Orbits and Galaxy Shape

• elliptical orbits• random orientations• different speeds in

different directions

Orbits and Galaxy Shape

Halo and Bulge:• elliptical orbits• random orientations

Disk:• nearly circular orbits• same direction• a little “bobbing” up and down

Thought Question:Which of the following mergers of spiral and

elliptical galaxies is most unlikely?

orderly orbits in spiral galaxies are disrupted during collisions:

• collisions between galaxies affect star orbits

• many examples – especially in galaxy clusters

Environment

Stars versus Gas

“Collisions” of star groups don’t result in star collisions…

stars are small with lots of space between

BEFORE AFTER

Collisions of gas clouds result in “shocks” … kinetic energy converted to heat and radiated away

Galaxy Properties

Spiral Elliptical

Shapes flat disk + round bulge

elliptical (football)

Overall color bluish (young, high-mass stars)

reddish (giant stars)

Gas, Dust yes – in disk little or none

Where? mostly in emptier regions of space

often in clusters of galaxies

Zoom in on a seemingly blank piece of sky…

Observe galaxies to try to learn…

• what does the universe look like?

• what was its past?

• what is its future?

Cosmology

How Does the Universe Look?

Isotropic?: Does it look almost the same in all directions to us?

count galaxies in different directions

Homogeneous?: Would it look almost the same to observers anywhere?

measure distances to many galaxies

How Does the Universe Look?I feel left

out…Isotropic?: Does it look almost the same in all directions to us?

count galaxies in different directions

Homogeneous?: Would it look almost the same to observers anywhere?

measure distances to many galaxies

Thought Question

Which of these 4 universes is homogeneous?

(Enter the letters for all correct answers.)

Thought QuestionWhich of these 4 universes is isotropic according to the stick figure?

(Enter the letters for all correct answers.)

Is the Universe Isotropic?isotropic: looks the same in all directions to us

All-sky surveys of millions of galaxies show there are no special directions

blocking by Milky Way

Is the Universe Homogeneous? harder to measure: need to measure galaxy distances and

survey faint, distant galaxies

some “clumping” of galaxies on small scales, but there seem to be no special places

Is the Universe Homogeneous?

galaxies line up in filaments, with clumps where they meet

EVIDENCE: Expansion of the Universe (1928)

Almost all galaxies have redshifts

appear to be moving away from us

more distant galaxies have larger redshifts…

BLUE RED

REDBLUE

COINCIDENCE? HMMM…

WHEW! DID SOMETHING DIE

IN HERE?

Hubble’s Law

vr : recession velocity

d: distance

H0: Hubble constant

22 km/s / Mly

(1 Mly = 1 Mega light-year = 106 light-years)

Hubble’s Law(fast forward)

Consequence #1Consequence #1

what we see:

alien galaxy feels stationary to them

they observe the same Hubble Law!

Consequence #2Consequence #2All distances change by

the same percentage in the same time:

implies space is stretching uniformly to make Hubble’s Law

Imagine running time backwards:

galaxies would meet in same place at same time t in past (BIG BANG)

Consequence #3Consequence #3

THEORY: A “Big Bang”

Idea:

• universe had a beginning

• early universe was hot, dense gas

Tests:

• Did “Big Bang” leave evidence?

THEORY: a scientific idea that has survived repeated tests of its predictions

Evidence for a “Big Bang”

What clues do we have that there was a BEGINNING to the Universe?

• Dark Sky at Night

• Expansion of the Universe (Hubble’s Law)

• Cosmic Microwave Background Radiation

• Abundances of Chemical Elements

The Observable Universe

If universe is NOT infinitely old, then…

only see galaxies whose light had time to reach us

universe may be MUCH larger, but there’s an edge (“HORIZON”) to what we can see

more distant galaxies look younger to us: “LOOKBACK TIME”

OBSERVABLE UNIVERSE

UNOBSERVABLE UNIVERSE

Thought Question:Imagine you are observing a high-mass star located in a

distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years.

• How long before we receive light from this star’s supernova?

100 MILLION LIGHT-YEARS

?

“I see a blue star right now.”

(Enter your answers in millions of yrs.)

Thought Question:Imagine you are observing a high-mass star located in a

distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years.

• When will or when did the supernova actually occur?

100 MILLION LIGHT-YEARS

?

“I see a blue star right now.”

(Enter your answers in millions of yrs. If it already happened, make your answer negative.)

Lookback

100 MILLION LIGHT-YEARS

40 MILLION LIGHT-YEARS

“The star just blew up!!!”

“The star has been

dead for 60 million years.”

“The star looks like it

has 40 million years

to live.”

“DARK AGES”

UNOBSERVABLE UNIVERSE

(early in history)

How far back in time can we possibly look?

“DARK AGES”: before galaxies and stars were born

Universe was once OPAQUE gas too dense to let light through

Universe History

US

Stretching and RedshiftsRedshift tells us about total amount of stretching

universe has done while the light has been traveling

EXPANDING UNIVERSE:

PRESENT

DISTANT PAST

MEDIUM PAST

LARGER REDSHIFT FOR MORE DISTANT GALAXY

Universe ScaleWe can’t measure size of universe (especially if infinite), so

compare distances at different times in history:

Distances between non-moving galaxies stretch in same way light wavelengths do:

PRESENT:

PAST:

density of the universe was larger in the past:

RU =1 z=0: present

RU<1, z>1: past

Thought Question:The most distant object that has been seen has a

redshift of about z = 8. Roughly how far was it from us when it released the light we see today?

A. 9 times its present distanceB. 8 times its present distanceC. 7 times its present distanceD. Its present distanceE. 1/7th its present distanceF. 1/8th its present distanceG. 1/9th its present distance

EVIDENCE: Cosmic Microwave Background Radiation (1965)

Microwave and radio from all directions in sky with nearly equal brightness:

MAP OF ENTIRE SKY:

thermal radiation with very low temperature

T = 2.728 K

EVIDENCE: Cosmic Microwave Background Radiation (1965)

If Big Bang was an explosion at one place in space, we should see a glow only from that place…

BIG BANG seems to have happened EVERYWHERE

Cosmic Microwave Background Radiation

thermal radiation with temperature T = 2.728 K

Expansion of universe stretches light, made temperature lower today:

The Horizon ProblemOpposite sides of observable

universe somehow “knew” to have almost the same temperature today

takes longer than age of universe for light to travel from one side to the other…

HORIZON

Choose a number between 1 and 1 million.

538,681

538,681

INFLATION: age 10-35 s to 10-32 s space expands faster than light

can travel by huge amount (1050 times)

parts of then-observable universe move so far away we never see them again

needed to explain:

isotropic microwave background

homogeneous universe

History of the Universe

OUR OBSERVABLE UNIVERSE TODAY

THEN-OBSERVABLE UNIVERSE (BEFORE INFLATION)

OBSERVABLE UNIVERSE AFTER INFLATION

BEFORE:

AFTER: