1 Announcements There will be a star map on the exam. I will not tell you in advance what month....
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Announcements• There will be a star map on the exam. I will
not tell you in advance what month.
• Grades are not yet posted, sorry. They will be posted by exam time on Wednesday.
• Grades including the 3rd midterm will be posted by Monday 5/3.
• Final is optional. I will announce the room Wednesday at exam time.
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Life History of a Star
Loss of Energy to SpaceGravitational Contraction of CoreContraction is halted temporarily
by nuclear fusionEnergy generation in core
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HOT COOL
BRIGHT
FAINT
HRDiagram
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Star Birth
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Small Mass Stars
become RED
GIANTS
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Large Mass Stars
becomeRED
GIANTS
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Small mass stars can not get hot enough to fuse Carbon
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White Dwarfs & Neutron Stars
• White Dwarfs– Supported by pressure of degenerate electrons– About the size of the Earth– Mass < 1.4 Msun
• Neutron Stars– Supported by pressure of degenerate neutrons– About the size of Lansing– Mass < 3 Msun
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Degenerate Pressure
• Pressure due to motion caused by squeezing particles very close togetherDepends only on density, not on temperature
• Uncertainty Principle location x speed ~ h/mass Means uncertainty in
h is a small constant number
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Formation of Black Holes
If the collapsing core of a massive star which produces the supernova explosion has more mass than the pressure of degenerate neutrons can support (> 3 Msun)
Nothing can stop its collapseThe escape velocity reaches the speed of lightNothing can go faster than the speed of lightBlack Hole
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Surface of a Black Hole
• Surface where escape velocity = speed of light is surface of a Black Hole, called Event Horizon
• Outside Event Horizon can escape,inside can not
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If nothing can escape from a BH, How do we know its there?
If gas falls into a BHBH gravity makes it speed upConservation of Angular Momentum
makes it form an Accretion Disk, orbiting at nearly the speed of light
Friction makes it very hotEmits X-Rays
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What can we know about Black Holes?• Nothing can escape from inside an Event
Horizon• Long range forces can exert influence
outside Event Horizon:1. Gravity2. Electric Force
• Can know:1. Mass2. Charge3. Spin
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HR Diagram for brightest
northern hemisphere
stars
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Test:Cluster
HR Diagrams
Same DistanceSame Age
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HR Diagram
for clusters
of different
ages
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Gas - Star - Gas Cycle
Interstellar Gas
H, He, C, N, O, Fe, etc.
StarSN orWind
Fusion of He, C, N, O& heavier atoms
Gas cloud contracts
Return ofgas enrichedin heavy atoms
SN fuse atoms > iron
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Halo Stars: 0.02-0.2% heavy elements (O, Fe, …) only old stars
Disk Stars: 2% heavy elements stars of all ages
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MilkyWay
Cartoon
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Density Excess?
Higher density proto-galactic clouds form stars more rapidly, use up all their gas before it can form a disk.
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Rotation?
Larger rotation produces more disk-like distribution of matter.
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Galaxies are close together
Mergers may make Ellipticals
Burst ofstarformation
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(hundreds to thousands of galaxies)
1. Denser cloud2. More collisions
Elliptical galaxies are much more common in
huge clusters of galaxies
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Distance & Age
Universe opaque
space
time
light
Here & Now
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Fig 22.18
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Relic Radiation
• Universe was once hot and opaque, see radiation from that time (as from surface of a star)– Comes at us from all directions
(inside fog bank)– Has a thermal spectrum– Cold now, expansion cools
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Spectrum is Thermal, T=2.7 K
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CMB Radiation
Radiation is nearly the same from all directions,
Doppler Shift due to motion of the Milky WayT/T ~ 10-3
After subtracting emission from MW, seePrimoridial fluctuations from when universe became transparent,T/T ~ 10-5
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Relic Elements
TheoryObservations
Universe is 75% H 25% He
Deuterium abundanceconstrains density ofordinary matter
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What do we know about Dark Energy?
• Constitutes 2/3 of energy in universe
• Is smoothly distributed and invisible
Doesn’t clump into galaxies likeMatter, visible or dark
• Has negative pressure
produces Acceleration
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Problems with the Big Bang Model
1. How can two pieces on opposites sides of the universe have the same temperature at the time the universe became transparent?
They are too far apart to have communicated with each other within the age of the universe, since light from them has just now reached us half way between.
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Problems with the Big Bang Model
2. Why is the space-time geometry of the universe so nearly flat, equivalent to the sum of the Ordinary Matter, Dark Matter and Dark Energy = Critical Density?
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Inflation
At very beginning of Big Bang, the Universe underwent a tremendous expansion (inflation).
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Inflation
Before Inflation the two parts of the universe were close enough together to communicate with each other
Fig 23.14
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Inflation
• Expansion smoothes out fluctuations and makes things appear flatter (e.g. blowing up a balloon).