What is the diameter of the Milky Way?
100,000 light years
What is the structure of the Milky Way?
Thin disk with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk contains most of the gas and dust, while halo contains only small amount of hot gas and no cold gas.
How do stars orbit in our galaxy?
Stars in the disk: Same plane, same direction
Halo and bulge stars: orbit the center of galaxy, but orbits are randomly inclined to the disk of the galaxy
Orbital motions of stars allow us to determine...
distribution of mass in our galaxy.
How far is the Sun from the bulge of the Milky Way?
28,000 light years
Most prominent stars in the halo are
globular clusters
The interstellar gas and dust is known together as
interstellar medium
How is gas recycled in our galaxy?
Gravitational collapse of gas clouds in molecular clouds= stars are born
Supernovae leads to hot bubbles in interstellar medium. This cools and mixes into surrounding interstellar medium and then cooling further into molecular clouds, which then form stars.
Star-gas-star cycle!
Where do stars tend to form in our galaxy?
Preferentially in spiral arms
Why? Represent regions where a spiral density wave has caused gas clouds to crash into each other. (more likely star formation)
What clues to our galaxy's history do halo stars hold?
Stars of bulge and halo (spheroidal population) are old hags of low-mass stars w/ small amounts of heavy elements.
Halo stars must have formed EARLY in galaxy's history, before gas settled into the disk!
How did our galaxy form?
Protogalactic cloud!!!
Gravity caused to shrink and conservation of angular momentum caused the cloud to form the spinning disk of our galaxy.
Stars in the halo formed before the gas finished collapsing into the disk.
What lies in the center of our galaxy?
Motions of stars near the center of our galaxy suggest that it contains a black hole about 3-4 million times as massive as the Sun. The black hole appears to be powering a bright source of radio emission known as Sgr A.
What are the Large and Small Magellanic Clouds, and the Sagittarius and Canis Major Dwarfs?
...
How can we use orbital properties to learn about the mass of the galaxy?
We can use Newton's version of Kepler's third law to calculate the mass of the galaxy WITHIN an object's orbit. We use the law in the form that is valid when one object is much more massive than the other.
p^2= (4pi^2/G
M)
a^3
where M is the mass of the massive object, p and a are the orbital period and semimajor axis of a smaller orbiting object; and G is gravitational constant.
We can convert this formula to a form that uses velocity (v), instead of period.
v=2pia/v
Since it is a circular orbit, we can replace "a" (semimajor axis) with the orbital radius "r".
ORBITAL VELOCITY LAW:
Mr= r*v^2/G
We can calculate a galaxy's mass from the orbital velocity of a star or star cloud.
Why do star clusters make superbubbles?
The bubble created by a single supernova can grow to a diameter of 100 LY's before it slows and merges w/ surrounding gas. Some cavities of hot gas are actually 1,000's of LY's across. MANY INDIVIDUAL BUBBLES COMBINED TOGETHER.
Imagine the star-gas-star cycle in your head. What does it look like?
See Figure 19.3
What are the proportions of elements in our galaxy?
70% hydrogen
28% helium
2% heavy elements
How does a galactic fountain help circulate new elements w/n MW?
When a superbubble breaks out of the disk (very large), nothing can stop its expansion. The result is blowout. Gas cools, forms clouds, and falls back into disk. See figure 19.19.
Cosmic rays
...
Ionization nebulae
Near hot stars, we often find colorful, wispy blobs of glowing gas- glow because electrons in their atoms are raised to high energy levels or ionized when absorb UV
Spiral arms- how do we know they don't rotate like pinwheels and what makes them bright?
More like swirling ripples in a whirlpool; difference in orbital periods from inside to outside of galaxy
What is Sgr A*?
Contains mass of 3 to 4 million solar masses within a tiny region of space. Has faint X-ray emisson= puzzling= matter falls into it in big chunks, not smooth
What are the three major types of galaxies?
Spiral, elliptical, irregular.
How do we measure the distances to other galaxies?
It's a chain of methods!
1) Radar ranging in our solar system and parallax measurements of distances to nearby stars
2) Relies on standard candles to measure greater distances
How did Hubble prove that galaxies lie far beyond the Milky Way?
Using big ass telescope...
identified individual Cepheid variables in the Andromeda Galaxy. He was then able to use the period-luminosity relation for Cepheids to determine their luminosities and then distances.
What is Hubble's Law?
It's a law that tells us that more distant galaxies are moving away faster:
v= H0 * d
It allows us to determine a galaxy's distance from the speed at which it is moving away from us, which we can measure from its Doppler Shift.
How do distance measurements tell us the age of the universe?
Combine distance measurements+velocity measurements= Hubble's constant
Inverse of this tells us how long it would have taken the universe to reach its present size if the expansion rate had never changed.
14 billion years old!
How does the universe's expansion affect our distance measurements?
Distances between galaxies are always changing because of the expansion of the universe.
It's best to express distance to a faraway galaxy in terms of its "lookback time".
Look back time
the time it has taken for the galaxy's light to reach us. We can figure out the look back time by looking at its red shift as the light is stretched while universe expands.
Cosmological Redshift and Stretching Light
1+z= wlobserved/wlrest
How the average distances between galaxies change with time.
The differing colors of galaxies arises from . . .
different kinds of stars that populate them.
Elliptical galaxies lack hot, young, blue stars, but have plenty of older, cooler red/yellow stars.
