Study Guide 3
Astronomy 105
Study Guide - Exam III
I. The Quiet Sun
A. Core - energy generation region
1. Hydrogen converted to helium
2. Electromagnetic repulsion of protons overcome by
large velocities (temperature) so that strong force
binds them together
B. Radiative Zone - steady, uniform transfer of energy
C. Convective Zone - hot "blobs" of gases rise to transfer
energy
D. Photosphere - visible surface
1. Sharp limb tells us that we can only see 700 km into
photosphere
a. density falls dramatically in photosphere
2. Limb darkening tells us that temperature falls
rapidly in photosphere
3. Granulation is evidence of convection from below
E. Chromosphere
1. Temperature reversal (temp rises with height)
a. Heated by dumping of energy by shock waves
2. Region ends when H lines no longer observable
F. Corona
1. Temperature very high
a. lines of highly ionized iron observed
b. corona is a source of x-rays
II. The Active Sun
A. Sunspots
1. Darker (cooler) than photosphere
2. Structure
a. umbra, penumbra, no granulation
3. Strong magnetic fields
a. measured by splitting of spectral lines
(Zeeman Effect)
b. pairs have opposite poles
c. opposite polarity on either side of equator
d. complete reversal between cycles
4. Usually occur in pairs
5. Maunder
a. 11-year cycle
b. Butterfly diagram
B. Babcock theory
1. Requires general magnetic field and differential
rotation
2. Wrapping of field lines will amplify magnetic field
3. Convective zone shock waves cause field to rise to
surface, producing spots
C. Sunspots related activity
1. Plages
2. Filaments, Prominences
3. Flares
D. Solar-Terrestrial Relationships
1. Light/Heat
2. Solar Wind and flares cause aurorae
3. Solar activity may affect climate
a. periodic droughts in SW USA
b. "Little Ice Age"
III. Stellar Evolution - general principles
A. Compressed gas (due to gravity) will heat up
1. Object will brighten and emit shorter wavelengths
B. Hydrostatic Equilibrium
1. Gravity competes with the force of radiation
pressure
2. These must balance for star to be stable
C. Conservation of Angular Momentum
1. Contraction of Pre-Solar Nebula causes increased
rotation rate
2. Increased rotation causes flattening at the poles
IV. Pre-Main Sequence Evolution
A. Contraction begins
1. Density wave
2. Shock wave from a supernova
B. Object spins more rapidly, heats up, and flattens
C. As protostar heats up dust is expelled into a shell
D. Strong "T Tauri winds" push back the lighter elements
away from the forming object
E. Hydrogen ignition in core quickly brings star onto Main
Sequence
1. Dust shell vaporizes
2. Planetary formation ceases
F. Mass determines the time for pre-Main Sequence phase
1. More massive stars collapse faster
G. T Tauri stars are identified with pre-Main Sequence
objects
1. Strong IR sources
2. Found in or near nebulae
3. Dust shells observable
V. Main Sequence Evolution
A. Hydrogen "burning" starts at the center and follows these
steps
1. Hydrogen depleted in a particular zone of the core
2. This core zone can no longer support its mass and
collapses
a. the collapse heats up this zone
b. excess energy from collapse causes envelope to
expand slightly and ignites hydrogen in the next
zone of the core
c. hydrogen "burning" again stabilizes the core
3. Observable effect is that the star brightens slightly
during its Main Sequence lifetime since it is growing
slightly bigger
VI. Post Main Sequence Evolution for a Low Mass Star(<2.5 Suns)
A. Hydrogen depleted in the last available core zone
1. Core collapses again, but cannot be halted because
there is no core H.
a. core in uncontrollable gravitational collapse
b. as core collapses it heats up
c. the excess energy expands the envelope to the
red giant stage
d. as the envelope expands, it cools
e. star brightens (due to its large size), but
reddens (due to its cooling)
B. When temperature in the core reaches 100 million K,
He is ignited in the core (triple-alpha process)
1. By this time, however, core has become degenerate
a. He fusion takes place all over the core at once
(Helium flash)
b. Excess energy produced breaks degeneracy
c. Normal He "burning" proceeds
2. Star may be identified as an RR Lyrae variable
C. He is exhausted in the core, and second red giant phase
begins
1. At almost the same time, the outer 20% of the star's
mass is lifted away in a planetary nebula
2. Core collapses again, but no nuclear process can
halt it (mass is less than 1.4 solar masses).
D. Core will collapse until electron degeneracy halts it
VII. Intermediate Mass Star (2.5 - 10 Suns)
A. All the steps in VI. the same except core does not become
degenerate at He fusion and there is no He flash.
