion
(Main Ref.: Lecture notes; FK Sec.17-1)
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II-1a. Stellar Distance
Stellar Parallax:
= Apparent
motion of a star due
to Earth’s annual motion
= Angular size of semimajor axis of the orbit of
Earth around Sun
Fig. II-1: Parallax
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Fig. II-2: Stellar Parallax
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Units of Distance:
Use mks system: length=meter, mass =kgm,
time=sec
Astronomical Unit (AU): Distance from the earth to the sun
= semi-major axis of the orbit of Earth around Sun
1 AU = d(sun) = 1.5 x 1011 m
Parsec (PC): Distance at which 1 AU subtends Angle of 1
second
1 pc (parsec) = 206625 AU = 3.086 x 1016 m = 3.262 ly
Light Year (ly): Distance light travels in 1 year
1 light year (ly) = 63240 AU = 9.46 x 1015 m
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DISTANCE
d (pc) = 1 / p(sec.)
Eqn (1)
•Distances to the nearer stars can be determined by parallax,
the apparent shift of a star against the background stars
observed as the Earth moves along its orbit
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EX 1: Alpha Centauri
•p = 0.76 sec
•d = 1 / p = 1 / 0.76 = 1.32 pc = 4.29 lys
See class notes for details
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EX 2: Barnard’s Star
Barnard’s star has a parallax of 0.547 arcsec
See class notes for details
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II-1b Stellar Motion
Fig. II-3: Stellar Velocity
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V
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vr
Doppler shift: see class notes and
FK Sec. 5-9, and Box 5-6
vT
d
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•RADIAL VELOCITY vr
vr / c = ( – 0) / 0 =  / 0
Eqn(2a)
Non-relativistic (see FK 5-9)
• TRANSVERSE VELOCITY vT
vT = 4.74  / p
Eqn (2b)
• vT in km/s;  in arc second/year; p in arc second
• SPACE VELOCITY v
v2 = vr2 + vT2
Eqn(2c)
Study Examples in FK Box 17-1 (Non-science majors
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optional)

 for
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II-2. Stellar Brightness, Magnitude, and
Luminosity
(Main Ref.: Lecture notes; FK Sec.17-2, 17-3)
II-2a. Brightness and Luminosity
(Main Ref.: Lecture notes; FK Sec.17-2, Box 17-2)
Definitions:
Luminosity: L = energy/sec = Power Output
(Watts = W)
Brightness: b = Luminosity/surface area (W/m2)
Area: A = 4  d2
d = distance
Eqn (3)
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Inverse Square Law
b = L / A = L / (4  d2)  1/d2
Eqn (4)
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Fig. II-4a: The Inverse-Square Law
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EX 3: Candle at 10 m and 100 m
Ans: At 10m 100 times brighter
See class notes for details
EX 4: Sun
L(sun) = 3.86 x 1026 W ; d(sun) = 1.5 x 1011 m;
Use Eqn (4), and get
Ans: b(sun) = 1370 W/m2
See class notes for details
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From Eqn (4)  L = 4  d2 b
Eqn (5a)
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Divide Eqn(5a) for star by that for sun 
L / L(sun) = (d / d(sun))2 (b / b(sun))
Eqn (5b)
Do the same for Star *1 and Star *2 
L1 / L2 = (d1 / d2 )2 (b1 / b2)
2
*2
1
d1
d2
Eqn (5c)
Fig. II-4b: The
Inverse-Square
Law (conti.)
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EX 5: Sirius A: d = 8.61 ly; L = 26.1 L(sun);
What is brightness b?
Ans: 8.79 x 10-11 brightness of Sun
(See class notes for details.)
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EX 6: Star *1 and Star *2
(same brightness: b1 = b2 = b)
Star 1: L1 = 1 L(sun); Star 2: L2 = 9 L(sun)
How far is Star 2 compared with Star 1?
Ans: 3 times further away.
(See class notes for details.)
Study more examples in FK Box 17-2.
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