Cosmological Principle
Mater in the universe is evenly distributed, without a center or an edge
How old is the universe using the inverse of Hubble's Law?
13.6 billion years
How do we observe the life histories of galaxies?
Telescopes enable us to to detect light from objects with look-back times almost as large as the age of the universe
How did the galaxies form?
Most successful model-
Galaxies formed as gravity pulled together regions of the universe that were ever so slightly denser than their surroundings.
Gas collected in protogalactic clouds, and stars began to form as the gas cooled.
Why do galaxies differ?
1) Conditions in their protogalactic clouds
2) Collisions with other galaxies
Slowly rotating or high-density protogalactic clouds may form elliptical rather than spiral galaxies. Ellipticals may also form through the collision and merger of two spiral galaxies.
What are starbursts?
Galaxies that form new stars at a very high rate, more than 100 times that of MW.
What can starburst galaxy's rapid star formation lead to?
Supernova-driven galactic wind. Many starbursts apparently result from collisions between galaxies. Some starbursts may occur as a result of close encounters with other galaxies rather than from direct collisions.
What the hell is a quasar?
A particularly bright active galactic nucleus.
(Some galaxies have unusually bright centers known as active galactic nuclei)
Where are quasars found?
Very great distances, they were much more common early in the history of the universe
What is the power source for quasars and other active galactic nuclei?
SUPERMASSIVE BLACK HOLES
HOW do black holes provide power to quasars?
As matter falls into a supermassive black hole through an accretion disk, its gravitational potential energy is transformed with enormous efficiency into thermal energy and then into light.
Do supermassive black holes really exist?
It's possible that all galaxies harbor them. It could be tied to process of galaxy evolution, since they often share properties with gally around them.
How do quasars let us study gas between the galaxies?
Each cloud of gas through which the quasar's light passes on its long journey to Earth produces a hydrogen absorption line in the quasar spectrum. Study of these absorption lines in quasar spectra allows us to study matter.
90% of quasars are weak sources of
radio waves
10-40% of the mass energy of matter falling into a black hole can be radiated away as energy . . . if 10% is radiated away:
Then the amount of energy radiated by mass falling into a black hole is E=1/10mc^2. We can use this fact to determine how much mass is accreting around the black hole in an active galactic nucleus, because we can determine the amount of energy radiated into space from the object's luminosity and distance.
E= m= 10 * E/c^2
A black hole radiating 10% of its mass energy outwards needs _____ as much matter going in as a black hole radiating 5% of its mass energy outwards.
1/2
10/5
What is the primary source of a quasar's energy?
Gravitational potential energy
(matter collapsing into black holes)
What do we mean by dark matter and dark energy?
Proposed to exist, because it seems the simplest way to explain a set of observed motions in the universe.
Dark matter
name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds
Dark energy
name given to whatever may be causing the expansion of the universe to accelerate
What is the evidence for dark matter in galaxies?
Orbital velocities of stars and gas clouds in galaxies do not change much with distance from center of galaxy. Applying Newton's law of gravitation and motion to these orbits leads to conclusion that total mass of galaxy is far larger than the mass of its stars. But matter doesn't have light= dark matter.
What is the evidence for dark matter in clusters of galaxies?
Three methods of determining whether dark matter is there:
1) galaxy orbits
2) temperature of the hot gas in clusters
3) gravitational lensing
Cluster Masses from Galaxy Orbits- the orbital velocity law!
Mr= r*v^2/G
Mr is total mass contained within a distance r of a galaxy's center
V is average velocity of objects orbiting the center of the galaxy
G is the gravitational constant
Use to calculate mass of a galaxy!!!
Cluster Masses from Gas Temperature
vH= 140 m/s x sqrt (T)
where vH is the average orbital speed of the hydrogen nuclei and T is the temperature in Kelvin.
After you solve this, you use vH to find the cluster's mass using the orbital velocity law. (SEE PREVIOUS)
Gravitational lensing
a cluster's gravity bends light from a single galaxy so that it reaches Earth from multiple directions
result?
through a telescope, we see multiple images of what is really just a single galaxy
light bending angle of a gravitational lens depends on the mass of the object doing the bending
What might dark matter be made of?
Some of the dark matter could be ordinary or baryonic matter in the form of dim stars or planetlike objects, but there does not appear to be enough ordinary matter to account for all the dark matter. Most of it is probably extraordinary or nonbaryonic matter consisting of undiscovered particles that we call WIMPS.
WIMPS
Dark matter might be subatomic particles that don't interact strongly with anything.
MACHOS
trillions of faint red stars, brown dwarfs, etc. left over from MW formation that could still roam our galaxy's halo, providing much of its mass. when one of these pass in front of a more distant star, gravitational lensing temporarily makes the star appear brighter.
it has been proven that these objects don't constitute the majority of the galaxy's dark matter
What is the role of dark matter in galaxy formation?
The gravity of dark matter is probably what formed proto-galactic clouds and then galaxies from slight density enhancements in the early universe.
(same gravity)
What are the largest structures in the universe?
Galaxies in gigantic chains and sheets that surround great voids
Critical density
average matter density that the universe would need for the strength of gravity to eventually halt the expansion
the overall mass density of the universe appears to be only about 25% of the critical density
Is the expansion of the universe accelerating?
Yes, but we don't know why.