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Unit One
1-8-19
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We do science to figure out why things are the way they are
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All models can change a lot, like the model of the atom
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1-15-19
Si units

Us customary system

Meter

foot

seconds

second

kilogram

slug

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Must be able to Convert back and forth
How many meters in a mile?
o A mile is 5280 ft, a meter is 3
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o There are 1,609 meters in a mile
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1-17-19
• Primary paint colors: magenta, cyan and yellow
o Primary light colors: Red, green and blue
• Different colors have different intensities, the amount of the light effects this
• Frequency - how many crests pass by per second
• Longer wavelength, shorter frequency
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• Light is an Inverse square law
o Gets dimmer by the square
• What determines the wavelength of the emitted light?
o The sun
▪ It’s emitting a lot of light and it’s in the visible light of the spectrum
o Temperature of an item
▪ People are warm, the sun is hot
• If temperature matters why does it make sense?
o Higher temperature, the molecules are moving faster
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o All color’s speed is different
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How we organize the periodic table is how they behaved
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1-22-19
• Rutherford gold foil disproves that atoms are empty space and they have a nucleus
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▪ Between 400 and 700 nm
• Light particle
o Called a photon
o Moves at c
o Has a specific energy that’s dependent on frequency?
• Spectra (spectroscopy)
o Emission - black with colors
o Absorption - rainbow with missing colors
o Continuous - rainbow of colors
o Each spectrum is unique, to each element
• Periodic table
o Organization
• Thomson's plum pudding model didn’t explain the spectra or the periodic table
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• Rutherford came and did an experiment that proved an atom more and created a new
model
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• When a photon is emitted, a certain amount of energy is loss, the orbit gets smaller
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• Rutherford’s model predicted that all atoms would collapse & that was wrong
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It did prove that atoms are mostly empty
space and humans have never really touched anything
1-24-19
• Bohr’s assumption
o Angular momentum is quantized, can only have energy (full electrons, not
halves)
o An electron does not radiate
o A single electron is emitted when it jumps from higher orbit to lower
• Angular moment gives us some allowed values of energy
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The energy in one orbit is constant
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If a
photon comes in, it must have the exact amount of energy, it then can jump up
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• Ionizing levels - adding the exact amount of voltage to cancel it out and make it go
away
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• Emission has more lines
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• Absorption spectra is a cool gas
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• Bohr’s model explains the periodic table
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• How many electrons are in the outside shell - how the periodic table is set up/organize
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o They’re stable if they’re in their allowed shell - why elements do not collapse
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• When something passes you with a sound, you heard the frequency change
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You must be moving
in or out
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1-29-18
• Telescopes collect light
• Bigger is better
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• Important things
o Collect light
o Focusing light to form an image
• Telescopes will use a lenses or a mirror to collect light
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• Your eye Focuses light on the back of the retina and sent to the brain
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• The sun is mostly hydrogen
• The sun is mostly hydrogen and helium and at first, the people did not believe it
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• They found that iron was missing 13 electrons, so it was very hot because that’s the only
way iron can be ionized that much
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▪ Takes year for light to travel through this layer
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▪ This layer is very dense, much like the core
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▪ Transferring energy from the bottle layers to the top
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▪ We don’t see the other layers
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The sun has granules
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The darker,
the less energy
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▪ 25-kilometer thick
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▪ 10k kelvin is the temperature
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o Transition region
▪ Transitions from ten k degrees to a flaming million degrees
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o Corona
▪ We knew this existed because there have always been solar eclipses
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▪ Particles coming out of it are going very fast
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▪ Low density
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Solar wind
o Particles from the sun
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Sunspots
o The sunspots are darker because the spots are colder
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o Can travel around, move like storms, they do rotate with the sun
o We can see the sun rotate by using the sun’s spots
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Magnetic fields are probably coming from the convection era