B. The most massive in this class (8 - 10 Suns) may have some
carbon fusion before the planetary nebula ends their life.
VIII. Post Main Sequence Evolution for a High Mass Star (> 10 Suns)
A. Same steps take place as in VI A above.
B. Temperatures reach 100,000,000 K before the core becomes
degenerate.
1. He "burning" proceeds normally.
C. After He is exhausted in the core, the core still has a
mass greater than 1.4 solar masses.
1. Heavier nuclear fuels can be processed.
2. Each new fuel requires higher temperature and
therefore takes less time.
Evolutionary Time Scales for a 15 Solar Mass Star
Fused Products Nuclear Reaction Time(yrs) Temperature(K)
-----------------------------------------------------------------
H He Hydrogen Burning 10 million yrs 4 million
He C Helium Burning few million 100 million
C O, Ne,
Mg,He Carbon Burning one thousand 800 million
Ne + O, Mg Neon Burning A few yrs 1 billion
O Si, S Oxygen Burning One year 2 billion
Si + Fe Silicon Burning days 3 billion
Fe Neutrons Core Collapse < 1 second >3 billion
-----------------------------------------------------------------
D. The core collapse in the final stage causes a supernova.
1. Fe fusion requires more energy than the reaction
produces
VIII.Stellar Remnants
A. Forces responsible for halting core collapse
1. White Dwarf - From Main Sequence Stars < 10 solar
masses.
a. electron degeneracy (electromagnetic force)
2. Neutron Stars - From Main Sequence stars between 10
and 40 solar masses.
a. neutron degeneracy (strong nuclear force)
3. Black Hole - (From Main Sequence stars > 40 solar
masses)
a. no known force can halt collapse
IX. White dwarfs
A. Should have less than 1.4 solar masses.
B. Radius about the same as earth.
C. Temperatures 35,000 - 50,000 K
D. Quite faint because of their small size.
E. Discovered in great abundance
X. Neutron Stars
A. Masses 1-2 solar masses
B. Radius of typical city
C. Rapidly rotating
D. Powerful magnetic field
E. Pulsars
1. Sources of very precise pulses - mostly radio sources
2. Light Time argument
a. can only be white dwarfs or neutron stars
3. Pulse mechanism
a. stellar oscillations
white dwarfs too slow
neutron stars too fast
b. binary star orbits
white dwarfs too large
neutron star would emit measurable gravity waves
that have not been detected
c. rotation
white dwarfs ruled out after Crab Pulsar found
neutron stars work
4. pulsars all slow down - allows us to date the pulsar
a. energy lost in slow down of Crab Pulsar makes
nebula glow
XI. Black Holes
A. Measurable properties
1. Mass
2. Rotation
3. Charge
B. Event Horizon
1. Surface of object when escape velocity equalled the
speed of light
2. Radius (in kilometers = 3 X Mass (in solar masses)
C. Most probable BH would be rotating BH (Kerr solution)
D. Can be detected by X-rays given off as matter falls into
event horizon
E. Predicted types
1. Stellar Size (masses at least 2-3 solar masses)
a. end fate of massive stellar evolution
2. Supermassive BH
a. may have been detected in the cores of galaxies
b. hundreds of millions of solar masses
XII. Quasars
A. Observable properties
1. Almost point-like
2. Extreme redshift
a. Hubble's Law implies extreme distance
3. Short period variations
a. Light Time argument limits size to about size
of solar system
B. Cosmological Hypothesis
1. Assumes redshift is due to the general expansion of
the universe, Hubble's Law works, and QSO's must be
extremely distant
2. Problem - must produce light of 1000 Milky Way
galaxies in the volume of our solar system
a. possible mechanism is accretion disk about a
supermassive BH
C. Local Hypothesis
1. Solves energy requirements by making QSO's nearby
2. Problem - what causes the redshift?
a. ejection - why no blueshift?
b. gravitational - requires exotic BH's that the
hypothesis was designed to avoid
D. Related objects?
1. Seyfert galaxies - active galactic cores
2. BL Lacertae objects - very active galactic cores
3. QSO's are probably the very young, extremely active
cores of galaxies
E. New evidence
1. Gravitational lens
2. No low redshift QSO's - they are all far away
XIII. Cosmology
A. Hubble's Law
1. General expansion of the universe
2. Both Big Bang and Steady State Theories could be
used
B. Three degree cosmic background radiation
1. The remnant of the Big Bang radiation cooled by
the expansion
2. Steady State theory abandoned after this discovery