THE COSMOS:
STRUCTURE &HISTORY
• Where are we in the universe?
• What do we know about the scale of the
universe?
• What do we know about the past history
of the universe?
Billions and Billions
(Powers of Ten)
HST image of the center of our Galaxy:
each star is comparable to our Sun
[This image is linked to Study Guide1.
Try counting the stars here.]
Informal Survey:
Estimate the number of songs (mp3 files) you have
stored on your iPod or other digital memory unit.
(A) Less than 500
(B) About 1000
(C) About 1500
(D) About 2000
(E) More than 2500
Clark Hall could accommodate
students carrying one million song files
It would take 1000 Clark Halls
to accommodate one billion song files
It would take 1000 Clark Halls
to accommodate one billion song files
It would take 1000 Clark Halls
to accommodate one billion song files
It would take 1000 Clark Halls
to accommodate one billion song files
--- 12x more than shown here
Astronomers use light travel time
to characterize distances.
Light speed: 186,000 miles per second, or
300,000 km per second, or
1 foot per nanosecond
• Separation of Earth and Moon?
Using light travel time as a measure of
distance to the Earth’s Moon
Earth and Moon to scale; separation is 238,000 miles
Light travel time is 1.26 sec.
Therefore, we can say the distance to Moon is also
“1.26 light-seconds”
Earth-Moon System
1.3 light seconds
Inner Solar System
The “Light Year”
• One light year is the DISTANCE light travels in one year
• That is: 186,000 mps x 3600 s/h x 24 h/d x 365 d/yr
• = 6 trillion miles, or 6x1012 miles in powers of ten notation
• Written out: 6,000,000,000,000 miles
• This is a convenient measure of distances between stars
Nearest star:
4.2 LY distant
Distribution of nearby stars
P
Puzzlah #5
The signal from the classic TV program ”Gilligan’s Island” has
been propagating outward from the Earth since 1965 at the
speed of light. Suppose there was an intelligent alien species
in a nearby planetary system that picked up the signal and
sent back a message of lavish praise. (OK—maybe not so
intelligent.) Suppose the return signal was received in 2005.
What is the maximum distance of the aliens from Earth?
(A)
(B)
(C)
(D)
(E)
1 billion miles
40 billion miles
1 light year
20 light years
40 light years
Puzzlah #5
The signal from the classic TV program ”Gilligan’s Island” has
been propagating outward from the Earth since 1965 at the
speed of light. Suppose there was an intelligent alien species
in a nearby planetary system that picked up the signal and
sent back a message of lavish praise. (OK—maybe not so
intelligent.) Suppose the return signal was received in 2005.
What is the maximum distance of the aliens from Earth?
(A)
(B)
(C)
(D)
(E)
1 billion miles
40 billion miles
1 light year
20 light years
40 light years
Distribution of nearby stars
P
Distribution of nearby stars
Structure of local regions
in the Galactic disk
Our "Milky Way" Galaxy:
A spiral galaxy
Face-on
25,000 ly
A flattened disk
Central bulge
Spherical halo
Bright stars form spiral
structure
Sun far from center
Edge-on
Our "Milky Way" Galaxy:
A spiral galaxy
Face-on
A flattened disk
Central bulge
Spherical halo
Bright stars form spiral
structure
Sun far from center
Contains about
100 billion stars
Edge-on
P
Puzzlah #6
Have we been able to photograph our own Galaxy
(the Milky Way Galaxy) from outside?
(A) Yes
(B) No
Puzzlah #6
Have we been able to photograph our own Galaxy
(the Milky Way Galaxy) from outside?
(A) Yes
(B) No
Milky Way Galaxy from Outside (Artist’s)
Galactic plane seen
from inside the plane
Summer sky view (180 degrees) of the "Milky Way"
The Local Group of Galaxies
Magellanic Clouds
Milky Way
(our galaxy)
Magellanic Clouds
(other galaxies)
The Andromeda Galaxy ("M31")
Foreground stars
The Andromeda Galaxy ("M31")
The "Lookback Effect":
An Astronomical Time Machine
The finite speed of light combined with
the huge distances to galaxies means
that we see them as they were in the past
The Andromeda Galaxy
Distance: 2 Million Light Years
The Andromeda Galaxy
Lookback time: 2 Million Years
By looking out in space, we are looking back in time.
Astronomy is the only human endeavor in which we
are actually able to see the past.