2-5-19
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We cannot predict come storms
The most we could predict was a couple of hours to a day
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We don’t know when something will hit us
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o More sunspots are more active
The little ice ages
o Europe
The overall temps don’t get affected by the sun
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If have less oz by the stratosphere by the poles, it will cool them
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The sun is 4
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o Bright
o Powerful
o How can the sun produce this much energy for this long?
Energy
o There are many different types of energy
o Conservation of energy: energy cannot be created or destroyed
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Not an easy thing to do, find and place where energy went
People thought it was a ball of burning wood - early 1800’s
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People thought the sun lasted by gravitational potential energy to kinetic energy
o The sun will contract, gas will fall and that will change the bottle layers hot
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▪ Undetectable - we couldn’t tell if this was happening
o This would make the sun last 100 million years
Darwin thought that evolution was happening longer than 100 million years
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Space and time are the same thing and we convert back and forth between them with
the speed of light
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o e=mc^2
o Take mass and multiply it by the speed of light squared
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o They cannot be broken up to quarks
It’s hard to put two like charges together because they repel
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When particles bind, they release energy
Where does the energy come from?
o It doesn’t take a lot of mass to produce a lot of energy
Fission
o Heavy Atoms are broken up
o This is releasing energy
o Not in the sun
o How do we know this is not happening in the sun?
▪ There are no heavy atoms in the sun

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Means to come apart
The only way we know how to use it
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You must have the atoms going very fast for them to combine
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o Takes a while to happen
o The second collision does not take long
Last step - two of the three helium nuclei must slam together to make a
Why is the sun shining all the time if the sequence of a hydrogen four to happen why do
we see sunlight all the time?

First Unit In-Class Review
Light
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How do you make an emission spectra - black background with lines of color?
o Making gas hot, hot gas
• How do you make an absorption spectra - colors with missing parts?
o White light goes through a cold gas
o The gas in ground state, grabs colors and jumps to higher levels
• We can tell by the spectra
o The composition
• Doppler effect
o Redshift and blueshift - redshift, going away from you, blue shift coming towards
you
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• Rutherford made the gold foil experiment and the tiny planet model

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Atoms are mostly empty space
Discovered nuclei
Also wrong- doesn’t explain periodic table, atoms collapsed after emitting light,
spectra patterns
Bohr’s (photons emitted/absorbed when jumps between allowed orbited occur)
o Electrons only allowed in specific orbits
o They don’t radiate light in an allowed orbit
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Sun
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Strong nuclear force holds nuclei together
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0071 kg)
electricity and magnetism
• Very similar
• Fluctuating of either produces other
• Moving charges create a magnetic field and can affect other field
Telescopes
• Reflective
o mirror
• Refraction
o lens
• Refraction - bending of light
• Bigger is better
o More diameter, the more light it can take in
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com/367707446/stars-galaxies-theuniverse-unit-one-flash-cards/

Unit Two
2-12-19
• Luminosity - brightness, the total amount of energy radiating all wavelengths per
second
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• Bigger numbers are dim, smaller numbers, including negative numbers are
brighter
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• The spectra looks so different because it depends on how ionized it is in the
stars
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• There are 7 spectral classes
• The light year is a unit of distance, not time
• 21 light years away - took 21 years for the light to get to you
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o We also have spectroscopic binary
▪ You can only tell by looking at the spectrum, but it looks like one
point of light
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o High mass star will have to be closer to the center of mass, the little one
will be moving around the bigger one in a bigger orbit
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• How are we getting the speeds of the stars
o Looking at the spectra
o Apply kepler’s laws and then we have mass
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• Bigger stars are rare, a lot smaller than the sun
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▪ If not, then its’ a dwarf
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2-21-19
• Finding distances
In-class conversion exercise between planets
• KATt
o Nose
o Claw- 5 noses
o Paw- 10 claws
o Belly- 20 paws
• 1 speedy - 20 bellies
• 4 bellies- 1 chain
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Metric system was from the french
Meter is 3
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2-26-19
• How does triangulation work?
o Take two points and look at one thing and get the angle and go to the
other point and look at the same object and get the angle and those two
angles will get the distance of the third object
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• We cannot measure stars far away, we can send things out to space
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o Small distance in the galaxy
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• Variable stars
o A star that varies in brightness over a period
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▪ Example : polaris the north star
▪ The longer it takes for the variable to go thru it’s cycles, the brighter
the luminosity is
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• The clouds were so far away, they’re basically all the same
distance
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They all have similar distances & they all have the same brightness
and same cluster
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Both are pulsating, changing with time, going thru and unstable phase in my life,
their diameter is changing all the time
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o You see red and blue shift and the lines are going to be thinner
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How does the pressure of star help us determine how big it is ?
o Thickness of spectra lines
o If they’re thick, high pressure
If the pressure if low of star, is it on the main sequence?
o Probably is not
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o The sun is a g2 V

