Announcements Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14 Angel...
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Transcript of Announcements Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14 Angel...
Announcements• Reading for next class: Chapter 20• Cosmos Assignment 2,
Due Wednesday, April 14Angel 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
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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?
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
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Mass warps Space - TimeWarped Space - Time tells
Mass how to Move
Forget time, think just about warped space
Orbits in Warped Space - Timec = circular, e = elliptical, u = unbounded
Elevator & Rocket
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Gravity = AccelerationLight Beam in an Elevator or Gravity
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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
• 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
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?• Nothing can escape from inside an Event
Horizon• Long range forces can exert influence
outside Event Horizon1. Gravity2. Electric Force
• Can determine: 1. Mass2. Charge3. Spin
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
Beginning of Unit IV: Cosmology
Milky Way - chapter 19
What does my building look like?
Milky Way
small portion from the
winter sky
First Idea: Count stars in different directions,
more stars -> larger extent.What Assumption is made?
apteyn Model of Milky Way, 1922
Sun
Question 1:What Assumptions were made?
A. Stars are clustered in a disk
B. Stars are evenly distributed in space
C. Stars are clustered near the Sun
D. We see all the stars in the Milky Way
Question 1:What Assumptions were made?
A. Stars are clustered in a disk
B. Stars are evenly distributed in space
C. Stars are clustered near the Sun
D. We see all the stars in the Milky Way
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)
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All-Sky View
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Mosaic View, does give good idea of MW structure
Familiar Constellations
Dusty gas clouds obscure our view because they absorb visible light
This gas is the interstellar medium that makes new stars
Infrared light passes more easily through dusty gas clouds
This gas is the interstellar medium that makes new stars
Infrared Light
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
MilkyWay
Cartoon
Stars in the disk orbit in the same direction with a little bobbing up & down
Stars in the bulge &
halo have randomly oriented
orbits
Question 2:
Why do orbits of disk stars bob up and down?
A. They’re stuck to the interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk
Question 2:
Why do orbits of disk stars bob up and down?
A. They’re stuck to the interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk
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 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 KSTAR FORMATION
Gas
Coo
ls
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X-ray map of sky shows hot gas high above and far below galactic disk
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Hot gas eventually cools to form atomic hydrogen and settles into disk
X-ray map of sky shows hot gas high above and far below galactic disk
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Hot gas eventually cools to form atomic hydrogen and settles into disk
Additional cooling makes the gas cold enough to form moleculesQuickTime™ and a
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Darkest regions of Milky Way correspond to these dense clouds.
Additional cooling makes the gas cold enough to form moleculesQuickTime™ and a
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Hot, Blue, Massive MS starsheat and ionize the gas around them
Massive, hot, blue MS stars ionize the gas around them
Produce ionization nebula
Recycling Stellar
Material-Low mass stars blow
off winds & eject
envelopes
Recycling stellar
Material -High mass
stars explode as supernova
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
Gas -> Star -> Gas cycle
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Disk: ionization nebula & blue stars = star formationHalo: no ionization nebula or blue stars = no star formation
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Halo Stars: 0.02-0.2% heavy elements (O, Fe, …) only old stars
Disk Stars: 2% heavy elements stars of all ages
Much of star formation in disk happens in spiral arms
Whirlpool Galaxy
Where do stars form?
Question 3: How can we study the Center of the Milky Way?
A. By observing it in x-rays
B. By observing it in ultraviolet light
C. By observing it in visible light
D. By observing it in infrared light
E. By observing it in radio light
Choose all that apply
Question 3: How can we study the Center of the Milky Way?
A. By observing it in (very energetic) x-rays
B. By observing it in ultraviolet light
C. By observing it in visible light
D. By observing it in infrared light
E. By observing it in radio light
Choose all that apply
Center of the Milky Way
Question 4: Will stars move faster closer to the
center or farther away from it?
A. Closer
B. Farther
C. The same at all distances
What happens
in the Solar
System?
Star Motions near the Center of the Milky Way
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Star Motions near the Center of the Milky Way
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What did we observe?
• Stars moved fastest closest to the center• Similar Solar System• Speed decreases as 1/DPoint 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