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440 Cards in this Set

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  • Back
White Dwarves are...
the remaining cores of dead low-mass stars
What supports white dwarves against Gravity?
Electron degeneracy pressure protects them against gravity.
A white dwarf is much hotter than a main-sequence star, therefore it...
it emits more at shorter wavelengths (Wien’s law).
With time, white dwarfs....
cool and grow dimmer
What happened to most of the Hydrogen of the Original Star (before it became a white dwarf)?
it was fused or else dejected as a part of the planetary nebula.
White dwarfs with the same mass as the sun are about the same size as ____.
Earth. Although higher mass white dwarves are smaller.
How can a more massive white dwarf be smaller?
More mass = more gravitational force on the degenerate core. The core compresses but doesn't fail (yet).
How much would one sugar cube of 1 Msun white dwarf material weigh on earth?
18,000 kg, or 40,000 pounds (about the same as a large truck)
what happens as a white dwarf’s mass approaches 1.4 MSun?
its electrons must move at nearly the speed of light.
So why can't a white dwarf be more massive than 1.4 MSun?
Because nothing can move faster than light. (white dwarf limit, or Chandrasekhar limit)
As it gets more massive, a white dwarf will....
What can happen to a white dwarf in a close binary system?
Mass transfer from a red giant to a white dwarf.
In a mass transfer, why can't the matter fall directly onto the white dwarf?
Conservation of angular momentum. Mass falling toward a white dwarf from its close binary companion has some angular momentum
Because of Conservation of Angular Momentum, Matter orbits the white dwarf in what is called a...
Accretion Disk
Gas in accretion disks at different radii orbit at

a. the same velocity
b. different velocities
b. different velocities.
The difference in velocity between parcels of gas in an Accretion Disk causes...
friction in an accretion disk.
Friction between orbiting rings of matter in the accretion disk
angular momentum transfers outward and causes the disk to heat up and glow.
What would gas in an accretion disk do if there were no friction?

A. It would orbit indefinitely.
B. It would eventually fall in.
C. It would blow away.
A. It would orbit indefinitely.
What is the key ingredient in making matter move inward toward the white dwarf?
Friction is the key ingredient.
As matter piles up on the surface of the white dwarf, temperature and pressure goes up until…
BOOM. supernovae occurs.
As angular momentum is transferred from the inner region of the accretion disk to the outer region...
matter can begin to fall onto the surface of the white dwarf.
What causes a Nova?
The temperature of accreted matter eventually becomes hot enough for hydrogen fusion (10 million K)
Fusion begins suddenly and explosively
Novae occur when hydrogen burning occurs on the..

a. core
b. surface
b. the surface.
What is more energetic, a Nova or a Supernova?
a supernova is more energetic, despite the similar names.
The Nova Star system temporarily appears
Much brighter, up to 100,000 Lsun (not naked eye)

The explosion drives accreted matter out into space.
Can a Nova be recurrent?
Yes, the process can repeat over and over.
a common outburst timescale is every 10,000 years.
What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?

A. It explodes
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its core
A. It explodes into a white dwarf supernova.
How can you tell the difference between a nova and a white dwarf supernovae?
SN are 10 million times as luminous.
In a Nova, what happens to a white dwarf?

A. H to He fusion of a layer of accreted matter leaves a white dwarf intact.
B. Complete explosion of a white dwarf leaves nothing behind.
C. nothing happens.
A. H to He fusion of a layer of accreted matter, white dwarf left intact
In a Supernova, what happens to a white dwarf?

A. H to He fusion of a layer of accreted matter leaves a white dwarf intact.
B. Complete explosion of a white dwarf leaves nothing behind.
C. nothing happens.
B. Complete explosion of a white dwarf leaves nothing behind.
When the White dwarf threshold of 1.4 Msun is surpassed, what happens?
Electron degeneracy fails and the core rapidly heats and contracts.

Carbon starts to violently fuse, tremendous energy is released.
In a Massive Star Supernova, what happens?
Iron core of massive star collapses into a
neutron star, causing explosion
>>neutron star left behind
In a White Dwarf Supernova, what happens?
-Carbon fusion suddenly begins.
-Whitedwarf in close binary system reaches white dwarf limit, causing total explosion
>>nothing left of the star!
What is the simplified order of events in a Massive star supernova?

B. accretion onto a white dwarf star, thermonuclear supernova explosion, supernova remnant without a neutron star.
B. Core implosion, supernova explosion, Supernova remnant, Neutron star left behind.
B. Core implosion, supernova explosion, Supernova remnant, Neutron star left behind.
What is the simplified order of events in a White dwarf supernova?

B. accretion onto a white dwarf star, thermonuclear supernova explosion, supernova remnant without a neutron star.
B. Core implosion, supernova explosion, Supernova remnant, Neutron star left behind.
B. accretion onto a white dwarf star.

thermonuclear supernova explosion.

supernova remnant without a neutron star.
How does a light curve aid in telling supernova apart?
it shows how luminosity changes with time.
White Dwarf Supernovae Light Curves are....

a. very similair.
b. not similair.
c. very different.
d. Each is unique and varying.
explosion mechanism is quite similar.

yielding very similar light curves (unlike massive star supernovae).

Because they are so similar, they are used as “standard candles”
Supernova Type: Massive Star or White Dwarf?
light curves differ.
spectra differ.
Location differ.
Why do the spectra differ between Massive star supernovas and white dwarf supernovas?
exploding white dwarfs don’t have hydrogen absorption lines because the progenitor white dwarf had no hydrogen
Why does the location differ between Massive star supernovas and white dwarf supernovas?
no massive star supernovae in regions where there are no young stars (e.g., in elliptical galaxies)
A Neutron star is....
the ball of neutrons left behind by a massive-star supernova.
What supports a neutron star against gravity?
degeneracy pressure.
Electron degeneracy pressure goes away because...
electrons combine with protons, making neutrons and neutrinos.

