Midterm Exam #2Tuesday, 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 exam!

• 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!!!!!)

DubheMerak

Phecda

Megrez

Alioth

Mizar

Alkaid

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 past

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 Quinteta group of galaxies

Interacting & Colliding Galaxies

Importance of Galaxy Collisions for Establishing Morphology

• 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 decompressor

are needed to see this picture.

Galaxy Collisions in a Distant Galaxy Cluster

“lookback time” is about 9 billion years

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

“Active” GalaxiesMore 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 over-exposed 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

QUASARs

Originally: quasi-stellar radio source (looked like “points” of radio light, like “radio stars”)

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 highly-redshifted lines of hydrogen

At the time (1960’s), quasars were the most distant objects ever discoveredMarch 11, 1966

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 galaxies.

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