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The Stars
HNRT 227 Chapter 14
22 October 2015
Great Idea:
The Sun and other stars use nuclear fusion
reactions to convert mass into energy. Eventually,
when a star’s nuclear fuel is depleted, the star must
burn out.
1
Chapter Outline
•
•
•
•
The Nature of Stars
The Anatomy of Stars
The Variety of Stars
The Life Cycles of Stars
2
The Nature of Stars
3
The Nature of Stars
• Astronomy
– Oldest science (?)
• Star
– Ball of gas
– Fusion reactor
• All stars have a
beginning and an ending
4
Measuring the Stars with
Telescopes and Satellites
• Electromagnetic radiation
• Measurement of photons
–
–
–
–
Wavelength
Intensity
Direction
Variation
5
Telescopes
6
Orbiting Observatories
• Great Observatories Program
– Hubble Space Telescope
– Spitzer Infrared Telescope
– Chandra X-Ray Observatory
7
The Anatomy of Stars
8
The Structure of the Sun
• Structure
–
–
–
–
–
–
Stellar core
Radiative zone
Convection zone
Photosphere
Chromosphere
Corona
• Solar Wind
– Stream of
particles
9
Our Sun
10
More On Solar Structure
• Hydrogen fusion takes place
in a core extending from
the Sun’s center to about
0.25 solar radius
• The core is surrounded by a
radiative zone extending to
about 0.71 solar radius
– In this zone, energy travels
outward through radiative
diffusion
• The radiative zone is
surrounded by a rather
opaque convective zone of
gas at relatively low
temperature and pressure
– In this zone, energy travels
outward primarily through
convection
11
How do we know the Sun’s interior?
• Helioseismology is
the study of how the
Sun vibrates
• These vibrations have
been used to infer
pressures, densities,
chemical
compositions, and
rotation rates within
the Sun
12
Magnetic Fields
13
Northern Lights (Aurora Borealis)
14
The Sun’s Energy
Source: Fusion
•
•
•
Sun’s Energy Source
–
–
Historical
Current
•
hydrogen
Fusion
–
3-steps-hydrogen burning
1) P + P  D + e+ + neutrino + energy
2) D + P  3He + photon + energy
3) 3He + 3He 4He + 2protons + photon + energy
Life expectancy
–
11-12 billion years
15
The Variety of Stars
• Differences
– Color
– Brightness
• Distance
• Absolute
brightness
– Energy output
– luminosity
• Apparent
brightness
• Behavior
– Total mass
– age
16
The Cosmic
Distance Ladder
• Distance
– Light-years
• Measurement
– Triangulation
(parallax)
– Spectroscopic
parallax
– Cepheid variable
– Tully-Fisher
– Supernovae Type Ia
– Hubble’s Law
17
The Hertzsprung-Russel
Diagram
• Star Groupings
– Main-sequence stars
– Red giants
– White dwarfs
18
Step 1 to an H-R Diagram
20
15
10
Absolute Magnitude
• Plot for
the 20
nearest
and
brightest
stars to
Earth
Not an H-R Diagram
5
0
0
5000
10000
15000
20000
25000
30000
-5
-10
Temperature
19
Step 2 to an H-R Diagram
• Reversing
the y-axis
Getting Closer to an H-R Diagram
Temperature (K)
-10
-5
Absolute Magnitude
0
5
10
15
20
0
5000
10000
15000
20000
25000
2030000
Step 3 to an H-R Diagram
• Reversing the
x-axis
– Lowest
temperature
to the right
Now it's looking Like an H-R Diagram
Temperature (K)
-10
-5
5
10
15
20
30000
25000
20000
15000
10000
5000
0
21
Absolute Magnitude
0
A Standard H-R Diagram
22
The Life Cycles of Stars
23
The Birth of Stars
• Nebular Hypothesis
– Laplace
24
The Main Sequence and
the Death of Stars
• Stars much less massive
than the Sun
– Glows 100 billion years
• No change in size,
temperature, energy
output
– Brown dwarfs
• Some don’t consider these
stars
25
The Main Sequence and
the Death of Stars
• Stars about the
mass of the sun
– Hydrogen burning at
faster rate
–
–
–
–
• Move off main
sequence
Helium burning
Red giant
Begin collapse
White dwarf
26
The Life Cycle of a Star Like the
Sun
•
•
•
•
•
•
•
•
Gas cloud
Fragmentation
Protostar
Kelvin-Helmholz
contraction
Hayashi Track
Ignition
Adjustment to Main
Sequence
Hydrogen Core
Depletion
•
•
•
•
•
•
•
•
Hydrogen shell burning
Helium flash
Helium core burning
Helium core depletion
Helium shell burning
Helium shell flashes
Planetary nebula
White Dwarf
27
Sun’s Life Cycle on H-R Diagram
H-R Diagram with annotations for Stellar Stages
Temperature (log)
100000
10000
1000
-10
Planetary Nebula
Asymptotic
Branch
Horizontal
Branch
Helium
Flash
-5
Kelvin-Helmholtz
Contraction
Adjust to
Main
Sequence
5
Absolute Magnitude
0
Sub
Giant Hayashi
Core
Track
Contraction
10
White
Dwarf
15
28
20
The Main Sequence and
the Death of Stars
• Very Large Stars
– Successive collapses
and burnings
– Iron core
– Catastrophic
collapse
• supernova
29
Layers of Massive Star
H -> He
Layers
of
He -> C
C -> O
O ->
Ne
Ne
-> Mg
Nuclear
Fusion
Mg ->
in
High
Mass
Stars
Si
Si -> Fe
Fe
30
Neutron Stars and Pulsars
• Neutron Star
– Dense and small
– High rotation rate
– Little light
• Pulsar
– Special neutron star
– Electromagnetic radiation
– End state of supernova
31
Black Holes
• Black Hole
– Result of collapsed
large star
– Nothing escapes
from surface
– Cannot “see” them
• See impact on
other stars, dust,
etc.
• Detect x-rays,
gamma rays
32