(neutrons collapse to the center, forming a neutron star)
A neutron star is about the same size as...
a small city, about 10km in radius.
A paperclip made of neutron star material would weigh more than...
Mount Everest.
How did jocelyn Bell discover neutron stars?
- Used a radio telescope in 1967.

- noticed very regular pulses of radio emission coming from a single part of the sky.

- pulses were coming from a spinning neutron star—a pulsar
(a spinning neutron star)

beams radiation along a magnetic axis not aligned with the rotation axis.
Which is true?

A. All neutron stars are pulsars, but not all pulsars are neutron stars
B. All pulsars are neutron stars, but not all neutron stars are pulsars.
B. All pulsars are neutron stars, but not all neutron stars are pulsars.
Could there be neutron stars that appear as pulsars to other civilizations but not to us?

A. Yes
B. No
A. Yes.

If the orientation of the beams point toward them and not us.
Circumference of NS
2 * pi * (radius) ~ 60 km
Spin Rate of Fast Pulsars
1000 cycles per second
How does Conservation of Angular Momentum apply to pulsars?
Pulsars spin fast because massive star core’s spin speeds up as it collapses to a neutron star.
Pulsars are considered incredibly stable clocks, how can we see there orbital motion?
Doppler shifts! – it appears like the pulsar is increasing and decreasing in frequency in a periodic way!

They spin down, but slowly (over hundreds of years).
neutron degeneracy pressure will fail for neutron stars more massive than...
3Msun. The star will collapse into a black hole.
What can happen to a NEUTRON STAR in a close binary system?
Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary
Why is an Accretion disk around an NS much hotter than one around a White Dwarf?
more gravitational energy of infalling matter is released because the neutron star is much smaller than a white dwarf.
Can we see X-ray binaries in other galaxies?
Yes, we can see X-ray binaries in other galaxies.
What is an X-ray burst?
Matter accreting onto a NS can becomes hot enough for Helium fusion

sudden onset of fusion produces a burst of X-rays
Stages of Fusion Near the Surface of a Neutron Star...
1. hydrogen from accretion disk builds.
2. bottom layers fuse hydrogen into helium.
3. Helium builds in crust under surface.
4. Helium is fused into carbon, releasing energy in an X-ray burst.
In a hot accretion disk...
infalling matter loses gravitational energy...
Fusion of helium in a layer near the crust produces...
energetic, sporadic x-ray bursts lasting only a few seconds.
X-ray binaries emit X-rays via what two mechanisms?
Hot accretion disk

Fusion of helium in a layer near the crust
According to conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star?

A. The neutron star’s rotation would speed up.
B. The neutron star’s rotation would slow down.
C. Nothing, the directions would cancel each other out.
B. The neutron star’s rotation would slow down.
What is an object whose gravity is so powerful that not even light can escape it?
A black Hole!
Rundown of a Black Hole Formation
As Mass inreases, so does gravitational pull.
If Gravitational pull is such that light cannot escape, a black hole forms.
What happens to the escape velocity from an object if you shrink it?

A. It increases
B. It decreases
C. It stays the same
A. It Increases

(think of the force of gravity equation)
Escape Velocity of a Black Hole
300,000 km/s – speed of light
Escape Velocity of the Earth and the Sun
Earth - 11 km/s

Sun – 620 km/s
Would light be able to escape the earths surface if shrunk to <1 cm?

What would the escape velocity be?
11.53 km/s or 0.0038 % the speed of light.
Stellar collapse only makes black holes if the core of the star is more than
about 3 MSun
In theory, if you compress the Earth down to under a centimeter, it will become a black hole.

In reality, however...
no known force could compact the Earth (or any smaller body) to be a black hole.
What is the "surface" of a black hole?
the radius at which the escape velocity equals the speed of light.

It’s not a physical surface.
The spherical surface of a black hole is known as the...
event horizon.
The radius of the event horizon is known as the...
Schwarzschild radius.
The event horizon of a 3 MSun black hole is about the size of....
also a small city, about 9 kilometers in radius
A typical 10 MSun black hole has a Schwarzschild radius (event horizon) of...
30 km
Schwarzschild Radius of a typical 10 Msun black hole is expressed in an equation how?
RSchwarzschild = 3 * (M/MSun) km

3 * 10 8
in vicinity of event horizon, a black holes mass strongly...
warps space and time. Any large mass Creates a "gravity well" around it.
True or False. In a Black Hole, the Gravity well is bottomless?
Quantum mechanics says what about the neutron star limit?
neutrons in the same place cannot be in the same state.
Some massive star supernovae can make a black hole if...
enough mass falls onto core.
Nothing can escape from within the event horizon because....
nothing can go faster than light.
What does 'no escape' mean?
That there is no more contact with something that falls in. It increases the hole mass, changes the spin or charge, but otherwise loses its identity.
How do we know that Gravity can bend light?
A star’s light path can be bent by the Sun’s gravity – verified during total solar eclipses.
beyond the neutron star limit, what known force can resist the crush of gravity?
no known force can resist the crush of gravity beyond the neutron star limit.
gravity crushes all the matter into a single point.

a point of infinite density where the laws of physics break down.
How does the radius of the event horizon change when you add mass to a black hole?

A. Increases
B. Decreases
C. Stays the same
A. it increases

R schwarzschild = 3 * (M/MSun) km
If the Sun shrank into a black hole with the mass of the Sun, its gravity would be different only when?
once you move closer than the radius of the original Sun.

Gravitational Redshift
Light near (but not inside) the event horizon effectively loses energy as it attempts to climb out of the deep potential well in the vicinity of a black hole.
Why is a gravitational Redshift called a Gravitation Redshift?
Since energy = h x frequency, a lower energy implies a lower frequency, and therefore a longer wavelength - redshifting.
Special Relativity...
Pertains to moving objects.
High velocity - redshifting.
Time appears to slow for a moving object.
General Relativity...
Pertains to gravity.
Strong gravity - redshifting
Time appears to slow for an object near a strong gravity well.
Time passes more slowly near the...
event horizon - just like moving clocks run slow (recall the experiment of carrying atomic clocks on commercial airlines).
What does an observer see on a Journey to a Black Hole?
see's the adventurers clock slow down as black hole is approached.

the adventurer will look redder and redder.

adventurer appears to slow down, never quite reaching the black hole, growing redder and dimmer.

Goes back home to alert nasa of the lost astronaut.
What does the adventurer see on the journey to the black hole?
see's observers clock speed up as the black hole is approached.

the observer looks bluer and bluer.

passes right through the event horizon in real time, not noticing anything unusual.

Will never see this universe again.
If you moved through the event horizon noticing nothing unusual at this radius, what would happen to you?
you would soon be crushed to infinite density at the singularity at the center of the black hole.
what exists near the event horizon of a
3 MSun black hole that would be lethal to humans?
Tidal Forces.
If you are a space explorer traveling in a random direction, is it easy or hard to fall into a black hole?

If the Sun is a grapefruit in Washington, DC, the nearest star is in California…what are your odds of stumbling across an object 20,000 times smaller than a grapefruit somewhere in the United States?
How do we verify if something is a black hole?
Need to measure mass!!!!!

-Use orbital properties of companion

-Measure velocity and period of orbiting gas
-Use Newton’s Law to determine mass
It is a black hole if it is not a star and...
and its mass exceeds the neutron star limit (~3 MSun)
what is one famous X-ray binary with a likely black hole?
in the constellation Cygnus – a young, 18 MSun supergiant star and a 10 MSun unseen companion.
What do some x-ray binaries contain that leads us to believe they are likely black holes?
compact objects of mass exceeding 3 MSun
What is some further evidence for black hole existence?
Neutron stars exhibit X-ray bursts when helium burns near their surface – black holes do not have a surface, so no burst would be expected.
To date, has an X-ray burst ever been observed from a suspected black hole X-ray binary?
Why was the initial discovery of gamma ray bursts worrisome?
the military thought they resulted from other countries testing nuclear weapons.
What proved that gamma-ray bursts were not occurring in our Galaxy?
they were shown to be distributed uniformly over the sky.
Gamma Ray bursts are followed by what?
An X-ray and an optical afterglow that allow the precise position to be obtained (Gamma-ray telescopes don’t focus very well).

Gamma-ray bursts briefly shine as bright as several thousand galaxies (!)
Optical Afterglow of Gamma-Ray Bursts
Host galaxies of gamma-ray bursts are typically very distant, up to several billion light years away.

Optical afterglows can sometimes be bright enough to be seen with binoculars.
What causes at least some of Gamma-Ray bursts?
some gamma-ray bursts are produced by supernova explosions.
Some others may come from collisions between neutron stars.
the term Milky Way means
both our galaxy and the band of light
What is the interstellar medium?
makes new star systems. obscures our view of the milky way by absorbing visible light.
Do we see the Milky way from the inside looking out or the outside looking in?
the inside looking out.

Central bulge of galaxy should be bright, but is heavily absorbed by gas and dust.
What are the primary features of our galaxy as seen from the outside?
Primary features: disk, bulge, halo, globular clusters
If we could view the Milky Way from above the disk, we would see its...
spiral arms.
If the Milky Way is the size of a football field how far from the Sun is the nearest star?
4 millimeters!
Disk of Milky Way:
100,000 light year wide, 1000 light year thick spiral

structure of old and young stars, gas, and dust
Bulge of Milky Way:
10,000 light year radius sphere of old stars at the center of the galaxy, little gas or dust
Halo of Milky Way:
~50,000 light year radius sphere of old stars

surrounding the disk and bulge, little gas or dust
Globular Clusters of Milky Way:
compact collection of old stars orbiting the galaxy in the halo and bulge – each a few tens of light years in radius, roughly 200 total clusters in the galaxy
What suggested to us that the sun was near the center of the galaxy?
Early star-counting studies found roughly the same number of stars in all directions from the Sun.
How did Harlow Shapley correct the mistake of our assuming the sun was near the center of the galaxy?
calculated that the ~200 globular cluster systems were centered on a point well away from the position of our Sun.
in Shapley’s picture of the Milky Way...
Globular Clusters NOT centered about Sun

Shows the Sun is not at the center of the Milky Way

Shapley found a distance of 45,000 light years to the Galactic Center.
What was wrong with Shapley's picture of the Milky Way?
the shape was right but the distances were all wrong.
Why was it so hard to determine the distances in the milky way?
Thick interstellar gas and dust toward the real center of the galaxy blocked the optical light from the many stars lying in this direction.
How do stars orbit in our galaxy?
Stars in the disk (like our Sun) all orbit in the same circular direction with a little up-and-down motion.
Orbits of stars in the bulge and halo...
have random orientations,
some circular,
some radial (highly elliptical).
Why do the orbits of bulge stars bob up and down?

A. They are stuck to the interstellar medium.

B. Gravity of disk stars pulls toward the disk

C. Halo stars knock them back into the disk
B. Gravity of disk stars pulls toward the disk
To measure the mass of the Milky Way, what quantities do we need?
period and distance to use Newton’s Law for orbits.

