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Midterm Exam #2
Tuesday, April 20
Closed book
Will cover Lecture 15 (Stellar Evolution) through Lecture 21 (Galaxy
Evolution) only
If a topic is in the book, but was not covered in class, it will not be on the
Some combination of multiple choice, short answer, short calculation
Equations, constants will all be given
Standard calculators allowed (but not provided!)
Cell phones, PDAs, computers not allowed
Selected Questions from Minute Papers
• Are there such things as parallel universes?
• How is the initial state of the universe different from a black hole?
• Did the universe start expanding immediately after the big bang?
• What started the big bang anyway?
• Will the universe expand forever?
• Will the universe ever completely cool down?
Outline - April 13, 2010
• Big Bang predictions recap
• Galaxies “Here and Now” vs. “There and Then”
• The “faint blue excess” galaxies
• Nature vs. Nurture for establishing galaxy morphology
• Active Galaxies and QUASARS
The Big Bang Concept
The universe began in an extremely hot,
extremely dense state
A fantastic notion, but also a truly testable hypothesis.
Big Bang Predictions Scorecard (so far)
• The universe had a specific beginning (night sky is dark;
Hubble’s Law; approximate age = 1/H0)
• The universe is expanding (Hubble’s Law)
• The universe has evolved/changed over time
• Initially, the universe was extremely hot, dense, and opaque
• Cosmic objects (such as stars) should have a chemical
composition that is roughly 75% hydrogen and 25% helium
The Universe is a Time Machine!
Because the speed of light is finite (c = 3x105 km/s), we see all
objects as they looked some time in the past!
“lookback time” = distance / c
Examples of “lookback times”
• Earth-Sun distance is 1.5x108 km, so at this very moment we see the sun
as it looked 500 sec = 8.3 min ago
• distance to the closest star is about 3 light years, so when we look at
this star we see it as it looked 3 years ago
• distance to our “sister galaxy” (M31) is about 2.6 million light years, so
when we look at this galaxy we see it as it looked 2.6 million years ago
• distance to the farthest known galaxies in the universe is about 12 billion
light years, so we see these galaxies as they looked 12 billion years ago
(before the Earth even existed!!!!!)
The light that you see tonight from Dubhe
left the star before your Grandmother
was born! It also left Dubhe 43 years
before the light that you see tonight from
Alioth left Alitoth.
Note: this slide has been
changed in the on-line notes
How can we study the evolution of the universe?
(the universe is REALLY old - 13.8 billion years)
Problem: Humans live for a short time (100 years) compared to
the age of the universe
We don’t have the luxury of watching the universe undergo
changes over our lifetimes (it actually changes very little on
time scales less than a few million years)
Tactic: Study vast collections of objects in the universe (i.e.,
galaxies) that are located at different distances (= different
lookback times) and compare them to each other
Are the galaxies that are located 5 to 10 billion light years from
us significantly different from the galaxies that are only a few
million light years away?
The universe is full of galaxies!
An image of an essentially
random region of the sky.
There are over 2000
galaxies in the image, and
in the entire universe there
are at least 100 billion
galaxies in the
observable universe.
The lookback times to the
galaxies in this image
range from 0.5 billion to 9
billion years.
Galaxies “Here and Now”
Large “modern day” galaxies (those with the smallest lookback times)
are usually observed to be regular systems that don’t look particularly
disturbed. Most of the small “modern day” galaxies look quite irregular.
In very rough numbers, “large” galaxies have 10 billion stars or more,
“small galaxies” have 1 million to 100 million stars.
Examples of spiral galaxies, “here and now”
More galaxies, “here and now”
Examples of
elliptical galaxies
(“red and dead”)
Examples of irregular
galaxies (small)
Large Magellanic Cloud
Galaxies “There and Then”
When you back in time 5 to 7 billion years and study the galaxies at that
time, you find:
• spiral galaxies were intrinsically brighter than they are today (by about a
factor of 2)
• elliptical galaxies were intrinsically brighter than they are today (by about
a factor of 5) and they were much bluer in color due to presence of blue
stars in the past
• there was a higher fraction of “disturbed” looking galaxies compared to
the present day
• there were 2 to 3 times more bright galaxies in the universe than there are
today (Faint Blue Excess Problem)
Disturbed Galaxies, 5 to 7 billion years in the past
If you squint just right you can see hints of spiral arms in some cases, but
these galaxies are lumpy and bumpy compared to galaxies “today”.
