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Transcript
Chapter 11
Surveying the Stars
How do we measure stellar
luminosities?
Luminosity:
Amount of power a star
radiates
(energy per second =
watts)
Apparent brightness:
Amount of starlight that
reaches Earth
(energy per second per
square meter)
Brightness of a star depends on both distance and luminosity
Thought Question
These two stars have about the same luminosity—
which one appears brighter?
A. Alpha Centauri
B. The Sun
Thought Question
These two stars have about the same luminosity—
which one appears brighter?
A. Alpha Centauri
B. The Sun
1
Luminosity passing
through each sphere
is the same
The relationship between apparent brightness
and luminosity depends on distance:
Brightness =
Luminosity
4π (distance)2
Area of sphere:
4π (radius)2
Divide luminosity by
area to get
brightness.
We can determine a star’s luminosity if we can
measure its distance and apparent brightness:
Luminosity = 4π (distance)2 × (Brightness)
Thought Question
Thought Question
How would the apparent brightness of Alpha
Centauri change if it were three times farther away?
How would the apparent brightness of Alpha
Centauri change if it were three times farther away?
A.
B.
C.
D.
It would be only 1/3 as bright.
It would be only 1/6 as bright.
It would be only 1/9 as bright.
It would be three times as bright.
A.
B.
C.
D.
It would be only 1/3 as bright.
It would be only 1/6 as bright.
It would be only 1/9 as bright.
It would be three times as bright.
Parallax
is the
apparent
shift in
position of a
nearby object
against a
background
of more
distant
objects.
So how far away are these stars?
Introduction to Parallax
2
The parallax
angle
depends on
distance.
Apparent
positions
of the
nearest
stars shift
by about
an
arcsecond
as Earth
orbits the
Sun.
Parallax of a Nearby Star
Parallax Angle as a Function of Distance
Parallax is
measured by
comparing
snapshots
taken at
different times
and measuring
the shift in
angle to star.
Measuring Parallax Angle
Parallax and Distance
Most luminous
stars:
106 LSun
p = parallax angle
1
d (in parsecs) =
p (in arcseconds)
Least luminous
stars:
10−4 LSun
d (in light-years) = 3.26 ×
1
p (in arcseconds)
(LSun is luminosity
of Sun)
3
The Magnitude Scale
m = apparent magnitude
How do we measure stellar
temperatures?
M = absolute magnitude
apparent brightness of Star 1
m −m
= (1001/5 ) 1 2
apparent brightness of Star 2
luminosity of Star 1
M −M
= (1001/5 ) 1 2
luminosity of Star 2
An object of
fixed size
grows more
luminous as
its
temperature
rises.
Every object emits thermal radiation with
a spectrum that depends on its temperature.
Properties of Thermal Radiation
1.  Hotter objects emit more light per unit area at all
frequencies.
2.  Hotter objects emit photons with a higher average
energy.
Relationship Between Temperature and Luminosity
Hottest stars:
50,000 K
Coolest stars:
3,000 K
(Sun’s surface
is 5,800 K)
4
106 K
105 K
104 K
Ionized
Gas
(Plasma)
103 K
Neutral Gas
102 K
Molecules
10 K
Solid
Level of ionization
also reveals a star’s
temperature.
Absorption lines in a star’s spectrum tell us its ionization level.
Remembering Spectral Types
(Hottest)
O B A F G K M
(Coolest)
•  Oh, Be A Foolish Ghoul, Kick Monsters
Lines in a star’s spectrum correspond to a spectral type
that reveals its temperature:
(Hottest)
O B A F G K M
(Coolest)
Thought Question
Which of the stars below is hottest?
A.
B.
C.
D.
M star
F star
A star
K star
Thought Question
Which of the stars below is hottest?
A.
B.
C.
D.
M star
F star
A star
K star
5
Pioneers of Stellar Classification
How do we measure stellar
masses?
•  Annie Jump
Cannon and the
“calculators” at
Harvard laid the
foundation of
modern stellar
classification.
Types of Binary Star Systems
•  Visual binary
•  Eclipsing binary
•  Spectroscopic binary
About half of all stars are in binary systems.
Orbit of a binary star system depends on strength of gravity
Visual Binary
Eclipsing Binary
We can directly observe the orbital motions of these stars.
We can measure periodic eclipses.
Exploring the Light Curve of an Eclipsing Binary Star System
6
Spectroscopic Binary
We measure mass using gravity.
Direct mass measurements are
possible only for stars in binary
star systems.
p2 =
4π2
G (M1 + M2)
p = period
a3
a = average separation
Isaac Newton
We determine the orbit by measuring Doppler shifts.
Most massive
stars:
Need two out of three
observables to measure mass:
1.  Orbital period (p)
2.  Orbital separation (a or r = radius)
3.  Orbital velocity (v)
For circular orbits, v = 2πr / p
100 MSun
Least massive
stars:
v
r
0.08 MSun
M
(MSun is the
mass of the
Sun.)
What is a Hertzsprung–Russell
diagram?
Luminosity
An H-R
diagram plots
the
luminosities
and
temperatures
of stars.
Temperature
7
Most stars fall
somewhere on
the main
sequence of
the H-R
diagram.