Distance is known, but period is not measurable because it is so long.

We can measure the Sun’s velocity, and turn that into a period.
What does the suns orbital motion tell us?
(radius and velocity) tells us mass within Sun’s orbit:

1.0 x 10₁₁ MSun
in the solar system, the orbital velocity increases or decreases with radius?
Decreases. Mass is concentrated in the center (the sun)
Keplarian Orbits says that when all mass is concentrated in the sun, velocity is proportional to...
Radius -1/₂
Why is the milky way a flat rotation curve?
velocity ~ constant
The mass increases with radius!
How is gas recycled in our galaxy?
Star-gas-star cycle

Recycles gas from old stars into new star systems

The galaxy is the ultimate recycler.
High mass stars have strong stellar winds that...
blow hot bubbles of gas
What are cosmic rays made up of?
very high energy electrons, protons and atomic nuclei produced by supernovae and other energetic processes.
Why wouldn't you want to live in a star-forming region with many supernovae?
Cosmic rays are known to cause genetic mutations.

On average 1 cosmic ray hits you every second, but in an airplane, about 100 per second are hitting you.
What is a galactic fountain?
Multiple supernovae create huge hot bubbles that can blow out of the disk.

Gas clouds cooling in the halo can rain back down on disk.
Where does much of the star formation in the disk occur in our galaxy?
spiral arms.
Reflection nebulae
scatter the light from stars
Ionization Nebulae
Atoms (hydrogen, oxygen, neon, etc.) absorb ultraviolet photons from nearby luminous stars, causing an electron in the atom to move to a higher energy level.

When the electron returns to a lower level, a photon is emitted at a specific frequency.
Where are Ionization Nebulae found?
around short-lived high-mass stars, signifying active star formation.
What does long-wavelength infrared emission show?
where young stars are heating dust grains.
How do we show the locations of molecular clouds?
Radio waves from carbon monoxide (CO)
Which waves show where gas has settled and cooled into disk?
21 cm radio waves emitted by atomic hydrogen
Where do we observe x-rays in the milky way?
X-rays are observed from hot gas above and below the Milky Way’s disk – some is galactic fountain gas
Which type of light reveals stars whose visible light is blocked by gas clouds?
infrared light.
How do we observe the star-gas-star cycle occurring in the milky ways disk?
using many different wavelengths of light.
Where will the gas from star formation be in 1 Trillion yrs?

a.) Blown out of the galaxy
b.) Still recycling just like now
c.) Locked into low mass stars, white dwarfs, neutron stars, black holes, and brown dwarfs
C.) Locked into low mass stars, white dwarfs, neutron stars, black holes and brown dwarfs.

Eventually, star formation will cease when all the available free gas is gone.
Spiral density waves
regions in the disc where mass density greater than average by ~10-20%
What does the gravitational field of the spiral pattern in spiral density waves cause?
stars and
gas to slow down near the arm
Summary of Galactic Recycling
-Stars make new elements by fusion

-Dying stars expel gas and new elements, producing hot bubbles (~106 K)

-Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K)

-Further cooling permits molecules to form, making molecular clouds (~30 K)

-Gravity forms new stars (and planets) in molecular clouds
what completes the star-gas-star cycle once stars are created within the cloud?
Gravity forms stars out of the gas in molecular clouds
What is the fourth step of the star-gas-star cycle?
-Further cooling permits molecules to form, making molecular clouds (~30 K)
What is the third step of the star-gas-star cycle?
-Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K
Composition of Molecular Clouds
Mostly H2
About 28% He
About 1% CO
Many other
What is the second step of the star-gas-star cycle?
-Dying stars expel gas and new elements, producing hot bubbles (~106 K)
What is an important atomic hydrogen transition that does not involve an electron changing energy levels?
the 21-centimeter (radio wavelength) transition. This involves an electron changing only its spin value.
where do Radio emissions in supernova remnants come from?
particles accelerated to near light speed.
Where do cosmic rays probably come from?
As supernova remnants cool, they begin...
to emit visible light as it expands. New elements made by the supernova then mix into the interstellar medium.
How do lower mass stars return gas to interstellar space?
through stellar winds and planetary nebulae.
What is the first step of the star-gas-star cycle?
stars make new elements by fusion.
What lies in the center of our galaxy?
Orbits of stars indicate a mass of about 4 million MSun

(something massive but invisible)
Which law proves that there is a black hole at the center of the Milky Way?
Keplers 3rd law.
Hubble Deep Field
Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away
Nearly every point of light is a...
How many galaxies are in one hubble pointing? How many stars are in one galaxy?
Each pointing contains thousands of galaxies.

It would take about 30 million such Hubble pointings to cover the entire sky.

Each galaxy has on the order of 100 billion stars.
a galaxy's age and distance are closely related with what?
the age of the universe.
Cosmology is....
the study of the structure and evolution of the universe.
What are the three major types of galaxies?
Spiral Galaxy
Elliptical Galaxy
Irregular Galaxy
Disk Component of a Spiral Galaxy Contains....
stars of all ages,
many gas clouds
Spheroidal Component of a Spiral Galaxy Contains...
bulge & halo, old stars,
few gas clouds
What does Blue-white color indicate in a Spiral Galaxy?
ongoing star formation
What does Red-Yellow color indicate in a Spiral Galaxy?
older star population
Why does ongoing star formation lead to a blue-white appearance?

A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light
B. Short-lived blue stars outshine others
A Barred Spiral Galaxy has...
a bar of stars across the bulge, probably due to gravitational instabilities.
What is a Lenticular (S0) Galaxy?
Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical).
What do Lenticulars (S0) NOT have?
Lenticulars do not have spiral arms and show little evidence for recent star formation.
Properties of an Elliptical Galaxy
All spheroidal component, virtually no disk component.

