A. Our Galaxy, The Milky Way

B. Galaxies, Normal & Peculiar

C. Observations of the Universe

D. Evolution of the Universe

E. Formation of the Universe

F. Fate of the Universe

A. The MILKY WAY, OUR GALAXY

What do other nearby galaxies look like?

Mostly they come in 3 types:

Irregulars - Magellanic Clouds [BE 3052,3055]

Spirals - Andromeda [Be 2723, 2727]

Ellipticals - companions to Andromeda

Note: color of stars, dust, globular clusters

Which type is the Milky Way?

1. Recognition of the Nature of the Milky Way

a. Band of Light across the sky - Thin Disk of Stars [BE 2659]

b. Star distribution - sun near center
"Kapteyn Universe" of 1922

c. Globular Cluster distribution

BE 1963,1965,1966,1967,1968]

spherical distribution, diameter 10⁵ LY, centered off to one side

Sun =3x10⁴ LY from center of Galaxy

d. Interstellar dust - obscures distant stars

e. Island Universe vs. One of Many Galaxies

2. Components of the Milky Way

a. Disk

Stars of 0-10 billion-years old, includes Open (galactic) clusters.
[Visible Light BE 2659]

Clouds of dust. (IR shows dust BE 2669)

Clouds of gas. [Radio: neutral hydrogen BE 2661, cold H₂ BE 2664]

Orbits nearly circular, lie in common plane.[BE 2686]

(Similar to planets in solar system.)

Spiral Arms

Overhead view [BE 2650-2652], "Exploded" Structure

Apparent in

massive, young, blue, main sequence stars (not intermediate age stars like the Sun),

Massive stars have such short lives, don't have time to move from their birth place.

H gas distribution - radio observations

Birthplace of Stars

Density Wave Theory

spiral arms are compression waves that move through galaxy gathering stars and clouds.

"Disk in a Dishpan" demo showing disk stars move like a fluid

Rotation curve shows the speed of gas depends on place in disk.

b. Spherical Component

i. Halo

Old (10-18 Billion years) Stars, mostly red giants and red MS stars.

Globular Clusters. (Little gas or dust.) [Andromeda: BE 2726]

Orbits elongated ellipses, randomly oriented. (Similar to comets in solar system.)

ii. Nuclear Bulge (flattened spheroid)

Bulge at center of Galaxy [in direction of Sagittarius: BE 2686].

Old and young stars.

Hot Gas and Dust.

iii. Center of Galaxy

High velocity clouds of hot gas

5x10⁵ M_Sun within 10 AU. Black hole?

[BE 2585,2589,2602]

c. Companion Galaxies

Magellanic Clouds, dwarf spheroidals

Diagram of Milky Way

   

4. Mass of the Milky Way

Kepler's Law M=D³ / P²

Rotation curve

M(R)=V²R/G

5. Formation Models

Must explain the structure and ages of components

B. GALAXIES

1. Classification by Shape (Morphology)

Galaxy images

a. Spirals (~15%)
   

i. Disk

Young stars, gas and dust

Rotates

ii. Spherical component - Halo and Nucleus

Old stars, no gas or dust

S0 galaxies - unusual variety of spiral

i. Disk - Old stars, no gas or dust

ii. Spherical component - Nucleus and Halo

b. Ellipticals (~70%)

Spherical component - Nucleus and Halo

Old stars, no gas or dust

No disk. Some rotate.

c. Irregular galaxies - like Milky Way's Magellanic Clouds

d. Unusual Galaxies

look like one of the 3 types above, but disturbed

Chain or Merging #39 from HST DEEP

2. Classification by Spectra

a. Normal galaxies (~95%)

stellar spectrum - absorption lines in continuous spectrum

b. Active galaxies (~5%)

non-thermal emission

i. Radio Galaxies

Excess radio emission.

Synchotron radiation from very fast electrons moving through a magnetic field

Double lobed large emission regions. Often connected to galaxy by jets.
   

ii. Quasars

Broad emission lines. Large redshift -> large distance. None close.

Bright yet far away -> very luminous. Source of energy?

Brightness varies in time of hours -> small size (= solar system)

Quasars occur in galaxies

A gallery of quasar images

c. Center of Milky Way

Radio emission

Broad emission lines -> high speed (orbital or thermal) -> high mass (= 10^6 Msun in size of solar system)

d. Model

Need large amount of energy

Only source is Gravitational Potential Energy of supermassive Black Hole

Gas falls in, gets very hot, emits energetic photons, Rotates rapidly, collimates jets

Evidence: rotation speed of gas and stars near centers of galaxies.