What’s between all the stars
interstellar medium - all the stuff in between the stars,
o Mostly gases and some solids
• Nebula - giant gas clouds
o Two kinds
o Some will glow and others will reflect all the light
o Moves and changes over time
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• Important because dying stars will project their material into space
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• Solids
very small solids
o Ices
o Graphite
o Molecules
o Dust grains , not visible just bonded molecules
• Red - hydrogens
blue-reflecting light from nearby stars
• Density is low for stuff in between the stars, interstellar medium
o Dust grains are less dense
• One atom per cubic centimeter, there is so much space there is a lot of mass for
this material
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o Ionized hydrogen
o Uv ray comes in , kicks out the electron and then another one is caught
and then absorbed and emits a photon and that photon is in the red color
spectra
• Bohr model had electron shell
o Each shell has a different amount of energy
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You’re only going to get an h2 region if that area is hot enough to heat that cloud
up
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o At first, it was hard to figure out what was happening
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o We need a detector to see energy changes in this
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o Very low energy, radio wave low
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There is gas so hot it produces x-rays
What could make it so hot?
o Radiation
Supernova explosions happen every 100 years somewhere in the galaxy
o Only big stars
The gas that they explode will be moving very quickly and can heat colder gases
up
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o Inside the cloud, you can form molecules
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o Very dense
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More Likely to make molecules
o This is where new stars are formed
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Cosmic dust
o Some are close to the stars because they reflect light
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o The most common example is the pleiades cluster
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3-12-19
• Interstellar reddening and extinction
o What does it mean for light to be scattered?
▪ Collision with light and other dust grains
▪ Scattering it away from our view
o What our atmosphere does to blue light
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They’re red
because there’s no blue
• Bright stars appear red because of the dust grains
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• Why sunsets appear to be more red
o The most atmosphere between us and the sun is at sunset so it’s
scattering more blue and the sunset is more red
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o Where are the dust regions?
▪ Along the plane nad center of the galaxy
o We can use it to see further into the galaxy
• How do we know it’s dust and grains absorbing the light and not the gas?
o It’s gotta be particles
• How do the particle form? What are they made of?
o Should be made from the common things
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• How big are the grains?
o Deflecting blue wavelengths, red wavelengths are too big for them
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• Cosmic rays
o Last of the three types of material
o Discovered in 1913
o Made from the same thing that everything else is made from
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o Easy for us to study
o abundant
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▪ What caused them?
• See how old they are and see if they’re around some
explosion
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• We are the product of dead stars
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But we need dense
regions from star deaths
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Old stars had to die
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It’s not dense enough to feel the hotness
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We think this is caused by a supernova explosion
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(amino acids in ice region)
▪ Carbon based
• Diamond and graphite
▪ Silicon based
• Sand like
o Dust clouds block out the light
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o Sinks & sources - flows of gas
▪ Supernovas
▪ Black Holes
▪ Star death
▪ Star birthing
▪ Particles falling into the galaxy
Variable stars
H-r diagrams
Celestial distances
o Radio waves
o Variable stars
▪ Cepheids
▪ Rr Lyre

