Chapter 11
Surveying the Stars
Outline of Chapter 11
Part I: Properties of Stars
Not exactly like book
I.
Parallax and distance.
II.
Luminosity and brightness
Apparent Brightness (ignore “magnitude system” in book)
Absolute Brightness or Luminosity
Inverse-Square Law
III.
Stellar Temperatures
Color, Spectral lines, Spectral Classification:OBAFGKM
IV.
Stellar sizes (radius)
V.
Stellar Masses
Properties of Stars
Our Goals for Learning
• How far away are stars?
• How luminous are stars?
• How hot are stars?
• How massive are stars?
• How large (radius) are stars?
I. Parallax and distance.
p = parallax angle in
arcseconds
d (in parsecs) = 1/p
1parsec= 3.26 light
years
I. Parallax and distance.
Nearest Star: Alpha Centauri d = 4.3 light years
(since 1 parsec = 3.26 light years)
distance in parsecs = 4.3/3.26 = 1.32
What is the parallax of this star?
d=1/p hence p=1/d
p for nearest star is
I. Parallax and distance.
Nearest Star: Alpha Centauri d = 4.3 light years
(since 1 parsec = 3.26 light years)
distance in parsecs = 4.3/3.26 = 1.32
What is the parallax of this star?
d=1/p hence p=1/d
p for nearest star is 0.76 arcseconds
All other stars will have a parallax angle smaller
than 0.76 arcseconds
Question 1
1.
The distance of a star whose parallax is 0.25 arc
seconds is
Question 1
1.
The distance of a star whose parallax is 0.25 arc
seconds is
A. 4 parsecs
B. 40 light-years
C. 100 astronomical units
D. 0.25 parsec
Question 1
1.
The distance of a star whose parallax is 0.25 arc
seconds is
A. 4 parsecs
B. 40 light-years
C. 100 astronomical units
D. 0.25 parsec
II. Luminosity and Brightness
1.
Apparent Brightness (how bright it looks in the
sky)
2.
Absolute Brightness or Luminosity (energy/sec)
3.
Inverse-Square Law
Energy passing through
each sphere is the same
The further the observer
the lower the apparent
brightness proportional
to 1/d2
Energy passing through
each sphere is the same
The further the observer
the lower the apparent
brightness proportional
to 1/d2
How many times
fainter will the
Sun seem from
Jupiter (5AU)
than from Earth?
Energy passing through
each sphere is the same
The further the observer
the lower the apparent
brightness proportional
to 1/d2
How many times
fainter will the Sun
seem from Jupiter
(5AU) than from
Earth? 25 times
II. Luminosity and Brightness
1.
Apparent Brightness (how bright it looks in the
sky)
2.
Absolute Brightness or Luminosity (energy/sec)
3.
Inverse-Square Law
 apparent brightness=(absolute brightness)/d2
4.
Examples: light bulbs at different distances
II. Luminosity and Brightness
1.
Apparent Brightness (how bright it looks in the
sky)
2.
Absolute Brightness or Luminosity (energy/sec)
3.
Inverse-Square Law
 apparent brightness=(absolute brightness)/d2
4.
Examples of absolute brightness and apparent
brightness: light bulbs at different distances
a)
b)
c)
d)
10W, 1 meter away
100W, 10 meters away
20W, 2 meters away
90W, 3 meters away
II. Luminosity and Brightness

light bulbs at different distances
a)
b)
c)
d)
10W, 1 meter away
100W, 10 meters away
20W, 2 meters away
90W, 3 meters away

Use formula: apparent brightness=(absolute
brightness)/d2

Which one is faintest?

Brightest?
II. Luminosity and Brightness

light bulbs at different distances
a)
b)
c)
d)
10W, 1 meter away
100W, 10 meters away
20W, 2 meters away
90W, 3 meters away

Which one is faintest? b

Brightest? a and d

Watts is a unit of energy/sec (power)
Review of distance, apparent brightness, absolute
(intrinsic) brightness and luminosity
Distance: If you know the parallax “p” (in arcseconds)
you can calculate the distance “d” (in parsecs) d=1/p
(1parsec= 3.26 lightyears)
Apparent brightness: how bright a star looks in the sky
The inverse-square Law: light from stars gets fainter as
the inverse square of the distance (apparent brightness is
proportional to 1/d2).
If we know the apparent brightness and the distance to
a star we can calculate its absolute (intrinsic) brightness:
apparent brightness = (absolute brightness)/d2
Luminosity (energy/sec) is equivalent to absolute
brightness (analogy with light bulbs: Watts)



