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Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine tburbine@mtholyoke.edu 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.
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Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.eduwww.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 • 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 question #1 • What is approximately the temperature of the plasma in a sunspot? • 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? • 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? • 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? • 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 • 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? • 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
No • Black Holes do not emit any light • So you must see them indirectly • You need to see the effects of their gravity
The white area is the core of a Galaxy Inside the core there is a brown spiral-shaped disk. It weighs a hundred thousand times as much as our Sun. Evidence
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.