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Chapters 19 - 24
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What kind of galaxy do we live in?

We live in a DISK galaxy, not an elliptical or irregular galaxy
(The milky Way is our view of the disk)


Panoramic views of the Milky Way in different bands of the spectrum

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We can observe the star-gas-star cycle through these different wavelengths of light


What is the interstellar gas medium composed of?

It is composed of dust and gas --> It obscures out view because it absorbs visible light


We see our galaxy EDGE - ON.
Primary features: disk, bulge, halo with stars and globular clusters

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If we could view the Milky Way from above the disk (face-on), we would be able to see its spiral arms

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How do stars orbit in our galaxy?

-Stars in the disk all orbit in the direction, but will a little up-and-down motion
- Orbits of halo and bulge stars have random orientations


Why do orbits of bulge stars bob up and down?

The gravity of all stuff in the disk pulls them toward the midplane


Orbits of stars in the bilge and halo have random orientations

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What is the ORBITAL VELOCITY LAW and what does it tell us?

- M = (r x v^2) / G
- The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass (M)C WITHIN that orbit


How can we use the Sun's orbital motion (radius and velocity) to tell us the mass withing the Sun's orbit?

Sun's velocity: 220 km/s = 2.2 x 10^5 m/s
Distance: 27, 000 ly (8.5 kpc)
Mass within Sun's orbit: 1 x 10^11 MSun
* the galaxy has rotated more than 50 times in the life of the universe


What does our galaxy look like?

Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical halo


How is gas recycled in our galaxy?

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Through the star-gas-star cycle (It recycles gas from old stars into new star systems)
- Gas from dying stars mixes new elements into the interstellar medium, which slowly cools, making the molecular clouds where stars form
- Those stars will eventually return much of their matter to interstellar space


How does gravity affect the star-gas-star cycle?

Gravity forms stars out of gas in molecular clouds, completing the star-gas-star cycle


What are the characteristics of high-mass stars?

They have strong stellar winds that blow bubbles of hot gas into the interstellar medium


What are the characteristics of low-mass stars?

They return gas to interstellar space through stellar winds and planetary nebulae


What is the effect of X-rays in elements?

X-rays from hot gas in supernova remnants reveal newly made heavy elements


Galactic fountains

- Multiple supernovae create huge hot bubbles that can blow out of the disk
- Gas clouds cooling in the halo can rain back down on the disk


Summary of galactic recycling

- Stars make new elements by fusion
- Dying stars expel gas and new elements, producing hot bubbles (~10^6 K)
- Hot gas cools, allowing atomic hydrogen clouds to form (~100 - 10, 000 K)
Further cooling permits molecules to form, making molecular clouds (~30 K)
- Gravity forms new stars (and planets) in molecular clouds


Where are ionization nebulae found and why are they important?

- Ionization (red) nebulae are found living around short-lived high-mass stars, signifying active star formation


What is the purpose of the reflection nebulae?

Reflection nebulae scatters the light from stars


Why are reflection nebulae blue?

Blue light is scattered more than red light


Where is much of the star formation in the disk occur?

In the spiral arms because the gas clouds are compressed in the arms.


Are spiral arms wound material?

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No! They are density waves!
- Because density is higher, it increases the star formation in arms
1. Gas clouds get squeezed as they move into arms
2. Squeezing of clouds triggers star formation
3. Young stars flow out of arms


Where do stars tend to form in our galaxy?

Active star-forming regions contain molecular clouds, hot stars, and ionization nebulae
- It happens in the spiral arms


Halo stars

- 0.02 - 0.2% of heavy elements (O, Fe, etc.)
- Contains only old stars
- Formed early, then stopped


Disk Stars

- 2% heavy elements
- Contains stars of all ages
- Formed early, but continued to form


How did our galaxy form?

Our galaxy formed form a cloud of intergalactic gas
- Halo stars formed first as gravity caused gas to contract
- The remaining gas settled into a spinning disk
- Stars continuously form in disk as galaxy grows older


Where will galactic gas be in a trillion years?