Question 2
1.
Two stars have parallaxes of 0.1 arc seconds and 0.01
arc seconds, respectively, if the stars are equally
luminous, how much brighter will the near one appear
than the farther one?
Question 2
1.
Two stars have parallaxes of 0.1 arc seconds and 0.01
arc seconds, respectively, if the stars are equally
luminous, how much brighter will the near one appear
than the farther one? (Hint: calculate the distance first
and then estimate the apparent brightness)
Question 2
1.
Two stars have parallaxes of 0.1 arc seconds and 0.01
arc seconds, respectively, if the stars are equally
luminous, how much brighter will the near one appear
than the farther one? (Hint: calculate the distance first
and then estimate the apparent brightness)
A. 100
B. 1000
C. 10,000
D. 400
Outline of Chapter 11 Part I
I.
Parallax and distance.
II.
Luminosity and brightness
Apparent Brightness
Absolute Brightness or Luminosity
Inverse-Square Law
III.
Stellar Temperatures
Color, Spectral lines, Spectral Classification:OBAFGKM
IV.
Stellar sizes (radius)
V.
Stellar Masses
How hot are stars?
III. Stellar Temperatures
1.
Color ( hotter > bluer; cooler > redder)
2.
Spectral lines
3.
Spectral Classification:
OBAFGKM (from hottest to coldest)
Laws of Thermal Radiation
hotter  brighter, cooler  dimmer
hotter  bluer,
cooler  redder
(from Ch. 5)
Hottest stars:
blue
Coolest stars:
red
(Sun’s surface is
about 6,000 K)
Lines in a star’s spectrum correspond to a spectral type that
reveals its temperature:
O B A F G K M
(Hottest)
(Coolest)
Table 11.1
IV. Stellar sizes (radius)
Luminosity is proportional to surface area (how
large) x temperature (how hot): L= 4R2T4
If we can measure the Luminosity and the
temperature of a star we can tell how large its
raduis is.
IV. Stellar sizes (radius)
Luminosity is proportional to surface area x
temperature: L= 4R2T4
If we can measure the Luminosity and the
temperature of a star we can tell how large its
raduis is.
Summary of Ch 11 Part I
Distance: If you know the parallax “p” (in arcseconds) you can
calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26
lightyears)
Apparent brightness: how bright a star looks in the sky
The inverse-square Law: light from stars gets fainter as the
inverse square of the distance (brightness proportional to 1/d2).
If we know the apparent brightness and the distance to a star
we can calculate its absolute (intrinsic) brightness: apparent
brightness = (absolute brightness)/d2
Luminosity (energy/sec) is equivalent to absolute brightness
L= 4R2T4
If we can measure the luminosity and the temperature of a star
we can tell how large it is.
 Binary stars allow us to determine stellar masses
Summary of Ch 11 Part I
Distance: If you know the parallax “p” (in arcseconds) you can
calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26
lightyears)
Apparent brightness: how bright a star looks in the sky
The inverse-square Law: light from stars gets fainter as the
inverse square of the distance (brightness proportional to 1/d2).
If we know the apparent brightness and the distance to a star
we can calculate its absolute (intrinsic) brightness: apparent
brightness = (absolute brightness)/d2
Luminosity (energy/sec) is equivalent to absolute brightness
L= 4R2T4
If we can measure the luminosity and the temperature of a star
we can tell how large it is.
 Binary stars allow us to determine stellar masses
Binary Stars
•
•
Definition
Three main types of Binary Stars
•
•
•
•
Visual
Spectroscopic
Eclipsing
Stellar Masses and Densities
Binary Stars
•
Definition:
When two stars are in orbit around their center of mass
•
Three main types of Binary Stars
•
•
•
•
Visual: orbits
Spectroscopic: Review of Doppler effect, spectral lines,
double and single lines
Eclipsing: masses and radii of stars
Stellar Masses and Densities
Visual Binary
Visual
Binary
Doppler Effect

Radial Velocity

Approaching stars: more energy,

Receding stars: less energy,
Radial Velocity

Approaching stars: more
energy, spectral lines undergo a
blue shift

Receding stars: less energy,
spectral lines undergo a red
shift

/ = v/c
Spectroscopic Binary
Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
Eclipsing Binary
We can measure periodic eclipses
Eclipsing Binary: Masses and Radii
Radii of Stars
Stellar Masses
Stellar Densities
Low
High
Properties of Stars
Our Goals for Learning
• How far away are stars?
• How luminous are stars?
• How hot are stars?
• How massive are stars?
• How large (radius) are stars?
Outline of Ch 11 part 2: The H-R Diagram
I.
The Hertzprung-Russell (H-R) Diagram:

Surface Temperature

Luminosity

Analogy: horsepower vs weight

Where Stars plot is the H-R diagram

Main Sequence: 90% of all stars

Giants, Supergiants, White Dwarfs
Figure 11.5
Team Responsible for Stellar Classification in
Late 1800s and Early 1900s