3. Large Scale Distribution

Clusters of galaxies.

Spirals found mostly in field, small groups, and outer regions of clusters.

Ellipticals and S0 dominate in centers of rich clusters.

Galaxies distributed as on surfaces of intersecting bubbles.

Most galaxies on arcs where bubble surfaces intersect,

Next most galaxies on bubble surfaces (sheets),

Fewest galaxies in interior of bubbles (voids).

Virgo Cluster Coma Cluster

4. Formation

Clues:

different orbits of old and young stars

current interactions with satellite galaxies

1. Theory 1: Top Down

Largest mass clouds collapse first -> clusters of galaxies.

Fragment into galaxies.

Stars formed during contraction have elongated, spherically distributed orbits.

As gas contracts, rotates, forms disk. Stars formed from the disk have disk-shaped orbits.

2. Theory 2: Bottom up

Globular cluster mass clouds collapse first.

Gravitationally cluster into larger and larger hierarchies to form galaxies and clusters of galaxies.

Tests

Observations indicate may need both theories

Galaxy mergers common
Cannibal galaxy

Spirals probably form by collapse of radiatively cooling gas cloud.

Ellipticals probably form by mergers of spirals

  Simulation of Merger

Ancient Galaxies from Hubble

HST DEEP Field is a sample of galaxies back to 1 billion years after the big bang from HST. Section of sky about as big as the period after this sentence.

Interactive HST DEEP Field Same galaxies back to 1 billion years after the big bang from HST with information on individual galaxies available by clicking on them.

C. OBSERVATIONS of the UNIVERSE

1. Fair sample

Universe is uniform and isotropic

When look out in space are looking back in time, takes time for light to travel from far away to us

2. Expansion of Universe

All but nearest galaxies are red shifted - moving away from us (Doppler shift)

Universe is Expanding

Hubble Law

Velocity = Hubble constant x Distance

Determining the Hubble constant

measuring velocities: Doppler shift

measuring distances:

geometry - parallax

brightness -

cluster main sequences

Cepheid variables

Cepheid Variables in M100 from HST

globular clusters

HII regions

brightest galaxies in clusters

Redshift -

Age of Universe

V = D/t = H D or D = Vt = V/H

H = 1/t

Farther away, moving away faster, took same time to get there

Implication: everything started from same place at same time

t(age) = 1/H (must be careful of units)

Original Hubble distances 10 x too small. Found age ~ 2 billion years. Less than age of oldest rocks on Earth.

Current distance scale 10 x larger, age 15-20 billion years.

No center to universe

Demo: blowing up balloon, stretching rubber band

Space is expanding, not bound objects

Neither we, nor Earth, nor the Sun or Solar system, nor the Milky Way or any galaxy, nor the clusters of galaxies are expanding. Only the distance between clusters of galaxies and isolated galaxies is expanding.

Scale of the Universe

if Milky Way is size of earth, then distance to horizon is 17 AU (size of Uranus orbit)

3. Night sky is dark

Olber's paradox

If look out, line of sight should eventually hit a star

Like looking through forest - eventually blocked by a tree

Sky should be as bright as surface of star. Why isn't it?

Universe has only existed for finite time

Can't see far enough away to be blocked by star

Universe is expanding

Light from distant stars is red shifted to very low energy

D. EVOLUTION of the UNIVERSE

1. Steady State Model

Problem: Earth can't be older than universe.

expansion age originally determined by Hubble was 2 billion years,

age of oldest rocks on earth is 3.5 billion years.

Solution: Universe has existed forever. Average properties of the universe do not change.

Galaxies move apart, new hydrogen atoms form to fill space, condense into new galaxies.

Developed theory of heavy element formation in stars.

Disproved, but was productive

Prediction: universe does not change

Test: Quasars seen only long ago (far away).

2. Big Bang Model

Expansion -> cooling.

In the past, matter and radiation were hotter and denser.

Scenario

Era of Equilibrium

Very hot. All nuclear reactions very fast.

Photons have enormous energy, destroy nuclei. Too hot for nuclei.

Era of Primordial Nucleosynthesis, 3 min < t < 30 min:

10⁹ > T > 10⁷ K.

Cool enough for nuclei (photons have too little energy to destroy nuclei),

Fuse protons and neutrons -> deuterium and helium.

Scale of universe = 10^-9 to 10^-7 present

Era of Radiation, 30 min < t < million years:

Too cold for fusion, too hot for atoms.