Locked into white dwarfs and low- mass stars


Spirals suffer minor mergers, Elliptical suffers a few major mergers

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What clues does out galaxy's history do halo stars hold?

Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first


How did our galaxy form

Halo stars formed early in the galaxy's history; disk stars form later, after much of the galaxy's gas settled into a spinning disk


What lies in the center of our galaxy?

- 200 light years from center: infrared light
- 50 light years from center: radio emission
- 10 light years from center: swirling gas
- 1 light year from center: orbiting stars


Stars appear to orbit something massive and invisible, what can it be?

Black hole


Orbits of stars indicate a mass of:

About 4 million MSun


Can we use Kepler's 3rd Law to find the mass?

Yes! Stars orbit far from the black hole, so it's just like any other mass


what do X-ray flares from galactic center suggest?

That tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in


Hubble Deep Field

Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away - and thus young


Galaxies and Cosmology

- A galaxy's age, its distance and the age of the universe are all closely related
- The study of galaxies is thus intimately connected with cosmology - the study of the structure and evolution of the universe


What are the three types of galaxies?

Spiral, Elliptical, and Irregular


Spiral galaxy

Disk component: Has stars of all ages, and it has much gas and dust
- Blue-white color indicates ongoing star formation
Spheroidal component: Has a bulge, halo, old stars, little gas and dust
- Red-yellow color indicates older star population


Why does ongoing star formation lead to a blue-white appearance?

Short-lived blue stars outshine the others


Barred spiral galaxy

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Has a bar of stars across the bulge


Lenticular galaxy

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Has a disk like a spiral galaxy, but it is much less dusty/ gas (intermediate b/w spiral and elliptical)


Elliptical galaxy

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All spheriodal component, virtually no disk component
- Red-yellow color indicates older star population


Irregular galaxy

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Blue-white color indicates ongoing star formation


Hubble's galaxy classes

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The rounder the appearance, the spheroid component dominates; the higher the spiral arms, the disk component domincates


Peculiar Galaxies

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Interacting Galaxies

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How are galaxies grouped together?

- Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies, and they are also found on the outskirts of clusters)
- Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies)


What is the motion of galaxies in a group or cluster?

Any galaxy orbits under the pull of all the other galaxies


How do we measure the distances to galaxies?

Step 1: Determine the size of the solar system using radar
Step 2: Determine the distances of stars out to a few hundred light-years using parallax
Step 3: The apparent brightness of a star cluster;s main sequence tells us its distance
Step 4: Because the period of Cepheid variable stars tell us their luminosity, we can use them as standard candles
Step 5: The apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years)


Inverse-Square Law

The relationship between apparent brightness and luminosity depends on distance
- Brightness = Luminosity / [4pie x (Distance^2)]
We can determine a star;s distance if we know its luminosity and can measure its apparent brightness
- (Distance)^2 = Luminosity / 4 pie x brightness
*Luminosity passing through each sphere is the same


Standard candles

A standard candle is something whose luminosity we know without having to measure its distance
- It is tricky with stars, so it is better to use the whole main sequence for a cluster --> "Main sequence fitting"
- If you know a star cluster's distance, we can determine the luminosity of each type of star within it
- White-dwarf supernovae can also be used as standard candles because it can help measure distances greater than 10 billion light years



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- Star pulsates so brightness changes
- Cepheid variable stars with longer periods have greater luminosity
(Discovered by Henrietta Leavitt, Radcliffe College 1893)


Why are Cepheid stars great fro measuring the distance to very far away galaxies?