No blue stars, no recent star formation.
What is a way to remember that Red-Yellow indicates an older star population?
Red and Dead
Elliptical Galaxies lack...
a cold interstellar medium. Have a lot of x-ray emitting HOT Interstellar medium, with gas too hot to form stars.
What do most irregular galaxies show? How do you know?
recent star formation.Blue-white color indicates ongoing star formation
What is the Elliptical Hubble Galaxy sequence?
E0 (round) – to E7 (most elliptical)
What is the Lenticular Hubble Galaxy Sequence?
called S0s, or SB0s (barred)
What are the properties of an Sa or SBa Spiral in the Hubble Galaxy Sequence?
prominent bulge, weak arms
What are the properties of an Sb or SBb Spiral in the Hubble Galaxy Sequence?
average bulge, average arms
What are the properties of an Sc or SBc Spiral in the Hubble Galaxy Sequence?
small bulge, strong arms
What are the properties of an Sd or SBd Spiral in the Hubble Galaxy Sequence?
virtually no bulge, all arms.
Hubble Galaxy Sequence for Irregular Galaxies?
NOT on hubble galaxy sequence.
Where are Spiral Galaxies often found?
in groups of galaxies.
Where is the milky way galaxy located?
in the local group.
Elliptical Galaxies are much more commonly found where?
in huge clusters of galaxies (hundreds to thousands)
How do we measure the distance to galaxies?
the cosmic distance ladder.
What is the order of the cosmic distance ladder?
WD supernova
Main Sequence Fitting
Does brightness alone provide enough information to determine distances?
Step 1 in Determining Distances
Determine size of solar system using radar.

Determining the distance to the Sun is particularly important.
Step 2 in Determining Distances
Determine distances of stars out to a few hundred light-years using parallax
What is the name used to describe an object whose luminosity can be determined without measuring its distance?
Standard Candle
We can determine a star’s distance if we know its...
luminosity and can measure its apparent brightness:

Distance =
4 * pi * x Brightness
The relationship between apparent brightness and luminosity depends on...

Brightness =
4 * pi * (distance)2
Step 3 in determining distances
Apparent brightness of star cluster’s main sequence tells us its distance
the G stars in Hyades are brighter than the G stars in Pleiades, thus...
The Hyades must be Closer (apparent brightness of main sequence tells us this)
If a cluster is close enough, its distance can be measured using...
Stellar parallax. Know distance, measure apparent brightness, find luminosity of stars on the main sequence.
Which kind of stars are best for measuring large distances?

A. High Luminosity
B. Low Luminosity
A. High Luminosity
Cepheid Variable stars with longer_____have greater______.
periods, luminosity.
Step 4 in determining distances
because the period of a Cepheid variable stars tells us its luminosity, we can use them as STANDARD CANDLES.
So what can we use as standard candles?
WD supernovaes, Cepheid Variable Stars and Entire Galaxies.

(all WD S.N reach the same peak luminosity)
Step 5 in determining Distances
Apparent brightness of white-dwarf supernova tells us the distance to its galaxy.
Why can entire Galaxies be used as Standard Candles?
galaxy luminosity is related to rotation speed. (tully-fisher relation)
3 steps of the tully-fisher relation?
1. Measure Rotation Speed
2. Plot Rotation Speed on Horizontal Axis
3. Determine Luminosity from Rotation Speed
How do we see through gas and dust to stars in the galactic center?
observing in the infrared.
What do X-ray flares from the Galactic Center tell us about tidal forces?
that they occasionally tear apart chunks of matter about to fall into a suspected black hole.
cosmic rays?
particles such as electrons, protons, and atomic nuclei that zip though interstellar space at close to the speed of light
Critical Universe
The possible fate of our universe in which the mass density of the universe equals the critical density. The universe will never collapse, but in the absence of a repulsive force it will expand more and more slowly as time progresses.
consist of many clusters of galaxies, groups of galaxies and individial galaxies and are the largest known structures in the universe
critical density
the precise average density for the entire universe that marks the dividing line between a recollapsing universe and one that will expand forever.
hydrogen-shell burning
hydrogen fusion that occurs in a shell surrounding a stellar core.
the place at the center of a black hole where gravity crushes everything into an infinite density
electron degeneracy pressure (textbook definition)
degeneracy pressure exerted by electrons, as in brown and white dwarfs.
inflation of the universe
a sudden and dramatic expansion of the universe thought to have occurred at the end of the GUT era.
solar circle
the sunds orbital path around the galaxy, which has a radius of about 28,000 light-years
interstellar dust grains
tiny flecks of carbon and silicon minerals in cool interstellar clouds.

form in the winds of red giant stars
particles, including protons and neutrons, that are made from 3 quarks
accretion disk (textbook definition)
a rapidly rotating disk of material that gradually falls inward as it orbits a starlike object (white dward, ns or black hole)
radio galaxy
a galaxy that emits unusually large quantities of radio waves, thought to contain an active galactic nucleus powered by a supermassive black hole.
main sequence fitting
a method for measuring the distance to a cluster of stars by comparing the apparent brightness of the clusters main sequence with the standard main sequence
helium fusion
the fusion of three helium nuclei into one carbon nucleus; also called the tripla alpha reaction
ionization nebulae
colorful cloud of gas that glows because neighboring stars irradiate it with ultraviolet photons that can ionize hydrogen atoms
a sudden burst of gamma rays from deep space; apparently they come from distant galaxies but their precise mechanism is unknown
bulge (of a spiral galaxy)
central portion of a spiral galaxy that is roughly spherical and bulges above and below the plane of the galactic disk
dramatic brightening of a star that lasts for a few weeks and then subsides; occurs when a burst of hydrogen fusion ignites in a shell on the surface of an accreting white dwarf in a binary system.
particle era
the era of the universe lasting from 10-10th power second to 0.0001 second after the big bang, during which subatomic particles were continually created and destroyed and ending when matter annihilated antimatter
brown dwarf
an object too small to become an ordinary star because electron degen. pressure halts its gravitational collapse before fusion becomes self-sustaining.

