Announcements Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14 Angel Quiz.

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)



All-Sky View



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



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

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

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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are needed to see this picture. hat is where stars form!

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



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



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

Announcements Reading for next class: Chapter 20 Cosmos Assignment 2, Due Wednesday, April 14 Angel Quiz.

Documents

gravity slide

halo stars

warped space slide

milky way slide

unbounded slide

spin slide

superbubbles slide

important slide