susan cartwrightour evolving universe1 understanding stars n what do we know? n from observations of...
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Our Evolving Universe 1Susan Cartwright
Understanding Stars
What do we know? From observations of nearby stars:
luminosity/absolute magnitude colour/spectral class/ surface temperature chemical composition
From observations of binary systems: mass
How do we use these to develop understanding? look at relations between quantities (luminosity &
temperature, mass & luminosity) compare with expectations from theoretical models
Our Evolving Universe 2Susan Cartwright
Luminosity & temperature: the Hertzsprung-Russell diagram Plot absolute magnitude
(or log L) against spectral class (or colour, or log T)
the Hertzsprung-Russell diagram
for Ejnar Hertzsprung and Henry Norris Russell
key to understanding stellar evolution
The Nearest and Brightest Stars
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Colour Index
Absolu
te V
isual M
agnit
ude
The Nearest and Brightest Stars
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Colour Index
Absolu
te V
isual M
agnit
ude
Our Evolving Universe 3Susan Cartwright
Structure of the HR Diagram
main sequence
red giant branch
white dwarfs
Data from the Hipparcos parallax measuring satellite
Most stars lie on the main sequence
Note: faint stars are undercounted
bright
faint
hot co
ol
Our Evolving Universe 4Susan Cartwright
The sizes of stars
We know cooler objects are fainter (for given size)
But red giant branch contains cool objects which are very bright
Therefore these stars must be much larger than the faint, cool main-sequence stars
Blackbody radiation
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10
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1,000
10,000
100,000
1,000,000
10,000,000
0 200 400 600 800 1000 1200 1400
wavelength (nm)
flu
x (
W/s
q m
/nm
)3600
4600
5800
7000
10000
Our Evolving Universe 5Susan Cartwright
Giants and dwarfs
Red giants are up to 60 times larger than the Sun (~orbit of Mercury)
Supergiants (e.g. Betelgeuse) are larger still
Red main-sequence stars (“red dwarfs”) are only 1/3 as large as the Sun
White dwarf stars are roughly Earth-sized
Theoretical HR diagram
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33.544.55
log T (K)
log
(L/L
su
n)
Our Evolving Universe 6Susan Cartwright
Mass and luminosity
Luminosity of main sequence stars is determined by mass
Main sequence is a mass sequence(bright hot stars are more massive,cool faint stars are less massive)
Mass-luminosity relation
y = 2.2735x - 0.567
y = 3.8971x + 0.0781
y = 3.4557x + 0.4708
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log(mass)
log(l
um
inosit
y)
Mass-luminosity relation
y = 2.2735x - 0.567
y = 3.8971x + 0.0781
y = 3.4557x + 0.4708
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log(mass)
log(l
um
inosit
y)
HR Diagram
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15-0.5 0 0.5 1 1.5 2
B - V
Ab
so
lute
Vis
ua
l M
ag
nit
ud
e
HR Diagram
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15-0.5 0 0.5 1 1.5 2
B - V
Ab
so
lute
Vis
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l M
ag
nit
ud
e
10 times as massive10000 times brighter!
1/10 as massive1000 times fainter!
Our Evolving Universe 7Susan Cartwright
The sizes of stars
Why are more massive stars larger and hotter?
force of gravity pulls stellar material downwards
steadily increasing pressure pushes upwards
larger mass needs higher pressures
higher temperatures (increasing downwards)
central energy source
pressure
gravity
Our Evolving Universe 8Susan Cartwright
What is the energy source?
Chemical reactions? no: not powerful enough
Gravity? suppose the Sun is still
contracting… …would work for “only” 20
million years (remember Earth is 4.5 billion years old)
Nuclear power? YES: stars are fusion
reactors
Nuclear stability
0.998
0.999
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1.001
1.002
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0 50 100 150 200
Number of nucleons
Mass p
er
nucle
on
helium 4
hydrogen 1
iron 56
Nuclear stability
0.998
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1
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1.005
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0 50 100 150 200
Number of nucleons
Mass p
er
nucle
on
helium 4
hydrogen 1
iron 56
E=mc2
Our Evolving Universe 9Susan Cartwright
Evidence for nuclear fusion as stellar power source Theory: it works!
Sun can be powered for 5 billion years by converting 5% of its hydrogen to helium
Theory: it produces the elements we observe
discussed later Experiment: we see solar
neutrinos elementary particles
created as by-product of fusion Experiment: correct solar structure
as measured by helioseismology
Our Evolving Universe 10Susan Cartwright
Stellar lifetimes
A star 10 times as massive as the Sun has 10 times more hydrogen to power nuclear fusion
But it is 10000 times as bright Therefore it should use up its fuel 1000 times more
quickly
Massive stars are very short-lived
Can we test this? cannot watch stars age need sample of stars of different masses but the same age
stellar clusters groups of stars bound together by gravity formed together of the same material ideal test bed for models of stellar evolution
Our Evolving Universe 11Susan Cartwright
Stellar clusters
M67
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B-V
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Pleiades
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B-V
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NOAO/AURA
Pleiades: bright blue main sequence stars; no red giants
M67: many red giants; no bright blue main sequence stars
Our Evolving Universe 12Susan Cartwright
Globular clusters
Much larger than open clusters like the Pleiades
All have no upper main sequence stars
All have spectra showing low or very low heavy element content
The oldestclusters in ourGalaxy (and someof the oldestobjects)
M3
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22-0.3 0 0.3 0.6 0.9 1.2
B-V
V
Our Evolving Universe 13Susan Cartwright
What have we learned?
If we plot luminosity against temperature stars fall into well defined bands most stars are on the main sequence
If we plot luminosity against mass main sequence stars fall on one line more massive stars are brighter, bluer and have shorter
lives
If we look at the Sun stars must be powered by
nuclear fusion provides enough power produces neutrinos
If we look at stellar clusters
clusters with short-lived bright blue stars have few red giants
clusters with many red giants have no short-lived blue main sequence stars
clues to stellar evolution…next lecture