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Chapter 17
By reading this chapter, you will learn
17-1 How we can measure the 17-9 How we can use binary stars to
distances to the stars
measure the masses of stars
17-2 How we measure a star’s 17-10 How we can learn about binary
brightness and luminosity
stars in very close orbits
17-3 The magnitude scale for 17-11 What eclipsing binaries are and
brightness and luminosity
what they tell us about the sizes of
17-4 How a star’s color indicates stars
its temperature
17-5 How a star’s spectrum
reveals its chemical
17-6 How we can determine the
sizes of stars
17-7 How H-R diagrams
summarize our knowledge of
the stars
17-8 How we can deduce a star’s
size from its spectrum
Parallax only works for the stars that we
D = distance to a star in parsec can measure the apparent change in
position. 500lys is about the upper limit!
P = parallax angle
Hipparcos (89 – 93) contributed greatly
for this purpose!
If too far for Parallax:
Apparent brightness of the
stars and inverse square law!
Apparent magnitudes of
the stars in Pleiades and
Orion’s surroundings
Now you are somewhat familiar with the method to measure distances, Please
work on “Distant objects” worksheet (part of your homework for this week!)
Imagine that you have received pictures of children from near by
planetary systems. Picture shows a child on his or her 12th
birthday. The pictures were then send electronically to us on
the day of the child’s birthday. (lys = light years)
• Yvone lives on a planet orbiting Garg, which is 9.5 lys away
from the sun.
• RYAN lives on a planet orbiting NORDSTROM, which is 14 lys
away from the sun.
• Julie lives on a planet orbiting Bray-Ali, which is 21.5 lys away
from the sun.
• MALLEREY lives on a planet orbiting LEWIS, which is 6 lys
away from the sun.
• RAQUEL lives on a planet orbiting VASQUEZ, which is 4.5 lys
away from the sun.
• MICHELLE lives on a planet orbiting LA HUE, which is 8.2 lys
away from the sun.
Stellar Motions:
Proper / radial motions
Color -> Temp
(recall Wien’s law)
1. If a star has parallax angle of 1/5 arc-sec,
what would be the distance to the star? A)1/5,
b)1. c)5, d)25 pc
2. Star A and B have same luminosity. If star A is
4 times closer to Earth then star B, then
_____ to earthly viewer.: a=A is 4 x brighter,
b=B is 4x brighter, c=A is 16 times brighter
d=B is 16 times brighter, e=A is 64x brighter
3. Apparent magnitude is measure of: a=light
output, b=light received by the observer,
c=temperature of light source
4. Which color star is hotter? A=red. B=white,
Click “ enter to review the questions
C, c, b, c
Spectral types (O, B, A, F, G, K,
and M), based on the major
patterns of spectral lines
Williamina Fleming (standing): left,
Annie Jump Cannon: right
Temp (K)
30k – 50k
11k – 30k
A White
7500 – 11k
5900 – 7500
G Yellow
5200 – 5900
K Orange
3900 – 5200
2500 – 3900
1300 – 2500
L and T are brown dwarfs
Properties of Nearby Stars
Stefan – Boltzmann law
L = 4pR2sT4
Inverse square law
L = 4pd2b
L = Luminosity
b = brightness (W / m2)
d = distance
L = Luminosity
R = radius
s = constant
T = temp
Hertzsprung-Russell (H-R) diagrams
Spectroscopic Parallax
Inverse square law
L = 4pd2b
L = Luminosity
b = brightness (W/m2)
d = distance
Stefan – Boltzmann law
L = 4pR2sT4
L = Luminosity
R = radius
s = constant
T = temp
Use of these laws tell us
distance, radius and
How about mass?
1. Williamina Fleming and Annie Jump Cannon were
involved in developing: a=luminosity class,
b=spectral class, c=distance measurements
2. Which type of star is hotter? a=G2V, b=A1II, c=M4III,
d=O1Ia e=K7IV
3. The sun’s spectrum is perfect continuous spectrum.
a=ture, b=false, c=we don’t know!
4. Which type of star is more stable? a=G2V, b=A1II,
c=M4III, d=O1Ia e=K7IV
5. H-R diagram is a plot of: a=mass vs. temp.,
b=Hydrogen vs. radius, c=Luminosity vs. age,
d=mass vs. luminosity, d=luminosity vs. temp.
Click “ enter to review the questions
Binary Star System
Visual binary : 2 stars can be
observed as 2 with mutual gravity
Spectrum binary : appear to be one
star with incongruous spectrum.
Spectroscopic binary : appear to be
one but shows Doppler shifts
Optical Double : Appear to be close,
but just close line of sight.
Calculating mass from Binary Star System
Kepler’s 3rd law:
M total = a3 / p2
M = mass,
a = semimajor axis
p = orbital period
Distance between two stars
= 1.8AU
Time to orbit each other =
0.7 years
M = (1.8)3 / (0.7)2 =
Spectroscopy in
binary system
Light curves of eclipsing binaries
1. Binary system means: a=stars young, b=stars
are very large, c=stars are old, d=2 stars
2. Binary system let the astronomers measure
____ for the first time: a=mass,
b=temperature, c=age, d=Hydrogen content,
3. For the eclipsing binary, we can observe ____
to determine their motion. A=Doppler shift,
b=Luminosity, c=apparent magnitude
(brightness), d=temperature change
Click “ enter to review the questions
D, a, a&c