Because they are very luminous, they can be seen at great distance


The Puzzle of "Spiral Nebulae"

Before Hubble, some scientist argued that "spiral nebulae" were entire galaxies like our milky way, while others believed they were smaller collections of stars within the Milky Way
- Debate remained unsettled until Edwin Hubble finally measured their distances
- Hubble resolved this by getting the distance to the Andromeda galaxy using Cepheids


The Hubble Law

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The spectral features of virtually all galaxies are redshifted, which means that they are all moving away from us and move away faster the further they are
- Velocity = H0 x Distance


Your friend leaves your house. She later calls you on her cell phone, saying that she took a while to get up to speed, but then drove at 60 miles an hour directly away from you and is now 60 miles away. How long has she been gone?

More than 60 minutes


Age of the universe

- Hubble's constant tells us the age of universe because it relates the velocities and distances of all galaxies --> Age = Distance/ Velocity
- Not allowing for any acceleration or deceleration
- If the universe's expansion has been decelerating, this is an overestimate; if the universe's expansion has been accelerating, this is an underestimate


Is Ann Arbor the center of the universe? How can we understand that we appear to be at the center of an expansion? It seems to violate the Copernican Principle

-No! Space itself expands
- Expansion stretches photon wavelengths, causing a cosmological redshift directly related to distance (predicted by Einsteins' General Relativity


Cosmological principle

- The universe looks about the same no matter where you are within it
- Matter is evenly distributed on very large scales in the universe (it has no center or edges)
- Principle is not prove but it is consistent with all observations to date


The horizon

Distances between faraway galaxies change while light ravels
- We specify a galaxy's location as the distance NOW, not when the light sets out


The cosmological horizon marks the limits of the OBSERVABLE universe

Galaxies that were close in the early universe have been separating for its entire history and are now the most distant objects we can see: ~ 47 billion years
- It is a horizon in TIME rather than SPACE. Since looking far away means looking back in time, there must be a limit - the beginning of the universe!


How do the distance between measurements tell is the age of the universe?

- Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach its current distance.
- Measuring Hubble's constant tells us that amount of time: about 14 billion years.


How does the universe's expansion affect our distance measurements?

We specify the present location of objects; they were closer when light set out.


Why do galaxies differ?

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The gas density of a galaxy's protogalactic clouds may determine whether it ends up spiral or elliptical; The spin will determine size of disk


Conditions in a protogalactic cloud: Density

Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk


Conditions in a protogalactic cloud: Spin

The initial angular momentum of the protogalactic cloud could determine the size of the resulting disk


Distant Red Ellipticals

Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe --> That suggests that we should also consider the effects of collisions


Collisions in galaxies

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- collisions were much more likely early in time because galaxies were closer together
- Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical
- Shells of stars observed around some elliptical galaxies are probably the remains of past collisions
- Collisions may explain why elliptical galaxies tend to be found where galaxies are close together
*Giant elliptical galaxies at the centers of clusters seem to ahve consumed a number of smaller galaxies


How are quasars powered?

- If the center of a galaxy is unusually bright, we call it an ACTIVE GALACTIC NUCLEUS (Quasars are the most luminous examples)
- The highly redshifted spectra of quasars indicate large distance
- From brightness and distance, we find that luminosity of some quasars are greater than 10^12LSun
-Variability shows that all this energy comes from a region smaller than our solar system


What can you conclude from the fact that quasars usually have very large redshifts?

They are generally very distant, they were more common early in time, galaxy collisions might turn them on, and nearby galaxies might hold dead quasars


Quasars powerfully radiate energy over a wide range of wavelengths, indicating that they contain matter with a wide range of temperatures

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Radio Galaxies

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They contain active nuclei shooting vast jets of plasma that emits radio waves coming from electrons that move at near light speed
- the "base" of these jets reveal blobs moving at almost the speed of light
- The lobes of radio galaxies can extend over hundreds of thousands of light-years


Characteristics of Active Galaxies

- Their luminosity can be enormous (>10^12 LSun)
- Their luminosity can rapidly vary (come from a space smaller than solar system)
- They emit energy over a wide range of wavelengths (contain matter with a wide temperature range)
- Some galaxies drive jets of plasma at near light speed


How can you explain all the properties of quasars?