Free electrons, protons, helium nuclei and photons.

Universe opaque.

Recombination, Decoupling, t = million years:

T = 3000 K

Cool enough for atoms. Electrons and nuclei combine.

Universe becomes transparent.

Matter and radiation no longer in thermal equilibrium with each other

Scale of universe = 10^-3 present

Era of galaxies, t > million years:

Clouds of hydrogen and helium gas contract to form galaxies.

Predictions:

photons from when universe was hot

helium and deuterium from primordial nucleosynthesis

3. Tests: relics of the big bang

(i) 3 K Background Radiation - photons reaching us from when universe became transparent

a. Existence of 3 K background radiation means universe was once hot.

b. Present temperature allows us to calculate temperature of early universe.

c. Uniformity of background radiation shows universe was very uniform.
Cobe Results

d. Determine our motion through universe

(ii) Helium and Deuterium

E. FORMATION of the UNIVERSE

1. Problems with the Big Bang

a. Why Hot?

b. Why so uniform - 3 K radiation?

c. Why nearly flat?

d. Origin of irregularities that become stars and galaxies?

2. Solution -- Inflation

Very early universe (Equilibrium Era) expanded enormously

smooths out fluctuations, flattens space-time.

3. Singularity?

Infinite density, singularity, at beginning of universe implies gravity so strong quantum effects important

Classically: either existed forever, or else began in singularity

Quantum mechanics: possible for space-time to be finite, but have no boundary, no singularity

Example: Vertex of cone (origin of time) vs. N pole on sphere (not a special location)

Time just like spatial coordinates, if beginning of time rounded like sphere then not a special point

Implication: universe self-contained, Not created or destroyed. Just is.

F. FATE of the UNIVERSE

1. Expansion velocity vs. Escape velocity

Is universe expanding fast enough to continue forever, or will gravitational attraction of all matter for each other slow it down enough to bring expansion to halt and start universe recontracting?

How determine Fate of universe?

Measure Expansion Velocity

Determine Hubble Constant, H

Recent Hubble Space Telescope Results

Measure Escape Velocity (strength of gravity)

a. measuring mass density

• galaxies -> expansion velocity = 10 x escape velocity
• clusters of galaxies -> expansion velocity = 3 x escape velocity

b. measuring the deceleration due to gravity
number density of galaxies as a function of expansion velocity -> expansion velocity = escape velocity

c. deuterium abundance depends on density of matter
-> expansion velocity = 3 x escape velocity

2. Geometry of the Universe

a. If expansion velocity > escape velocity

Universe will expand forever

Universe is infinite

Universe has saddle like geometry

b. If expansion velocity = escape velocity

Universe will expand forever

Universe is infinite

Universe has flat geometry

c. If expansion velocity < escape velocity

Universe will stop expanding, and contract

Universe is finite, but has no boundary

Universe has sphere like geometry

3. A Detailed Prediction of Fate of the Universe under 2.(a) or 2.(b) Geometry

a. 10^0-4 billion years: Radiation-Dominated Era

assumes the Big Bang Formation Scenario as above

Ends at Recombination

b. 10^6-14 billion years: Stelliferous Era

Galaxies (groups of stars) are the building blocks of the Universe

Most star formation occurs when galaxies collide, which is common

We are now at 10^10.2

Ends when no new stars form

c. 10^15-37 billion years: Degenerate Era

all that's left is remnants
from stars are black holes, white dwarfs, neutron stars, planets and failed stars
from galaxies with super-massive black holes, only the black hole

Ends when the protons decay and destroy everyything with protons

d. 10^38-100 billion years: Black Hole Era

only black holes, as they have no protons

Ends when the black holes evaporate via Hawking radiation

e. 10¹⁰⁰⁺ billion years: Dark Era

photons (low-energy), electrons, postitrons, neutrinos

never ends

[American Scientist, May-June 1997, pp 223-225]

[Laughlin et al., Reviews of Modern Physics, April 1997]

"To doubt everything or to believe everything are two equally convenient solutions:
            both dispense with the necessity of reflection."

                            - Henri Poincare' (1854-1912)

Wendy Freedman - The Expansion Rate and Size of the Universe (Sci.Amer.)

mpeg movies of interacting galaxies

Hubble Space Telescope DEEP Field: Interactive Mode

Collision of Milky Way and Andromeda

Galaxies from Univ. Alabama

Cosmic Background Radiation

Index of Astronomy Pictures of the Day

The Big Bang by Mike Guidry

Introduction to Cosmology

Cosmology: a Research Briefing