“Faint Blue Excess” Galaxies
5 to 7 billion years ago, there were 2 to 3
times more bright galaxies than we see
around us at the present day
These were mostly very small, irregular
galaxies that were making a huge
amount of young stars in the distant
We don’t see these guys “here and now”
because they made so many stars so
fast that they burned themselves out
Galaxies 7 to 12 Billion Years Ago
• galaxies existed back then! (big surprise to theorists)
• spirals and ellipticals were extremely rare
• galaxies were much smaller than they are today (factors of 3 to 20
smaller) and almost all are badly disturbed/lumpy
• there were far fewer galaxies in the universe than there are today, and
the farther back you go (beyond 7 billion years), the fewer you find
Galaxies 7
to 9 billion
years ago
Galaxies 10
to 12 billion
years ago
Clearly, galaxies have changed over the age of the universe
Galaxy Formation
Things the Big Bang does not directly predict:
How did galaxies form?
How was the morphology (spiral, elliptical, irregular) established?
Which is more important: nature or nurture?
Note: compared to their diameters, galaxies are very close to each
other, so chances of them going bump in the night are very high.
Stephen’s Quintet
a group of galaxies
Interacting & Colliding Galaxies
Importance of Galaxy Collisions for Establishing
• If you leave a large “protogalaxy” alone, it will most likely turn
into a spiral galaxy (by conservation of angular momentum)
• Disks of galaxies are very fragile; if they get hit with anything
bigger than 10% of their own mass, they are destroyed (and
don’t come back)
• Collisions of galaxies in clusters was much more common in
the past than at present
Formation of a Spiral Galaxy
Collision of Two Spiral Galaxies
QuickTime™ and a
are needed to see this picture.
Galaxy Collisions in a Distant Galaxy Cluster
Note: most elliptical
galaxies probably
formed by the
collision of many (5
to 10) clumps of gas
and not simply the
collision of two spirals
“lookback time” is about 9 billion years
“Active” Galaxies
More evidence for galaxy evolution
At the present day, the light from most galaxies comes primarily from
ordinary processes: stars, gas, dust
In the distant past, many large galaxies emitted a large amount of light from
processes associated with the supermassive black holes at their centers
(“Active” Galaxies)
Active Galaxies were common in the past (5 to 10 billion years ago), but
they are rare at the present day
As supermassive black holes consume material nearby to them, tremendous
amounts of light are emitted, both from the “accretion disk” and from jets
of material that are spewed out of the center of the galaxy
M87 - one of the biggest galaxies in the universe
M87 lives at the heart of
the Virgo Cluster and is
actually quite close by (the
lookback time is small)
This is a badly overexposed optical image
M87 - Short Time Exposure
Taking a short time exposure
photograph reveals a jet of material
emerging from the interior of M87
Supermassive Black Hole at the Center of M87
Speed of rotation of disk of
material nearby the black
hole shows that its mass is
about 3 billion times the
sun’s mass
M = (v2 r) / G
Centaurus A
another nearby active galaxy
If the dust weren’t there,
this would probably be
classified as an elliptical
galaxy. Instead it’s
usually called “peculiar”.
“Normal” galaxies don’t have jets of
material spewing out from their centers!
What’s what?
The X’s show the images
of “point sources” of light
Stars are “point sources”
of light, but they’re not the
only point sources
Originally: quasi-stellar radio source
(looked like “points” of radio light, like “radio
Puzzle in the 1960’s: “weird” spectral lines
(did they originate with some bizarre new
chemical element)
Maarten Schmidt (one of few astronomers
ever to make the cover of TIME magazine)
discovered that the lines were highlyredshifted lines of hydrogen
March 11, 1966
At the time (1960’s), quasars were the
most distant objects ever discovered
Radio Light Image
Lengths of the jets is
usually much larger than
the size of a typical galaxy
The “lobes” can be as large
as 100,000 ly
QUASAR-Galaxy Connection
The central region of a
quasar can produce 1,000
times as much light as an
ordinary galaxy of the size of
the Milky Way.
The central region
“outshines” the
underlying “host” galaxy.
Host galaxies imaged for the
first time in 1995.
QUASAR power source - supermassive black holes at
the center of the galaxies
The light is generated by material
that is either in the process of being
swallowed by the black hole, or is
ejected from the region near the
black hole (probably moving along
magnetic field lines)
Where are the quasars today?
If you want to see a “dead”
quasar today, best to look in
the center of a large cluster of
Quasars mostly seem to have a
lot of galaxies around them in
the past.
Why were quasars more
prevalent in the past?
Why did they “shut off”?
Next Time (not on MT#3)
What will be the fate of our universe?
To answer, we need to know:
How much energy is in the form of mass (including dark matter)
How much energy is in the form of light
How much “weird” energy (not mass, not light) is there