have less than 0.08Msun
era of nuclei
the era of the universe lasting from about3 minutes to about 5000,000 yrs after the big bang, during which matter in the universe was fully ionized and opaque to light. The cosmic background radiation was released at the end of this era.
Neutron degeneracy pressure
degeneracy pressure exerted by neutrons, as in neutron stars.
spherical region surrounding the disk of a galaxy.
recollapsing universe
the possible fate of our universe in which the collective gravity of all its matter eventually halts and reverses the expansion. the galaxies will come crashing back together and the universe will end in a fiery big crunch.
standard candle
an object for which we have some means for knowing its true luminosity, so we can use its apparent brightness to determine its distance with the luminosity distance formula!
Low-mass stars
born with masses less than about 2Msun, end their lives by ejecting a planetary nebula and becoming a white dwarf
Grand unified Theory (GUT)
a theory that unifies three of the four fundamental forces--strong, weak, electromagnetic (but not gravity)--in a single model.
the inseperable, fourdimensional combination of space and time
electroweak era
the era of universe during which only 3 forces operated (gravity, strong force and electroweak force) lasting from about 10-38power second to 10-10 second after big bang
huge volumes of space between superclusters that appear to contain very little matter
era of atoms
the era of the universe lasting from about 500,000 years to about 1 billion years after the big bang, during which it was cool enough for neutral atoms to form
massive star supernova
occurs when a massive star dies, initiated by catastrophic collapse of its iron core
spheroidal component of a galaxy
the portion of a galaxy that is spherical in shape and contains very little cool gas. contains only very old stars. elliptical galaxies have only the spheroidal component while spiral galaxies have a disk too.
active galactic nuclei
unusually luminous centers of some galaxies, thought to be powered by accretion onto supermassive black holes. Quasars are brightest, radio galaxies have some too.
mass-to-light ratio
the mass of an object divided by its luminosity, usually stated in units of solar masses per solar luminosity. Objects with high mass to light ratios must contain substantial quantities of dark matter.
Disk component (textbook definition)
the portion of a spiral galaxy that looks like a disk and contains an interstellar medium with cool gas and dust; stars of many ages are found in the disk component.
cosmological redshift
redshifts we see from distant galaxies, caused by the fact that expansion of the universe stretches all the photons within it to longer, redder wavelengths.
molecular clouds (textbook)
cool, dense IS clouds in which the low temperatures allow hydrogen atoms to pair up into hydrogen molecules (H2)
cosmological horizon
boundary of our observable universe, which is where the lookback time is equal to the age of the universe. beyond this boundary in spacetime, we cant see anything at all.
intracluster medium
hot, x-ray emitting gas found between the galaxies within a cluster of galaxies
hubbles constant (textbook)
# that expresses current rate of expansion of the universe, designated H0, it is usually stated in units of km/s/mpc. the reciprocal of hubbles constant is the age the universe would have if the expansion rate never changed.
galactic fountain (textbook)
refers to a model for the cycling of gas in the milky way galaxy in which fountains of hot, ionized gas rise from the disk into the halo and then cool and form clouds as they sink back into the disk.
the brightest type of active galactic nucleus
helium-capture reactions
fusion reactions that fuse helium nucleus into some other nucleus; such reactions can fuse carbon into oxygen, oxygen into neon, neon into magnesium and so on.
era of galaxies
present era of the universe, which began with the formation of galazies when the universe was about 1 billion years old!
hubbles law
mathematically expresses the idea that more distant galaxies move away from us faster.

formula: v=H0 x d, where v is a galaxies speed away from us, d is its distance and H0 is hubbles constant.
closed universe
the universe is closed if its average density is greater than the critical density, in which case spacetime must curve back on itself to point where its overall shape is analogous to that of the surface of a sphere.

in the absence of a repulsive force, a closed universe would someday stop expanding and begin to contract.
planck era
era of universe prior to the planck time!
Tully-Fisher Relation (textbook)
relationship among spiral galaxies showing that the faster a spiral galaxys rotation speed, the more luminous it is. Its very important b/c it allows us to determine the distance to a spiral galaxy once we measure its rotation rate and apply the luminosity distance formula.
CNO cycle
the cycle of reactions by which intermediate and high mass stars fuse hydrogen into helium.
accelerating universe
possible fate of universe in which a repulsive force causes the expansion of the universe to accelerate with time. its galaxies will recede from one another increasingly faster and it will become cold and dark more quickly than a coasting universe.
olbers paradox
say, how can the night sky be dark if the universe is infinite and full of stars?
x-ray binary
binary star system that emits substantial amounts of x-rays, thought to be from an accretion disk around a neutron star or black hole.
a star that is between being a main-sequence star and being a giant, subgiants have inert helium cores and hydrogen burning shells.
planetary nebula
glowing cloud of gas ejected from a low mass star at the end of its life.
supermassive blackhole
giant fucking blackhole, mass millions to billions of time that of our sun, thought to reside in centers of many galaxies and to power active galactic nuclei.
nonbaryonic matter
exotic matter not part of normal atom composition, like neutrinos or hypothetical WIMPS.
radio ranging
method of measuring distances within the solar system by bouncing radio waves off planets.
radiation pressure
pressure exerted by photons of light
baryonic matter
ordinary matter made from atoms. (nuclei of atoms contain protons and neutrons, which are both baryons)
forming star that has not yet reached the point where sustained fusion can occur in its core.
disk component of a galaxy
portion of spiral gal. that looks like a disk, contains IS medium with cool gas and dust.