By the accretion of gas onto a super massive black hole


Energy from a black hole

- Gravitational potential energy of matter falling into a black hole turns into kinetic energy
- Friction in an accretion disk turns kinetic energy into thermal energy (heat)
- Heat produces thermal radiation (photons)
-This process can convert 1- to 40% of E = mc^2 into radiation


What do we think comes from twisting the magnetic field in the inner part of accretion disk

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The active galactic nuclei zoo

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Do super massive black holss really exist?

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Orbits of stars at center of Milky Way galaxy indicate a black hole with mass of 4 million MSun
- The orbital speed and distance of gas orbiting the center of Galaxy M87 indicate a black hole with mass of 3 billion MSun


What is the size of the super massive black hole in M87, relative to our Solar System?

A little bigger than Pluto's orbit


Black holes in galaxies

Many nearby galaxies (perhaps all of them) have super massive black holes at their centers.
- These black hols seem to be dormant active galactic nuclei
- All galaxies may have passed through a quasar-like stage earlier in time


Galaxies and Black holes

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The mass of a galaxy's central black hole is closely related to the mass of its bulge.
- The development of the central black hole must be somehow related to galaxy evolution


How do quasars let us study gas between the galaxies?

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- Gas clouds between a quasar and Earth absorb some of the quasar's light
- We can learn about photogalactic clouds by studying the absorption lines they produce in quasar spectra


How are quasars powered?

- Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type
- The only model that can adequately explains the observations is that super massive black holes are the power source


What key features of the universe are explained by inflation?

- The origin of the structure, the smoothness of the universe on large scales, the nearly critical density of the universe
- Structure comes from inflated quantum ripples
- Inflation flattened the curvature of space, bringing expansion rate into balance with the overall density of mass-energy


the geometry of the universe is closely related to ...

total density of matter and energy (density = critical [flat])


Origin of inflation

Inflation can make all the structure by stretching tiny quantum ripples to enormous size. These ripples in density then become seeds for all structures in the universe


What is the horizon problem?

Despite the distance between objects, microwave temperature can be nearly identical on opposite sides of the sky because they were close together but inflation pushed them further apart


What is the flatness problem?

Inflation of the universe flattens its overall geometry like the inflation of a balloon, causing the overall density of matter plus energy to the very close to the critical density


Did inflation really occur?

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We can compare the structures we see in detailed observations of the microwave background with predictions for the "seeds" that should have been planted by inflation. So far, our observations of the universe agree well with models in which inflation planted "seeds."


What do we infer from the CMB?

- The overall geometry is flat [ Total mass + energy has critical density]
- Ordinary matter is ~ 27% total [Dark matter is ~23% of total and dark energy is 73% of total]
- Age is 13.8 billion years


Why is it dark at night?

Because the universe has finite age


Why is the darkness of the night sky evidence for the Big Bang?

The night sky is dark because the universe changes with time. As we look out in space, we can back to time when there were no stars.


What is Olbers' Paradox?

If the universe was INFINITE, UNCHANGING, AND EVERYWHERE THE SAME, them stars would cover the night sky


What is dark matter?

An undetected form of mass that emits little or no light, but whose existence we infer from its gravitational influence.


what is dark energy?

An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate


What is the contents of the universe?

- Ordinary matter: ~4.4%
- Dark matter: 23%
- Dark energy: 73%
* We saw that these numbers come from studying the CMB, the abundance of light elements and structure formation


What is the evidence for dark matter in galaxies?

We measure the mass of the solar system using the orbital period and and average distance of orbital planets. We would predict that velocity falls as square root of distance.
[Rotational curves of galaxies are flat, indicating that most of their matter lies outside of their visible region]


What is the rotation curve?

A plot of orbital velocity versus orbital radius. The solar system's rotation curve declines because the Sun has almost all the mass
- The rotation curve of the Milky Way stays flat with distance. The mass must be more spread out than in the solar system and over a larger region than its stars.
*most of the Milky Way's mass seems to be dark matter!