Triangulation (parallax) - works to 30,000 light years, most accurate for
stars
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▪ Classifying it to the spectrum
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Luminosity - how bright a star is
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o Apparent vs actual
Binary stars
o Spectroscopic
o Visual
o Eclipsing
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Red shift & Blue shift
o We can learn rotation, speed compared to us, period of expansion of
contraction
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o Can tell us apparent vs
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The most abundant star in the galaxy
o Is M type
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o 90% of a star’s life are on the main sequence
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Why wasn’t Williamina’s classification system ordered by temperature? Because
she created a system on how strong the hydrogen absorption lines were
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2
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It has weak hydrogen lines
a
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If it has calcium and ionized metals, then what class could it be? K or G
3
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4
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It
only works if the star is rotating in and out of our plane
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If one star is three times more massive than another, how much bright is it?
3 =81
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2
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The three types of binary
systems are visual, spectroscopic and eclipsing
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Suppose you want to determine the average education level of people
throughout the nation
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Will you get an accurate answer? Will your survey be distorted by a
selection effect? Explain
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Yes, the survey
will be distorted by the selection effect
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4
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5
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Will you
design your telescope to detect light in the ultraviolet or infrared part of the
spectrum? Why? Infrared because they’re not giving off enough energy
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Describe what a typical star in the Galaxy would be like compared to the Sun
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It’d be red and less bright
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How do you distinguish stars from brown dwarfs? How are brown dwarfs
distinguished from planets? A brown dwarf doesn’t fuse helium and planets don’t
fuse anything
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Are supergiant stars also extremely massive and dense? Explain
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4
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Which star has the largest radius? Which star
has the smallest radius? Which star is the most common in our area of the
Galaxy? Which star is the least common? Explain
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White dwarfs have the smallest radius
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13 Celestial distances Part 1
1
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Should you believe her?
Why or why not? No, because it’s gas and it’s going to get absorbed
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Most distances in the Galaxy are measured in light-years instead of meters
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3
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Why does this need to be the case? The
earth’s orbit isn’t perfectly circular
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Would it be better to make parallax measurements from Pluto? Why or why not?
Yes, it has a bigger orbit, so it has a bigger difference
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Describe the process of triangulation
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14 Celestial Distance Part 2
4

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What steps would
you take to determine its distance? First you’d determine its period and use it to
find its luminosity and compare it to the luminosities we see and find its distance
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Which method would you use to obtain the distance to each of the following?
a
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A star astronomer believed to be no more than 50 light years from the sun
parallax or triangulation
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A tight group of stars in the Milky Way Galaxy that includes a significant
number of variable stars
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A star that is not variable but for which you can obtain a clearly defined
spectrum-R Diagram
3
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For centuries astronomers wondered whether comets were true celestial objects
like the planets and stars, or a phenomenon that occurred in the atmosphere of
Earth
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If it goes beyond
earth’s atmosphere, then it doesn't happen inside of earth’s atmosphere
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A molecular cloud is about 1,000 times denser than the average of the interstellar
medium
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Air has a density of about 1 kg/m3, so something 1,000 times denser would have
a density of 1,000 kg/m3
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)
Is the density difference between a molecular cloud and the interstellar medium
larger or smaller than the density difference between air and water or granite?
The density of water is close to 1,000 times denser than air, like the cloud to the
average interstellar mediums
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2
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Suppose you wanted to search for some recently formed stars
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This is because it’s a high density of dust and gas, it can block
visible light
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Why do molecules, including H2 and more complex organic molecules, only form
inside dark clouds? Why don’t they fill all interstellar space? Molecules such as
h2 and other complex organic molecules only form inside dark clouds because
it’s hot
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4
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Why is this line such an
important tool for understanding the interstellar medium? The 21-cm line of
hydrogen is formed by the flip of the electrons
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This line is so
important because it tells us the difference of the radio wavelength
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16 Interstellar Medium Part 2
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2
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The appears to be red at sunset
because at sun set, there’s more atmosphere between us and the sun
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3
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Make a table listing the major
sources and sinks, and briefly describe each one
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Sinks would be star forming or the
formation of planet
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In big cities, you can see much farther on days without smog
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Because it’s made of particles
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Suppose that the Sun were inside a giant molecular cloud instead of the Local
Bubble
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The infrared sky would
be us seeing everything
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Quiz Questions
1
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The star spends 90% of its life in the main sequence
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When luminosity and temperature were plotted on a graph, a relationship
between the two were found
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If we wanted to find a star, if it cannot fuse it cannot be a star
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Stars form in molecular clouds
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o The high dense region will collapse to form the star
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o Clumps - more dense than other regions
▪ There are cores that are even more dense
• Low temp
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is low and pressure is low
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Once a star has formed, they’re more likely to form afterwards
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o Takes 1,000 years for a star to be born
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3-19-19
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How stars get to old age
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o They watch a lot of stars
• Plot the stars on an h-r diagram
o It tells us everything we need to know
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• Why do they stay on the main sequence for most of their lives?
o They have a lot of hydrogen to fuse
• Gravitational contraction - when they’re a star, they’re making energy using this process
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o Core becomes dense
o Temp rises
▪ The particles will move faster, more collisions, more energy generation
o Luminosity will increase
• Bigger stars die faster
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• Once all the hydrogen is used up, there is only helium
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• Normal main sequence star
o A force trying to pull it in and one pulling it out and it should be equal
o Pushing it out - energy generation, pressure
o Pushing it in - gravitational energy
• The pressure from the collapse is making the outer layers bigger, the temp decreases
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o They all formed around the same time
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o Roughly the same chemical elements
• Age of the cluster, when they were formed, if some of the stars have made it to the main
sequence
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o Spherical shape
o Very old
• Open clusters
o Found throughout the disc
o Smaller
o Appear more separate from each other
o Move and escape their neutral gravitational pulls
• Stellar associations
o O -type stars
o Younger
o Low mass stars but they’re hard to see, can see them much further away