Beyond the Local Group
A “slice” of the nearby universe (Sloan Digital Sky Survey)
Each dot is a galaxy. Note lookback timescale.
2 Byr
lookback
Local Group
Interactive cosmic scale (APOD)
AMNH Trip to the Edge of the Universe
"Hubble Ultra Deep Field"
600-hour exposure of distant galaxies
"Hubble Ultra Deep Field"
600-hour exposure of distant galaxies
"Hubble Ultra Deep Field"
The faintest galaxies are ~13 BLY away
Puzzlah # 7
The number of stars in the observable universe is
(A) about the same as the number of grains in
a bucket of sand
(B) about the same as the number of grains of
sand on a typical beach on Earth
(C) about the same as the number of grains of
sand on all the beaches on Earth
(D) infinite, because the universe is infinite.
What is the total number of stars
in the observable universe?
P
What is the total number of stars
in the observable universe?
• The maximum distance to which we can make
observations is about 14 billion light years.
What is the total number of stars
in the observable universe?
• The maximum distance to which we can make
observations is about 14 billion light years.
• Within this volume, there are about 100 billion
galaxies, containing (on average) about 100
billion stars each.
What is the total number of stars
in the observable universe?
• The maximum distance to which we can make
observations is about 14 billion light years.
• Within this volume, there are about 100 billion
galaxies, containing (on average) about 100
billion stars each.
• The total number of stars in the
observable universe is therefore about
10 billion trillion or 1022
The number of stars in the observable universe
is about the same as the NUMBER OF
GRAINS OF SAND
on ALL THE BEACHES on the face of the Earth.
Imagination
Mathematics
Intuition
Infinity: a reality?
PUZZLAH #8
If the universe is infinite in spatial extent, as we believe
it to be, what fraction of its volume are our most
powerful telescopes able to probe?
(A)
(B)
(C)
(D)
90%
50%
A very small amount, a few %
Zero
PUZZLAH #8
If the universe is infinite in spatial extent, as we believe
it to be, what fraction of its volume are our most
powerful telescopes able to probe?
(A)
(B)
(C)
(D)
90%
50%
A very small amount, a few %
Zero
Earth in the Context
of
Cosmic History
Puzzlah #8
In what way does life on Earth depend on the stars?
(A) A star (the Sun) provides essential energy
inputs to our biosphere.
(B) Planets form as byproducts of star formation
(C) Essential organic materials (e.g. C, N, O)
originate inside stars.
(D) All of the above
(E) None of the above
Puzzlah #8
In what way does life on Earth depend on the stars?
(A) A star (the Sun) provides essential energy
inputs to our biosphere.
(B) Planets form as byproducts of star formation
(C) Essential organic materials (e.g. C, N, O)
originate inside stars.
(D) All of the above
(E) None of the above
Cosmic History “Top Ten”
• The universe began in a “Big Bang”, an ultra-hot/dense state, 14
billion years ago, and has been expanding ever since
• Present day structure = initial irregularities amplified by gravity
• Basic “building block”: galaxies (ours is not special)
• Stars form continuously from interstellar gas
• The Sun is a star with average properties
• Stars generate energy by fusion of hydrogen nuclei
• Chemical elements are synthesized by nuclear reactions in stars
• Planets are a byproduct of star formation; there are billions of
planetary systems in our galaxy
• Earth is a planet in orbit around the Sun
• Earth’s biosphere is highly vulnerable to cosmic phenomena
Cosmic History “Top Ten”
The universe began in a “Big Bang”, an
ultra-hot/dense state, 14 billion years
ago, and has been expanding since. The
universe was then and is now spatially
infinite.
Present day structure = initial
irregularities amplified by gravity
Basic “building block”: galaxies (ours
is not special)
Cosmic History “Top Ten”
• Stars form continuously from interstellar
gas
• The Sun is a star with average
properties
• Stars generate energy by fusion of
hydrogen nuclei
Cosmic History “Top Ten”
• Chemical elements (above helium) are
synthesized by nuclear reactions in
stars
• Planets are a byproduct of star
formation; there are billions of
planetary systems in our galaxy
• Earth is a planet in orbit around the
Sun
Cosmic History “Top Ten”
• Earth's biosphere is highly vulnerable
to cosmic phenomena
Cosmic History “Top Ten”
• Earth's biosphere is highly vulnerable
to cosmic phenomena
-- End --