Announcements• Reading for next class: Chapters 21 & 22• Cosmos Assignment 3,

Due Monday, April 191. Astronomy Place tutorial “Measuring

Cosmic Distances”, review and complete lessons, submit exercises

2. Astronomy Place tutorial “Hubble’s Law”, do all lessons and submit exercises

• Email me questions you would like me to address in class

Questions:

• Why do we bother measuring the distance of objects in the universe if they are all just constantly moving away from us? What is the signifigance?

• Differences between types of galaxies

Questions:

• Hubble’s law– Hubble’s constant– Its units– How to use it– What it means

• How can the universe expand if its size is infinite?

• What is a standard candle?

• Measuring cosmic distances

MilkyWay

Cartoon

Life of a Galaxy:Gas -> Star -> Gas cycle

Gravity clumps gas, makes stars Stars produce heavy elements by fusion Stars die and return enriched gas to ISM

Gas -> Star -> Gas cycle

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Halo: 0.02-0.2% heavy elements (O, Fe, …) only old stars, only hot no cold gas

Disk: 2% heavy elements, gas & dust, stars of all ages, spiral arms

Much of star formation in disk happens in spiral arms

Whirlpool Galaxy

Where do stars form?

Center of the Milky Way

• 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

The Larger Universe

1. Other galaxies

2. Measuring Cosmic Distances

3. Hubble’s Law

4. Age of the Universe

Hubble Ultra Deep Field

Space is crowded

with Galaxies

Spiral Galaxy

Ellipical Galaxy

Irregular Galaxy

What are galaxies?

• This was a question in the early 20th century

• Are they large collections of stars, like the Milky Way?

• Are they small objects, gaseous nebula or small collections of stars, that are part of the Milky Way?

• Great debate at the National Academy of Sciences in 1920.

What are Galaxies?

Galaxies are vast collections of stars (~1011) and sometimes gas and dust as well

Spiral Galaxy

disk bulge

halo

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

SpheroidalComponent(halo & bulge)

DiskComponent

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Spheroidal Component:bulge & halo, old stars,few gas clouds

Disk Component:stars of all ages,many gas & dust clouds

Spheroidal Component:bulge & halo, old stars,few gas clouds

Disk Component:stars of all ages,many gas clouds

Blue-White color indicates ongoing star formation.

Red-Yellow color indicates older stars

Question 1:

• Why does the blue-white regions indicate ongoing star formation

A. Young stars are bluish & whitish

B. Ionization nebula are bluish & whitish

C. Bluish stars have short lives

D. Reddish stars are old

Question 1:

• Why does the blue-white regions indicate ongoing star formation

A. Young stars are bluish & whitishB. Ionization nebula are bluish & whitishC. Bluish stars have short lives. Bluish stars

are hot, massive, highly luminous main sequence stars. Not many, but provide most light because so luminous.

D. Reddish stars are old

Question for next class:

Why are there different types of Galaxies?

Measuring Cosmic Distances

How do you ESTIMATE distances when you can’t use a ruler or tape measure?

Size (compared to expected size)

Location (in front or behind other objects)

Brightness (compared to expected brightness)

Astronomers use mostly BRIGHTNESS,

Need to know actual Luminosity

• For nearby stars in our own galaxy use PARALLAX (geometry not brightness)

• For remainder of our galaxy FIT MAIN SEQUENCE

• For nearby galaxies use bright pulsating stars, called CEPHEID VARIABLES

• For distant galaxies use white dwarf SUPERNOVA

Stellar Parallax

Main Sequence Fitting

Cepheid Variables

White Dwarf Super-nova

Why are WD supernova standard candles?

All happen for WD at mass limit of 1.4 Msun,

so properties similar.

Hubble Diagram

• V. M. Slipher first found all but few nearby galaxies are red-shifted.

• Edwin Hubble measured distances and red-shift = velocity away from us for nearby galaxies (1929)

Doppler Shift

Redshiftof

Galaxies

Hubble’sLaw

Hubble’s Results

Hubble Law Results (1994)

What does Hubble’s Law Mean?

C

What happens if it expands?

View

graph Experim

ent

Question 2:

If we now measure distances from E, will the points in the plot of change in distance vs. original distance fall along the same line or not?

A. Same Line

B. Different Line

View

graph Experim

ent

D=constant x D

D/D = slope = H

Space is Expanding

Expansion of the Universe

• The universe IS expanding• You and I are NOT expanding• The solar system is NOT expanding• The Milky Way Galaxy is NOT expanding• Our local group of Galaxies is NOT

expanding• Nothing that is bound together by a force is

expanding• SPACE between groups of galaxies IS

expanding

Question 3:

Suppose your friend calls you on their cell phone and says, “I have been driving at 70 mph straight away from you and I am now 140 miles distant.” How long did your friend take to get there, assuming they drove at a constant speed?

A. 1 hour

B. 2 hours

C. 3 hours

D. Can’t tell how long

Question 3:

Suppose your friend calls you on their cell phone and says, “I have been driving at 70 mph straight away from you and I am now 140 miles distant.” How long did your friend take to get there, assuming they drove at a constant speed?

A. 1 hour

B. 2 hours

C. 3 hours

D. Can’t tell how long

Question 4:

Suppose another friend calls you at the same time on their cell phone and says, “I have been driving straight away from you at 35 mph (broken down car) and am now 70 miles distant.” When did you second friend start?

A. Before your first friend

B. At the same time as your first friend

C. After your first friend

D. Not possible to tell when they started

Question 4:

Suppose another friend calls you at the same time on their cell phone and says, “I have been driving straight away from you at 35 mph (broken down car) and am now 70 miles distant.” When did you second friend start?

A. Before your first friend

B. At the same time as your first friend

C. After your first friend

D. Not possible to tell when they started

Question 5:

Suppose yet another friend with a hyperspatial communicator calls you from their galaxy and says, “I have been traveling away from you at a constant speed of 2x1011 km/yr and am now 2.6x1021 km distant from you.” How long have they been travelling?A. 13 million yearsB. 13 thousand yearC. 13 billion yearsD. 13 trillion years

Question 5:

Suppose yet another friend with a hyperspatial communicator calls you from their galaxy and says, “I have been traveling away from you at a constant speed of 2x1011 km/yr and am now 2.6x1021 km distant from you.” How long have they been travelling?

A. 13 million years

B. 13 thousand year

C. 13 billion years

D. 13 trillion years

Question 6:

Suppose another friend in a different galaxy tells you that they have been travelling twice as fast and are twice as far away, how long have they been travelling?

A. A shorter time

B. The same time

C. A long time

D. We can’t tell

Question 6:

Suppose another friend in a different galaxy tells you that they have been travelling twice as fast and are twice as far away, how long have they been travelling?

A. A shorter time

B. The same time

C. A long time

D. We can’t tell

Hubble’s Law

Velocity = Hubble’s Constant x Distance V = HDmeans the same thing. If you are twice as far away,you are moving away twice as fast, so you started moving away at the same time!

How long ago was that?