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Found in regions where they are a lot of gas
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That tells you how they evolve
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When we have carbon in the core, it’s going to contract again
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Helium shell flashes then start
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3-16-19
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Planetary nebulae
o Matter from a dying star
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• The visible nebula must be 50,000 years at most because if it’s older, we cannot see it as well
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• Heavier elements are made when the massive stars die in supernova explosions
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Death of stars
• Death of low and high mass stars differ
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4 solar masses when they die
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• White dwarf
o Low mass stars stop at fusing carbon and become white dwarfs
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o Bright and small, hot
o It’s hot at first and when it dies, it cools off and we can still see it
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o To maintain this state, it will stay here forever if nothing disturbs it
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4-3 solar masses are a neutrino star
• Less than 1
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• More than 3 solar masses will descend into a black hole
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o Very dense, the densest thing in the universe
• Supernova explosions make all the rich materials in the star go back to space
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o Where cosmic rays are from
▪ They’re stuck inside the galaxy because of the magnetic fields
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o 260 light years is safe for us
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Pulsars must be aligned with our sight for us to see them
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• Supernova type 2 -when a massive star of ten solar masses or greater explodes this makes a
neutron star
• Nova- when a white dwarf gets bigger from a companion star & it will get enough mass in
hydrogen, burn the hydrogen and explode
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White dwarf gets too much material from other star and the
white dwarf is gone
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4-2-19
Riddles:
• Two twin brothers
o The space twin doesn’t have the proper time because he’s in many reference frames
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o The space twin is younger because time slowed down for him
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• Garage doors and long pole
o The person with the pole has the correct length
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o This wasn’t a paradox
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• Time and space are the same thing
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• You can convert between space and time using the speed of light
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• Black hole- no light can escape or anything
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• Equivalence principle
o Fundamental fact of nature
• Light bends
• Space time curves
4-4-19
• inertia frame - constant velocity
o Non intertie frame - some acceleration
• Time dilation- time is relative
• Equivalence principle- you can’t tell if you’re in free fall or far from a planet
o Because of this, this causes light to bend
• Barn paradox - length contraction
o Length is relative
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• Equivalence principle
• Light will take the shortest path, light will bend around
gravitational objects
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Weight will be the same between both floors
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Gravitational red shifting - proves that general relativity is correct
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Escape speeds vary
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o Theory & model - we cannot see what’s inside the black hole
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o Think that black holes will continue to collapse
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▪ Space time
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▪ Space time does not exist
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As the star continues to collapse, the horizon doesn’t get any smaller
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Formation: obliterate any original information about what star it came from
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But they’re really all moving and spinning
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If anything gets into a black hole it would get spaghettified
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Around the black hole
o It can suck some material away
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When it gets hot, it will emit x-rays
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Going to see x-rays shoot out of the poles
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Two black holes colliding - bigger black hole
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The molecular clouds are too dense to see them with visible light
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• It if takes more than three million years for the dust to begin clearing out of the inner regions of
the disks surrounding protesters, would you expect to find a planet around a star that is 100 times
more massive than the sun? No, because they don’t take as long to form, forms too fast
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18 Stellar Evolution
• If you are looking for life on another planet, would you search for it on planets around an O or B
type star? Neither because their lifetimes are too short
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• Are red giant stars expanding, shrinking or doing a little of both? Explain both because stars
expand to a red giant and then shrink
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• Based on the table in the slides, when Betelgeuse was on the main sequence, was it more massive,
less massive or the same mess as our Sun? Still more massive, mass barely changes
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• If all the stars in a cluster have nearly the same age, why are clusters useful in studying
evolutionary effects (different stages in the lives of stars) because all the stars would have
different masses, so they move at different rates
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Would you believe him? No, because O type stars are
large enough to fuse heavier elements
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• What is a planetary nebula? Will the sun have one? Planetary nebula is dying star’s material and
the sun will have one
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• Suppose no stars more massive than about 2 time the mass of the sun had ever formed
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• Describe the formation of a white dwarf
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It’s core cannot continue to collapse
because of the Pauli exclusion principle which says that element cannot get any closer
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No, because they’re dense and massive
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They also come from massive stars
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• Based upon the evolution of stars, place the following elements in order of least to most common