How can we measure the rotation curves of other spiral galaxies?

By using the Doppler shift of the 21 cm line of atomic hydrogen.
- Spiral galaxies tend to have flat rotation curves, indicating large amounts of dark matter.
- Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting (broader = fastest)


What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?

It is specially rich in dark matter


What is the evidence for dark matter in clusters of galaxies?

We can measure the velocities of galaxies in a cluster form from their Doppler shifts.
* The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars
- Clusters contain large amounts of X-Ray emitting gas. temperature of hot gas (particle motions) tells us cluster mass.


What is gravitational lensing?

It is the bending of light rays by gravity. They can tell us a cluster's mass.


Clusters: Final count

All three methods of measuring cluster mass indicate similar amounts of dark matter in galaxy clusters (85% dark matter, 13% hot gas, 2% stars)


Cosmic flows

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We find similar proportions of dark to regular matter here: regular matter is only 15% the whole, and stars a few percent


Does dark matter really exist?

Dark matter really exists, and we are observing the effects of its gravitational attraction.


What is dark matter?

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Particle dark matter: Weakly interacting massive particles
- Measurements of light element abundances indicate that ordinary matter cannot account for all of the dark matter


Why believe in WIMPs?

There is not enough ordinary matter. Physics suggests the existence of "supersymmetric particles." These could be left over from Big Bang. Models involving such particles explain how galaxy formation works.


What is the role of dark matter in the formation of our galaxy?

Dark matter doesn't interact with radiation, therfore local over densitties of dark matter could start to collapse early: ~1,000 years after Big Bang.
- Regular matter (baryons) interacted with radiation until 380, 000 years (that kept it from collapsing)
- Universe would not look as it does if there had been so little time for structure to form


Baryon radiation Interaction



Why do stars and globular clusters in the halo of our galaxy not collapse to form a bulge or disk?

They have no way to lose orbital energy


What is the role of radiation?

Once a dark matter "halo" has formed, dark matter can't sink further because it can't radiate away its orbital energy, but the baryons can.
[The gravity of dark matter seems to be what drew gas together into protogalactic clouds, initiating the process of galaxy formation]


What are the largest structures in the universe?

[Galaxies appear to be distributed in gigantic chains and sheets that surround great voids.]
- Maps of galaxy positions reveal extremely large structures: superclusters and voids
- Models show that gravity of dark matter pulls mass into denser regions - the universe grows lumpier with time.


Why is accelerating expansion evidence for dark energy?

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- The fate of the universe depends on the amount of dark matter. Since the amount of dark matter is about 25% of the critical density, we expect the expansion of the universe to overcome its gravitational pull. In fact, expansion speeds up (V)
* estimated age depends on the amount of dark matter and dark energy (V <-- oldest!)


Supppose that the universe has more dark matter than we think there is today. How would this change the age we estimate from the expansion rate?

The estimated age would be smaller


White dwarf supernovae

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The brightness of distance white dwarf supernovae lets us extend the Hubble diagram to great distance. An accelerating universe best fits the supernova data.


Why is flat geometry evidence for dark energy?

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Measurements of the cosmic microwave background indicate that the universe has a flat geometry, thus dark energy is needed to fill out the remaining mass-energy


What is the fate of the universe?

The eventual fate of the universe depends upon the rate of the acceleration of the expansion. If the universe does not end in a Big Rip, it should keep expanding for a very long time. All matter will eventually end up as part of black holes, which will eventually evaporate.


What have we learned?

In the absence of the repulsive force of dark energy, the expansion of the universe will not be accelerating. Also, evidence from the CMB indicates that the universe is very near critical density, requiring an additional contribution to the mass-energy of the universe. Lastly, the universe should keep expanding indefinitely, eventually consisting of a dilute sea of fundamental particles.