Generating an H-R Diagram
Large radius
Stars with lower
T and higher L
than main
-sequence stars
must have
larger radii:
Stellar Luminosity
•  How can two stars have the same temperature,
but vastly different luminosities?
•  The luminosity of a star depends on 2 things:
•  surface temperature
•  surface area (radius)
4
giants and
supergiants
Stars with
higher T and
lower L than
main-sequence
stars must have
smaller radii:
white dwarfs
Small radius
2
•  L = σ T 4 π R
•  The stars have different sizes!!
•  The largest stars are in the upper right corner of
the H-R Diagram.
A star’s full classification includes spectral
type (line identities) and luminosity class (line
shapes, related to the size of the star):
I
II
III
IV
V
— supergiant
— bright giant
— giant
— subgiant
— main sequence
Examples: Sun — G2 V
Sirius — A1 V
Proxima Centauri — M5.5 V
Betelgeuse — M2 I
8
H-R diagram
depicts:
C
B
Which
star is the
hottest?
Luminosity
Color
Spectral type
Luminosity
Luminosity
Temperature
Radius
D
A
Temperature
C
B
D
A
A
A
C
A
Temperature
C
B
Which star
is the most
luminous?
Luminosity
Luminosity
Temperature
C
D
Which star
is the most
luminous?
D
Temperature
B
C
B
Which
star is the
hottest?
Luminosity
Luminosity
Temperature
Which star is a
main-sequence
star?
D
A
Temperature
9
Luminosity
D
D
A
Temperature
Luminosity
Which star has
the largest
radius?
D
A
Temperature
C
B
C
B
Which star is a
main-sequence
star?
Luminosity
C
B
Which star has
the largest
radius?
What is the significance of the
main sequence?
C
D
A
Temperature
Main-sequence
stars are fusing
hydrogen into
helium in their
cores, like the
Sun.
High-mass stars
Luminous main
-sequence stars
are hot (blue).
Less luminous
ones are cooler
(yellow or red).
Mass
measurements of
main-sequence
stars show that
the hot, blue stars
are much more
massive than the
cool, red ones.
Low-mass stars
10
High-mass stars
The mass of a
normal,
hydrogen
-burning star
determines its
luminosity and
spectral type!
The core pressure
and temperature
of a higher-mass
star need to be
higher in order to
balance gravity.
A higher core
temperature
boosts the fusion
rate, leading to
greater
luminosity.
Low-mass stars
Hydrostatic Equilibrium
Stellar Properties Review
Luminosity: from brightness and distance
10−4 LSun–106 LSun
Temperature: from color and spectral type
3,000 K–50,000 K
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun–100 MSun
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Stellar Properties Review (Main Sequence)
Luminosity: from brightness and distance
(0.08 MSun) 10−4 LSun–106 LSun
(100 MSun)
Temperature: from color and spectral type
(0.08 MSun) 3,000 K–50,000 K
(100 MSun)
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun–100 MSun
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
11
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of a 10 MSun star:
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of a 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years × 10/104
10 million years ~ 10 billion years × 10/104
Life expectancy of a 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years × 0.1/0.01
Main-Sequence Star Summary
High-mass:
What are giants, supergiants, and
white dwarfs?
High luminosity
Short-lived
Large radius
Blue
Low-mass:
Low luminosity
Long-lived
Small radius
Red
Off the Main Sequence
•  Stellar properties depend on both mass and age:
those that have finished fusing H to He in their
cores are no longer on the main sequence.
•  All stars become larger and redder after
exhausting their core hydrogen: giants and
supergiants.
•  Most stars end up small and white after fusion has
ceased: white dwarfs.
Relationship between Main-Sequence Stellar Masses and
Location on H-R Diagram
12
A
Which star is
most like our
Sun?
Luminosity
D
B
C
Main-sequence stars (to scale)
Giants, supergiants, white dwarfs
Temperature
A
A
Which star is
most like our
Sun?
B
B
D
Luminosity
Luminosity
D
B
C
C
Temperature
Temperature
Luminosity
D
B
Which of these
stars will have
changed the
least 10 billion
years from
now?
C
A
D
Luminosity
A
Which of these
stars can be no
more than 10
million years
old?
B
C
Temperature
Which of these
stars will have
changed the
least 10 billion
years from
now?
C
Temperature
13
A
Luminosity
D
Which of these
stars can be no
more than 10
million years
old?
What are the two types of star
clusters?
A
B
C
Temperature
Open cluster: A few thousand loosely packed stars
How do we measure the age of a
star cluster?
Globular cluster: Up to a million or more stars in a dense ball bound
together by gravity
Massive
blue stars
die first,
followed
by white,
yellow,
orange,
and
red stars.
Visual Representation of a Star Cluster Evolving
14
Main-sequence
turnoff
Pleiades
now has no
stars with
life
expectancy
less than
around 100
million
years.
To determine
accurate ages,
we compare
models of stellar
evolution to the
cluster data.
The main
-sequence
turnoff point
of a cluster
tells us its
age.
Detailed
modeling of
the oldest
globular
clusters
reveals that
they are
about 13
billion years
old.
Using the H-R Diagram to Determine the Age of a Star Cluster
15