in the galaxy: gold, carbon, neon
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Carbon is more common so most stars can form it
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• How is nova different from a type 1a supernova? what is the difference between a type ii
supernova and a type 1a supernova? A nova doesn’t destroy the white dwarf
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Type 1a doesn’t have hydrogen in the explosion because it forms from a
white dwarf
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21 Relativity and Black Holes
• How does the equivalence principle lead us to suspect that space-time might be curved
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If you fall, light will always curve to keep up with you
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• Imagine you have built a large room around the people throwing the ball in the figure
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Galileo showed that if there is no drag, light and heavy
objects fall at the same rate
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Also suppose the man is twice as massive as the woman
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So the man would fall faster
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22 Relativity and Black Holes Part 2
• Why would we not expect to detect X-rays from a disk of matter about an ordinary star? Matter
must be moving very fast
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• You arrange to meet a friend at 5:00 PM on the observation deck of the Empire State Building
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She arrives 5 minutes late and says the
reason is that time runs faster at the top of a tall building, so she is on time but you were early
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QUIZ QUESTIONS
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You are observing two supernova explosions
Explosion one is brighter
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But they are not as
Strong
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• After core collapses it will get hotter and start fusion
o Fusion is the official birth of a star
• Brown dwarfs don’t fuse hydrogen
• After they form they are on the main sequence
• More mass → faster the fusion
Three possibilities when they die
• White dwarf - < 1
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The star itself can be up to 8 solar masses
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4 solar masses and three solar masses
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When a star dies, fusion will stop
T-Tauri Stars
• Between protostar and star
• Massive stars do not go thru this phase
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Star evolution
• When it starts to die, the core contracts the outer layers expand
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• High mass stars can fuse up to iron
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• Cores contract once they’re reached the last phase
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o Neutron gas
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General relativity
Special relativity
Reference frame
• Inertial- velocity is constant
Equivalence principle - can’t tell if you’re in free fall or not
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Length contraction
Harder to formulate
The person in the same reference as the object, will always have the true length
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Unit Four Notes - Stars, Galaxy & Universe
4-9-19
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Hershel concluded that the sun was in the center
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Stars are not confined in the center, so there’s a bulge in the center of the disk
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Thick disk have old stars 12-13 billion years old
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Galaxy doesn’t move like a cd, but it looks like a cd
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Differential galactic rotation- why the spiral galaxies form
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The core of the galaxy has a lot of mass
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Cannot see the center of the galaxy that well because there’s dust in the way
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o Clumps have cores and they contract and then collapse, it star heats up and then starts to
spin
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Cepheids : variable stars that you can use to measure
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The fuzzy things were galaxies and they have a shape
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1925 everything was in the galaxy and then after 1925, the universe got a lot bigger
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They spent nights observing the same object
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If there are a lot of bright emission lines, that means there is a lot of young stars
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All spirals are rotating
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Elliptical galaxies have a lot of old stars and does not have a lot of dusk
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o Can get a lot bigger than spiral galaxies
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Irregular
o Lots of star formation
o Range of young and old stars
How would you use the doppler effect to figure out the mass/rotation ?
o You’d see the bars get thicker
mass/light = one for the sun
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o Low mass stars- it’s greater than one
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The further the galaxy was from us, the faster they’re moving
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o It has a weird spectra- that’s how we know it’s not a star
o They’re very red shifted
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o Big blue star that is going very fast
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They’re all very far away and they’re all moving away from us
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Do they obey hubble’s law ?
▪ They could possible be in a galaxy- could suggest
▪ Give us an idea of how far away they are
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Particles will come out at the poles
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o The particles that are shooting out cannot go thru the disk
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All galaxies go thru the phase where they have qasaurs
More gas and dust allowed Quasars to be more present
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We must take a survey of what’s out there and make conclusions about how galaxies evolve
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The spectra will tell us a lot about galaxies
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The color indicates how old the stars are in it
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Everything started to form not long after the big bang
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o We don’t see blue type stars in ellipticals
There are a lot of heavy elements in the spectra of old galaxies
o Many stars have been born and died and they’ve done it enough to send gas into the
galaxy
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o Didn’t find out until 1950’s
Small galaxies must have collided with other galaxies
Stars do not collide because there is a lot of space in between them
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Merger - when galaxies collide
Big galaxy eats a small galaxy
Quasars have black holes in the center
o When they combine - the black hole can get bigger
The universe is isotropic and homogeneous
o Isotropic - same in every direction