stars of many ages found here.
dark matter
matter that we infer to exist from its gravitational effect but from which we have not deteced any light. dark matter evidently dominated the total mass of the universe.
chemical enrichment
process by which the abundance of heavy elements (heavier than helium, anyway) in the IS medium gradually increases over time as these elements are produced by stars and release into space.
helium flash
event marking the sudden onset of helium fusion in the previously inert helium core of a low-mass star.
intermediate mass stars
stars with about 2-8Msun. end their lives by ejecting planetary nebula and becoming a white dwarf.
massive compact halo objects. represent one possible form of dark matter in which the dark objects are relatively large, like planets or brown dwarfs.
refers to any particle with the same mass as a particle of ordinary matter but whose other basic properties, such as electrical charge, are precisely the opposite.
protogalactic cloud
huge collapsing cloud of intergalactic gas from which an individual galaxy formed.
coasting universe
possible fate of our universe in which the mass density of the universe is smaller than the critical density, so that the collective gravity of all matter cant halt the expansion. in the absence of repulsive force such a universe would keep expanding forever with little change in its rate of expansion.
a giant interstellar bubble, formed when the shock waves of many individual bubbles merge to form a single giant shock wave.
large-scale structure of the universe
refers to structure of the universe on size scales larger than that of clusters of galaxies.
era of nucleosynthesis
era of universe lasting from about 0.001 second to about 3 minutes after big bang, by the end of which virtually all of the neutrons and about one seventh of the protons in the universe had fused into helium.
carbon stars
stars whose atmospheres are especially carbon rich, thought to be near the ends of their lives, carbon stars are the primary sources of carbon in the universe.
rotation curve
graph that plots rotational or orbital velocity against distance from the center for any object or set of objects.
starburst galaxy
galaxy inwhich stars are forming at an unusually hight rate.
elliptical galaxies (textbok!)
appear rounded in shape, often longer in one direction, like a football, have no disks and contain a very little cool gas and dust compared to spiral ones, though they often contain very hot, ionized gas!
21-cm line
a spectral line from atomic hydrogen with a wavelength of 21 cm (in the radio portion of the spectrum)
what is the evidence for dark matter in clusters of galaxies?
galaxy motions

x-ray emissions from hot gas

gravitational lenses
How do we derive the mass estimate of a cluster?
using newtons law of motion
How do we derive the velocities of galaxies in a cluster?
their doppler shifts?
How many times larger is the mass we find from galaxy motions in a rich cluster than the mass in individual galaxies?
50 times larger

Galaxies are a very tiny constituent of galaxy clusters.
What gas is left over from the epoch of cluster formater?
X-ray emission from hot gas

Clusters contain large amounts of X-ray emitting hot (100 million K) gas.
What tells us the distribution of the cluster mass?
Hot gas sits in the gravitational well of the cluster.

Temperature of hot gas (particle motions) and its distribution tell us the cluster mass
What sits in the gravititational well of a cluster?
hot gas.
what tells us a clusters mass?
Temperature of hot gas (particle motions) and its distribution tell us the cluster mass.
Gravitational lensing
the bending of light rays by gravity, can also tell us a cluster’s mass
How do we detect Dark Matter?
since dark matter has mass, there is a gravitational force between dark matter and visible matter.

this forces increases the net force of gravity exerted on the stars and gas, causing faster orbital motions than if there were no dark matter
In the solar system, where are orbital speeds highest?
The sun. Objects closest to the sun feel the strongest force of gravity and therefore orbit fastest.
if a planet had a weak gravitational force but a high orbit speed, what would happen?
it would leave the solar system.
how would the rotation curve of the solar system differ if the mass of the sun were different?
if the suns mass were higher than it is, each planet would feel a stronger gravitational attraction. It would have to move faster in order to have the same circular orbit.
In which region of the galaxy do stars orbit most slowly?
near the center of the galaxy.
in the inner region of the galaxy, the orbital speeds of the stars _______ as the distance from the center increases.

- increases.
- decreases.
- stays the same.
in the outer parts of the galaxy, the orbital speeds of the stars and gas ______ with increasing distances from the center.

-stay the same.
stay the same. the rotation curve is flat in the outer region of the galaxy. So all the stars and gas have roughly the same orbital speeds.
How do we determine the orbital speeds of gas and stars?
by measuring the doppler shifts of hydrogen gas emission lines. Looking at doppler shifts for different distances from the galactic center gives us the rotation curves for edge on spiral galaxies.
if you use a telescope to look at a portion of the disk to the right of the central bulge, will emission lines be red or blue shifted?
redshifted because the stars and gas to the right of the central bulge are moving away from the earth.
the greater the doppler shift, the greater the objects...
What two things does the speed of an object that orbits in response to gravity depend on?
orbital radius -

the greater the O.R, the weaker the Gravitational Force the object feels.

