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 exam! Some combination of multiple choice, short answer, short calculation
The universe began in an extremely hot,
extremely dense state
A fantastic notion, but also a truly testable hypothesis.
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
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
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?
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.
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”
Examples of elliptical galaxies
(“red and dead”)
Examples of irregular galaxies (small)
Large Magellanic Cloud
When you back in time 5 to 7 billion years and study the galaxies at that time, you find:
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”.
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 10 to 12 billion years ago
Clearly, galaxies have changed over the age of the universe
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.
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
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 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
Taking a short time exposure photograph reveals a jet of material emerging from the interior 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
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!
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 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 discovered
March 11, 1966
Radio Light Image out from their centers!
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
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.
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)
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”?
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