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o Homogeneous - all the same
Rick clusters have a lot more galaxies in the center
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Cold moves slowly - formed after the big bang
o Helping galaxies form
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o After the big bang, it pushed space apart even faster
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We’re close to the mid plane
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• Suppose the milky way was a band of light extending only halfway around the sky
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Because the way we’d spin
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24 Galaxies
• Why can we not determine distance to galaxies by the same method used to measure the parallax
of stars we can’t use the same method because we look at star clusters, not specific stars and
they’re too far away for us to use the parallax method
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Suppose That galaxies were observed in larger numbers at distances greater
than 100 million light-years
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• What does it mean to say that a universe is expanding? What is expanding? Is the astronomy
classroom expanding, the solar system? Why or why not? When they say that the universe is
expanding, they mean that galaxies are moving away from us
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No, the classroom is not expanding, the solar system is not expanding
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The most
useful standard bulb would be the type 1A supernova
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25 Quasars
• Suppose you observe a star-like object in the sky
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One of the early Hypothesis to explain the high redshifts of quasars was that these objects had
been ejected at very high speeds from other galaxies
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Explain why we would expect to see quasars with both
blue shifted and red shifted lines if they were ejected from nearby galaxies
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You’d expect to see at least one coming at us
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No, because the galaxy is old
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26 The Evolution and Distribution
• What are the forms of evidence we have for the existence of dark matter? Some forms of
evidence we have for the existence of dark matter would be that we discovered Neptune because
of the orbit of Uranus, and It affects the motion of things that we can see
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• Suppose you are standing in the center of a large, densely populated city that I exactly circular,
surrounded by a ring of suburbs with lower- density population, surrounded in turn by a ring of
farmland
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• How are galaxies today different from the earliest galaxies? We know more about galaxies today
than back then with the help of technology
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Paper Notes
23 The Milky Way
• We use radio and infrared cameras to see the galaxies
• The brightest part of the galaxy is a thing rotating disk
• Our galaxy has a bar with spiral arms
o The bar’s made of old yellow-red stars
• The galaxy is a barred spiral
• In the center, there’s a bulge
o Two times longer than it is white
o Seen through infrared
• There’s a dark matter halo around the galaxy
• Our galaxy has two major arms from the bar
o The fainter arms are spurs
o The suns on a spire, Orion’s spur
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o That’s how we got the arms
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• Gravity also helped with the formation of arms
• There’s a black hole at the center of our galaxy - a supermassive black hole
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• We need radio waves to estimate the galactic center
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25 Quasars
• Quasars - quasi-stellar radio sources
o They look like stars, but they are not
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o Quasars are very red shifted
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o Quasars are not in our galaxy
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o There are no blue shifted quasars
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o Their brightness can give us their distances
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• Our galaxy does not have the mass for a quasar - not enough mass
• Black holes - amount of energy produced
o Mass
o How much is falling into one
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o The plumes/jets are moving very fast
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o Was not taken very seriously
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• Once technology got better, the fuzzy objects became star clusters
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• Hubble looked at M31, the Andromeda galaxy and saw that it was not in our galaxy
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• Now they were classifying and looking at other galaxies
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• Spiral galaxies
o Open clusters in arms
o Globular in the halo
o Mis of old and young stars
o ⅔ are barred
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o Central bulge is small and the arms are loose, it’s a “sc galaxy”
• Elliptical galaxies
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o They don’t have arms
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o The light it gives off is from red stars
o Dust and emission nebulae are not obvious
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o Doesn't have arms because the star don’t go in the same direction
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o Fwart ellipticals are common
▪ Faint and hard to see
• Irregular galaxies
o Everything that’s not a spiral or an elliptical
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Galaxies can change shape
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• Finding the mass of elliptical galaxy
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o Use spectra - see if there is line - broadening
• Mass to light ratio - tells us how much dark matter there is
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26 the evolution and distribution
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o We also survey them
o We look at the spectra
▪ Tells us their distance, rotation rate, mass and whether there’s a lot of gas or dust
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• Elliptical galaxies started forming a early
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• Wasn’t until telescopes that we could see far away
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• Galaxies collide but stars did not
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• Cannibalism- when a large galaxy eats another galaxy
• Starburst - if at least one of the galaxies contains a lot of interstellar material the collision can
cause extra star formation
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• The universe is isotropic and homogeneous
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o Spirals on the outskirts
• Poor clusters
o Spiral galaxies
• Voids - empty spaces in super clusters
• We know that dark matter exists because
o We found neptune because of it
o It affect the things we can see
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Dark matter pushes the universe apart

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