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Announcements. Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14 Angel Quiz. Questions:. Black Holes Star-Gas-Star cycle Halo, halo stars, halo vs. bulge, halo vs disk Spiral arms Do dying stars come back as the same star? superbubbles. Halo Stars:

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announcements
Announcements
  • Reading for next class: Chapter 20
  • Cosmos Assignment 2,Due Wednesday, April 14
    • Angel Quiz
questions
Questions:
  • Black Holes
  • Star-Gas-Star cycle
  • Halo, halo stars, halo vs. bulge, halo vs disk
  • Spiral arms
  • Do dying stars come back as the same star?
  • superbubbles
slide3

Halo Stars:

0.02-0.2% heavy elements (O, Fe, …)

only old stars

Disk Stars:

2% heavy elements

stars of all ages

what do your classmates see
What do your classmates see?

To answer this need to know a little of Einstein’s theory of Motion and Gravity:

  • Gravity is Motion in Warped Space - Time
  • You can’t tell the difference between acceleration by gravity and any other constant acceleration
  • E = mc2, energy and mass are same thing measured in different units
mass warps space time warped space time tells mass how to move
Mass warps Space - TimeWarped Space - Time tells Mass how to Move

Forget time, think just about warped space

orbits in warped space time
Orbits in Warped Space - Time

c = circular, e = elliptical, u = unbounded

gravity attracts light light generates gravity
Gravity Attracts LightLight generates Gravity

Reasonable since E = mc2

  • Black Holes Gravity attracts light
  • Light loses energy escaping from environs of a Black Hole. Convert radiation energy to gravitational potential energy.
  • Escaping Light is redshifted to longer wavelengths and periods
your classmates would see you slow down as you approached the bh event horizon
Your classmates would see you slow down as you approached the BH event horizon
  • Can use period of light as a clock
  • Redshifted light oscillates with a longer period
  • Time appears to run slower near event horizon
  • You would appear to stop and hover (& fade out) as you approached the Event Horizozn
what would you notice as you passed the event horizon
What would you notice as you passed the Event Horizon

Nothing special

  • For you time does not slow down in a BH.
  • You quickly crash into the previous matter inside the BH(But you couldn’t tell us about it)
what can we know about black holes
What can we know about Black Holes?
  • Nothing can escape from inside an Event Horizon
  • Long range forces can exert influence outside Event Horizon
    • Gravity
    • Electric Force
  • Can determine:
    • Mass
    • Charge
    • Spin
mini black holes can evaporate
Mini Black Holes can Evaporate

Mini BH produce strong tides (stellar BH don’t have strong enough tides)

  • Lose energy by work of tidal gravity on material outside the event horizon
  • Since energy = mass, they lose mass and get smaller
  • Evaporate
the milky way our galaxy
The Milky Way,our galaxy

Beginning of Unit IV: Cosmology

Milky Way - chapter 19

first idea count stars in different directions more stars larger extent what assumption is made
First Idea: Count stars in different directions,more stars -> larger extent.What Assumption is made?

Sun

Kapteyn Model of Milky Way, 1922

question 1 what assumptions were made
Question 1:What Assumptions were made?
  • Stars are clustered in a disk
  • Stars are evenly distributed in space
  • Stars are clustered near the Sun
  • We see all the stars in the Milky Way
question 1 what assumptions were made1
Question 1:What Assumptions were made?
  • Stars are clustered in a disk
  • Stars are evenly distributed in space
  • Stars are clustered near the Sun
  • We see all the stars in the Milky Way
what was missing
What was missing?
  • We don’t see all the stars, because some are hidden by interstellar clouds of gas & dust
  • The stars are not evenly distributed in space (but this is not as important)
slide23

Dusty gas clouds obscure our view because they absorb visible light

This gas is the

interstellar

medium that

makes new stars

slide24

Infrared light passes more easily through dusty gas clouds

This gas is the interstellar medium that makes new stars

milky way has disk shape
Milky Way has DISK shape
  • Stars are concentrated into a disk, but some stars above and below the disk
  • Neutral Hydrogen gas is concentrated in disk
question 2
Question 2:

Why do orbits of disk stars bob up and down?

A. They’re stuck to the interstellar medium

B. Gravity of disk stars pulls toward disk

C. Halo stars knock them back into disk

question 21
Question 2:

Why do orbits of disk stars bob up and down?

A.They’re stuck to the interstellar medium

B. Gravity of disk stars pulls toward disk

C.Halo stars knock them back into disk

life of a galaxy gas star gas cycle
Life of a Galaxy:Gas -> Star -> Gas cycle
  • Gas clumps together by gravity -> stars
  • Stars produce heavy elements by fusion
  • Stars die and return processed gas to space
  • Enriched gas clumps together by gravity
  • New stars
life of a galaxy gas star gas cycle1
Life of a Galaxy:Gas -> Star -> Gas cycle

Hot, ionized gas - one million K

Warm neutral gas - 10,000 K (most is here)

Cool neutral gas - 100 K

Molecular clouds - 30 K

Molecular cores - 6 K

Gas Cools

STAR FORMATION

slide37

X-ray map of sky shows hot gas high above and far below galactic disk

Hot gas eventually cools to form atomic hydrogen and settles into disk

slide38

Additional cooling makes the gas cold enough to form molecules

Hot gas eventually cools to form atomic hydrogen and settles into disk

slide39

Additional cooling makes the gas cold enough to form molecules

Darkest regions of Milky Way correspond to these dense clouds.

That is where stars form!

recycling stellar material
Recycling Stellar Material

Stellar Winds and Supernova return stellar material, as hot gas, to the Interstellar Medium (gas between the stars) ENRICHED in Heavy Elementsproduced during nuclear fusion & supernova explosions

slide46
Disk: ionization nebula & blue stars = star formationHalo: no ionization nebula or blue stars = no star formation
slide47

Halo Stars:

0.02-0.2% heavy elements (O, Fe, …)

only old stars

Disk Stars:

2% heavy elements

stars of all ages

where do stars form
Where do stars form?

Much of star formation in disk happens in

spiral arms

Whirlpool Galaxy

question 3 how can we study the center of the milky way
Question 3: How can we study the Center of the Milky Way?
  • By observing it in x-rays
  • By observing it in ultraviolet light
  • By observing it in visible light
  • By observing it in infrared light
  • By observing it in radio light

Choose all that apply

question 3 how can we study the center of the milky way1
Question 3: How can we study the Center of the Milky Way?
  • By observing it in (very energetic) x-rays
  • By observing it in ultraviolet light
  • By observing it in visible light
  • By observing it in infrared light
  • By observing it in radio light

Choose all that apply

question 4 will stars move faster closer to the center or farther away from it
Question 4: Will stars move faster closer to the center or farther away from it?
  • Closer
  • Farther
  • The same at all distances
what did we observe
What did we observe?
  • Stars moved fastest closest to the center
  • Similar Solar System
  • Speed decreases as 1/D
  • Point mass at Center of Galaxy
  • From velocity & distance can determine the MassM (inside distance D) = V2 D / G
  • Supermassive Black Hole, M ~3-4x106 Msun
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