Download Astronomy 100 Tuesday, Thursday 2:30

Astronomy 100
Tuesday, Thursday 2:30 - 3:45 pm
Tom Burbine
[email protected]
www.xanga.com/astronomy100
OWL assignment (Due Today)
• There is be an OWL assignment due on Tuesday
April 5 at 11:59 pm.
• There are 15 questions and a perfect score will
give you 2 homework points.
Exam
• 40 Questions
• Chapters 15, 16, 17, and 18
• Some of the questions are taken straight from
OWL questions
Other Room
• April 7th Goessmann 0020
2:30-3:45 PM
Last Names beginning with H, I, J, and K
Things you should know
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Layers of the Sun
Hydrogen Fusion
Hertzsprung-Russell Diagram
Stellar Classifications
Life Cycle of the Sun
Helium Fusion
CNO cycle
What happens to stars as they “die”
Review Session
• Wednesday-Review Session
– Hasbrouck 134 from 7-8 pm
– I will be there at 6 pm if you want to talk to me in a
much smaller group
PRS Questions
PRS question #1
• What is approximately the temperature of the
plasma in a sunspot?
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A) 2,000 K
B) 4,000 K
C) 6,000 K
D) 8,000 K
E) 10,000 K
PRS question #1
• What is approximately the temperature of the
plasma in a sunspot?
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A) 2,000 K
B) 4,000 K
C) 6,000 K
D) 8,000 K
E) 10,000 K
PRS Question #2
• Which of these spectral types have the strongest
hydrogen emission lines in their spectra?
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A) O
B) B
C) A
D) F
E) G
PRS Question #2
• Which of these spectral types have the strongest
hydrogen emission lines in their spectra?
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A) O
B) B
C) A
D) F
E) G
“Deaths” of Stars
• White Dwarfs
• Neutron Stars
• Black Holes
White Dwarfs
• White Dwarfs is the core left over when a star can
no longer undergo fusion
• Very dense
– Some have densities of 3 million grams per cubic
centimeter
– A teaspoon of a white dwarf would weigh as much as
an elephant
White Dwarfs
• Some white dwarfs have the same mass as the
Sun but slightly bigger than the Earth
• 200,000 times as dense as the earth
White Dwarfs
• Collapsing due to gravity
• The collapse is stopped by electron degeneracy
pressure
Electron Degeneracy Pressure
• No two electrons can occupy the same quantum
state
Electron Degeneracy Pressure
• As electrons are moved closer together
• Their momentum (velocity) increases
• Due to Heisenberg Uncertainty Principle
So What Does This Mean
• Electron Degeneracy Pressure balances the
gravitational force due to gravity in white dwarfs
One Interesting Thing
• More massive white dwarfs are smaller
White Dwarf Limit
• The mass of a White Dwarf can not exceed
approximately 1.4 Solar Masses
• Called the Chandrasekhar Limit
• Electrons would have velocities greater than the
speed of light
The Sun
• Will end up as a White Dwarf
Neutron Star
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Neutron stars are usually 10 kilometers acroos
But more massive than the Sun
Made almost entirely of neutrons
Electrons and protons have fused together
How do you make a neutron star?
• Remnant of a Supernova
How do you get a Supernova?
• A high-mass star keeps on fusing elements into
ones with larger atomic masses
• Is now a Red Supergiant
• Energy keeps on being released since the mass of
the new nucleus is less than the original ones
This stops with Iron
• Fusion of Iron with another element does not
release energy
• Fission of Iron with another element does not
release energy
• So you keep on making Iron
Initially
• Gravity keeps on pulling the core together
• The core keeps on shrinking
• Electron degeneracy keeps the core together for
awhile
Then
• The iron core becomes too massive and collapses
• The iron core becomes neutrons when protons and
electrons fuse together
Density
• You could take everybody on Earth and cram
them into a volume the size of sugar cube
Explosion
• The collapse of the core releases a huge amount
of energy since the rest of the star collapses and
then bounces off the neutron core
• 1044-46 Joules
• Annual energy generation of Sun is 1034 Joules
How do we know there are neutron stars?
• The identification of Pulsars
• Pulsars give out pulses of radio waves at precise
intervals
Pulsars
• Pulsars were found at the center of supernovae
remnants
Pulsars
• Pulsars were interpreted as rotating neutron stars
• Only neutron stars could rotate that fast
• Strong magnetic fields can beam radiation out
Black Holes
• If a collapsing stellar core has a mass greater than
3 solar masses,
• It becomes a black hole
Black Hole
• After a supernova if all the outer mass of the star
is not blown off
• The mass falls back on the neutron star
• The gravity causes the neutron star to keep
contracting
Black Hole
• A black hole is a region where nothing can
escape, even light.
Event Horizon
• Event Horizon is the boundary between the inside
and outside of the Black Hole
• Within the Event Horizon, the escape velocity is
greater than the speed of light
• Nothing can escape once it enters the Event
Horizon
How do calculate the
radius of the Event Horizon?
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It is called the Schwarzschild Radius
Radius = 2GM/c2
This is a variation of the escape velocity formula
Escape velocity = square root (2GMplanet/Rplanet)
Black Hole Sizes
• A Black Hole with the mass of the Earth would
have a radius of 0.009 meters
• A Black Hole with the mass of the Sun would
have a radius of 3 kilometers
Can you see a Black Hole?
No
• Black Holes do not emit any light
• So you must see them indirectly
• You need to see the effects of their gravity
Evidence
• The white area is
the core of a Galaxy
• Inside the core there
is a brown spiralshaped disk.
• It weighs a hundred
thousand times as
much as our Sun.
Evidence
• Because it is rotating we can measure its
radii and speed, and hence determine its
mass.
• This object is about as large as our solar
system, but weighs 1,200,000,000 times as
much as our sun.
• Gravity is about one million times as strong
as on the sun.
• Almost certainly this object is a black hole.
Questions