Announcements
• Reading for next class: Chapters 21 & 22
• Cosmos Assignment 3,
Due Monday, April 19
1. 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
Milky
Way
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
Halo:
0.02-0.2% heavy elements (O, Fe, …)
only old stars, only hot no cold gas
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Disk:
2% heavy elements, gas & dust,
stars of all ages, spiral arms
Where do
stars form?
Much of star
formation in disk
happens in
spiral arms
Whirlpool Galaxy
Center of the Milky Way
• Stars moved fastest closest to the center
• Similar Solar System
• Speed decreases as 1/D
Point mass at Center of Galaxy
• From velocity & distance
can determine the Mass
M (inside distance D) = V2 D / G
Supermassive Black Hole, M ~3-4x106 Msun
The Larger Universe
1.
2.
3.
4.
Other galaxies
Measuring Cosmic Distances
Hubble’s Law
Age of the Universe
Ellipical Galaxy
Space is
crowded
with
Galaxies
Hubble
Ultra
Deep
Field
Irregular Galaxy
Spiral 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
halo
disk
bulge
Spiral Galaxy
Disk
Component
Spheroidal
Component
(halo & bulge)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Disk Component:
stars of all ages,
many gas & dust clouds
QuickTime™ and a
TIFF (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 clouds
Spheroidal Component:
bulge & halo, old stars,
few 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.
B.
C.
D.
Young stars are bluish & whitish
Ionization nebula are bluish & whitish
Bluish stars have short lives
Reddish stars are old
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. 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
Supernova
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 redshift = velocity away from us for nearby
galaxies (1929)
Doppler
Shift
Redshift
of
Galaxies
Hubble’s
Law
Hubble’s Results
Hubble Law Results (1994)
What does Hubble’s Law Mean?
B
A
C
What happens if it expands?
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
DD=constant x D
DD/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.
B.
C.
D.
1 hour
2 hours
3 hours
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.
B.
C.
D.
1 hour
2 hours
3 hours
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.
B.
C.
D.
Before your first friend
At the same time as your first friend
After your first friend
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.
B.
C.
D.
Before your first friend
At the same time as your first friend
After your first friend
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.
B.
C.
D.
13 million years
13 thousand year
13 billion years
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.
B.
C.
D.
13 million years
13 thousand year
13 billion years
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.
B.
C.
D.
A shorter time
The same time
A long time
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.
B.
C.
D.
A shorter time
The same time
A long time
We can’t tell
Hubble’s Law
Velocity = Hubble’s Constant x Distance
V = HD
means 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?