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Measuring the Stars
How big are stars?
How far away are they?
How bright are they?
How hot?
How old, and how long do they live?
What is their chemical composition?
How are they moving?
Are they isolated or in clusters?
By answering these questions, we not only learn about stars, but
about the structure and evolution of galaxies they live in, and the
Universe.
Energy
mosquito lands on your arm = 1 erg
1 stick of dynamite = 2 x 1013 ergs
1 ton of TNT = 4 x 1016 ergs
1 atomic bomb = 1 x 1021 ergs
Magnitude 8 earthquake = 1 x 1026 ergs
Earth’s daily solar input = 1 x 1029 ergs
Planet cracker = 1 x 1032 ergs
Luminosity of the sun = 4 x 1033 ergs/sec
Review Burners and Stars
Size, Temperature, and Luminosity
Building the Hertzsprung-Russell (H-R) Diagram
Which is larger, X or Y? => A.) X B.) Y C.) same size D.) can’t tell
Building the Hertzsprung-Russell (H-R) Diagram
Which is larger, Y or T? => A.) Y B.) T C.) same size D.) can’t tell
The Hertzsprung–Russell Diagram
Once many stars are plotted on an H–R diagram, a
pattern begins to form:
These are the 80 closest stars to
us; note the dashed lines of
constant radius.
The darkened curve is called
the main sequence, as this is
where most stars are.
Also indicated is the white
dwarf region; these stars are
hot but not very luminous, as
they are quite small.
The Hertzsprung–Russell Diagram
An H–R diagram of the 100 brightest stars looks quite
different.
These stars are all more
luminous than the Sun. Two
new categories appear here –
the red giants and the blue
giants.
Clearly, the brightest stars in
the sky appear bright
because of their enormous
luminosities, not their
proximity.
The Hertzsprung-Russell (H-R) Diagram
Red Supergiants
Red Giants
Increasing Mass,
Radius on Main
Sequence
Sun
Main Sequence
White Dwarfs
The Hertzsprung–Russell Diagram
This is an H–R plot of
about 20,000 stars. The
main sequence is clear,
as is the red giant
region.
About 90 percent of
stars lie on the main
sequence; 9 percent are
red giants and 1 percent
are white dwarfs.
Extending the Cosmic Distance Scale
Spectroscopic parallax: Has nothing to do with
parallax, but does use spectroscopy in finding
the distance to a star.
1. Measure the star’s apparent magnitude and
spectral class.
2. Use spectral class to estimate luminosity.
3. Apply inverse-square law to find distance.
Extending the Cosmic Distance Scale
Spectroscopic parallax can extend the cosmic distance
scale to several thousand parsecs.
Extending the Cosmic Distance Scale
The spectroscopic parallax calculation can be misleading
if the star is not on the main sequence.
The width of spectral lines can be used to define
luminosity classes.
Star Classifications
Super Giant
=>
From 100 to 1000 times larger than the Sun
Giant
=>
From 10 to 100 times larger than the sun
Dwarf
=>
any star of size comparable
or smaller than the Sun
White Dwarf => about the size of the Earth
Figure 10.13: H-R diagram for nearest stars
Stadium Analogy
© 2013 Pearson Education, Inc.
Fig. 10.14: H-R diagram for brightest stars
Building the Hertzsprung-Russell (H-R) Diagram
Which stars have the same temperature? How can you tell?
Are stars of the same temperature always of the same type?
Building the Hertzsprung-Russell (H-R) Diagram
Which star(s) on this diagram (A – G) match this description?
This star is very bright (high luminosity) and very hot (high temperature)
Building the Hertzsprung-Russell (H-R) Diagram
Which star(s) on this diagram (A – G) match this description?
This star is very dim and very cool
Building the Hertzsprung-Russell (H-R) Diagram
Which star(s) on this diagram (A – G) match this description?
This star is very dim and very hot
Building the Hertzsprung-Russell (H-R) Diagram
Which star(s) on this diagram (A – G) match this description?
This star is very bright and very cool
Stellar Masses
Many stars are in binary
pairs; measurement of their
orbital motion allows
determination of the masses of
the stars. Orbits of visual
binaries can be observed
directly; Doppler shifts in
spectroscopic binaries allow
measurement of motion; and
the period of eclipsing binaries
can be measured using
intensity variations.
Stellar Masses
Mass is the main
determinant of
where a star will be
on the main
sequence.
How does a star's Luminosity depend on its Mass?
L  M 3
(Main Sequence stars only!)
Stellar Masses
Stellar mass distributions –
there are many more small
stars than large ones!
How Long do Stars Live
(as Main Sequence Stars)?
A star on Main Sequence has fusion of H to He in its core. How
fast depends on mass of H available and rate of fusion. Mass of H
in core depends on mass of star. Fusion rate is related to
luminosity (fusion reactions make the radiation energy).
So,
lifetime
α
Because luminosity 
lifetime 
mass of core
fusion rate

mass of star
luminosity
(mass) 3,
mass
or
3
(mass)
1
(mass) 2
So if the Sun's lifetime is 10 billion years, a 30 MSun star's lifetime is only
10 million years. Such massive stars live only "briefly".
Clicker Question:
The HR diagram is a plot of stellar
A: mass vs diameter.
B: luminosity vs temperature
C: mass vs luminosity
D: temperature vs diameter
Clicker Question:
What would be the lifetime of a star one
tenth as massive as our sun?
A: 1 billion years = 109 years
B: 10 billion years = 1010 years
C: 100 billion years = 1011 years
D: 1 trillion years = 1012 years
Summary of Chapter 10
• Distance to nearest stars can be measured by
parallax.
• Apparent brightness is as observed from Earth;
depends on distance and absolute luminosity.
• Spectral classes correspond to different surface
temperatures.
• Stellar size is related to luminosity and temperature.
Summary of Chapter 10, cont.
• H–R diagram is plot of luminosity vs. temperature;
most stars lie on main sequence.
• Distance ladder can be extended using spectroscopic
parallax.
• Masses of stars in binary systems can be measured.
• Mass determines where star lies on main sequence.
The Interstellar Medium (ISM) of the Milky Way Galaxy
Or: The Stuff (gas and dust) Between the Stars
Why study it?
Stars form out of it.
Stars end their lives by returning gas to it.
The ISM has:
a wide range of structures
a wide range of densities (10-3 - 107 atoms / cm3)
a wide range of temperatures (10 K - 107 K)
Compare density of ISM with Sun or planets:
Sun and Planets: 1-5 g / cm3
ISM average:
1 atom / cm3
Mass of one H atom is 10-24 g!
So ISM is about 1024 times as tenuous as a star or planet!
ISM consists of gas (mostly H, He) and dust. 98% of mass is in gas, but
dust, only 2%, is also observable.
Effects of dust on light:
1) "Extinction"
Blocks out light
2) "Reddening"
Blocks out short wavelength light better than
long wavelength light => makes objects appear redder.
Grain sizes typically 10-5 cm. Composition uncertain,
but probably silicates, graphite and iron.
Gas Structures in the ISM
Emission Nebulae or H II Regions
Regions of gas and dust near stars just formed.
The Hydrogen is essentially fully ionized.
Temperatures near 10,000 K
Sizes about 1-20 pc.
Hot tenuous gas => emission lines
(Kirchhoff's Laws)
Rosette Nebula
Lagoon Nebula
Tarantula Nebula
Red color comes from one
emission line of H (tiny
fraction of H is atoms, not
ionized).
Clicker Question:
What does does ionized Helium, He II,
contain?
A: He nucleus only
B: He nucleus and one electron
C: He nucleus and two electrons
D: He nucleus and three electrons
Why red? From one bright emission line of H. But that
requires H atoms, and isn't all the H ionized? Not quite.
Sea of protons and electrons
Once in a while, a proton and electron will rejoin to form H atom.
Can rejoin to any energy level. Then electron moves to lower levels.
Emits photon when it moves downwards.
One transition produces red photon. This
dominates emission from nebula.
Why is the gas ionized?
Remember, takes energetic UV photons to ionize H. Hot, massive
stars produce huge amounts of these.
Such short-lived stars spend all their lives in the stellar nursery of their
birth, so emission nebulae mark sites of ongoing star formation.
Many stars of lower mass are forming too, but make few UV photons.
Why "H II Region?
H I: Hydrogen atom
H II: Ionized Hydrogen
...
O III: Oxygen missing two electrons
etc.