The total mass lying inside the orbit -

(the encircled mass.) the greater this is, the stronger the gravitational force.
to compensate for gravity weakening with distance, the encircled mass (desired) must be...
increasing with distance.
How does encircled mass change with increasing distance?
it stays constant, as the E.M does not increase noticeably with increasing distance from the sun.
the mass of the solar system is:

-mostly in the outer region.
-concentrated at the center.
-spread evenly throughout
concentrated at the center. the encircled mass is essentially the same for all radii outside the sun.
as you go out from the sun, the orbital speed should_______ with increasing radius.
remain the same.

orbiting speed rises with increasing encircled mass but decreases as the orbit gets bigger. In the solar system, orbital speed decreases with distance.
in the solar system, orbital speed ______ with distance.
most of the mass in a spiral galaxy is distrubuted where?
outside the central region.
For a particular orbital radius, the encircled mass is proportional to....
the encircled luminosity.
how do we obtain dark matter mass?
by subtracting the stellar mass distribution from the total mass distribution.
Is there a region of a spiral galaxy where essentially of the mass is due to stars?
yes, the central region.
why does the expected rotation curve decrease beyond the galactic disk?
very few stars exist there.

thus, the force of gravity weakens with increasing distance.
what is the dark matter halo?
region far from the galactic disk of a spiral galaxy where mass is thought to be
What is responsibel for most of the mass (and size) of a spiral galaxy?
dark matter. although we don't know what the fuck that is.
Why does the rotation curve of a spiral galaxy increase with increasing distance near the galaxy's center?
the encircled mass increases so fast that gravity increases with radius.
What is the evidence that there is dark matter beyond the disk of a spiral galaxy?
the stellar mass distribution does not account for the rotation curve of galaxies.
the outer portion of the rotation curve of a galaxy is flat. What does this fact indicate?
where the curve is flat, the encircled mass increases with increasing distance.
if all of the dark matter were located in the disk of a spiral galaxy.

its rotation curve would still be flat
- its rotation curve would have higher velocities than if there were not dark matter, but the velocities would decrease with distance beyond the disk.
- its rotation curve would be decreasing with distance shortly beyond the center of the disk. its rotation curve would have higher velocities than if there were no dark matter, but the velocities would decrease with distance beyond the disk.
its rotation curve would have higher velocities than if there were no dark matter but the velocities would decrease with distance beyond the disk.
suppose we found out that there is no dark matter in galaxies. what is an alternative explanation of the rotation curve?
our understanding of gravity is wrong for great distances.
What is the evidence there is no dark matter in our solar system?
the rotation curve of the planets is consistent with all the mass being due to the sun.
What can you conclude from the fact that quasars usually have very large redshifts?
-they are generally very distant
- they were more common early in time
- galaxy collisions might turn them on.
-nearby galaxies might hold dead quasars.
Dark Energy (or quintessence)
An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate.
How is the rotation curve determined beyond the extent of stars?
From 21-centimeter emission from atomic hydrogen gas that lies well beyond the stellar content.
Why doesn’t the rotation curve decline at radii containing all the stars and most of the gas?
There must be large amounts of unseen matter at large radii whose presence is noticed by its gravitational effect on the stars and gas.
if the sun were twice as distant from milky way center as it currently is, its orbital velocity would be...
the same!
What makes velocity measurements more problematic in elliptical galaxies?
they lack significant amounts of atomic hydrogen gas, making it harder to use the doppler shift and the 21 centimeter emission line.
What can we use to determine a rotation curve for ellipticals?
stars and globular cluster radii's.
What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?

A. Its mass is concentrated at the center
B. It rotates like the solar system
C. It’s especially rich in dark matter
D. It’s just like the Milky Way
C. it is especially rich in dark matter.
What is the evidence for dark matter in clusters of galaxies?
- Galaxy motions
- X-ray emission from hot gas.
- Gravitational lenses.
The fate of the universe depends on what three things?
its expansion rate.
the total gravitational force any object experiences due to the distribution of matter in the universe
- dark energy, if it actually fucking exists.
If gravity is the only force that affects the evolution of the universe, then what two factors does the fate of the universe depend on?
only on the current rate of expansion and the current mass density.
the fate of the universe depends on whether the _________ _________ exceeds or falls short of the _________ _______ _______.
whether the mass density exceeds or falls short of the critical mass density.
The critical density of the universe can be estimated from the ___________. Determined by the _________ _________.
current rate of expansion, determined by the hubbles constant.
Why is dark energy said to act like a repulsive force?
it causes the expansion rate of the universe to speed up over time.
if dark energy is strong enough relative to gravity, the universe may _________ _______ at an ___________ ___________ ______. What is this possible fate called?
if dark energy is strong enough relative to gravity, the universe may expand forever at an ever-increasing rate. This is called an accelerating universe.
With the existence of dark energy (and the average mass density remaining constant with the current estimate) would the universe be older or younger?
the universe would be older than it would if there were no dark energy.

the universe must have expanded more slowly in the past than if there were no dark energy. (the galaxies would need more time to reach their current average seperation.
if dark energy does not exist...
the age increases as the mass density decreases
if dark energy does exist...
the age would be higher for a particular value of the mass density
How have astronomers been able to determine how the rate of expansion of the universe has changed during the past five billion years or so?
by observing white dwarf supernovae in faraway galaxies, since they are good standard candles they can be used to determine the distance and lookback time to host galaxies.
What does lookback time tell us?
how long ago a supernova occured.
the average distance between galaxies five billion years ago would be smallest for which type of universe? What would that result in now?
the increase in average distance between then and now would be greatest, resulting in the largest redshift.
an accelerating universe must have...
dark energy, bitch.
the critical mass density of the universe would __________ than currently thought if the Ho constant were found to be higher than current estimates.

- higher than
- lower than
- the same as
higher than.
for which model of the universe is the age of the universe shortest?

-one expanding forever at an ever-slowing rate.
-recollapsing universe.
What do we mean by a protogalactic cloud?
It is a cloud of matter that contracts to become a galaxy.
the older the star, the lower...
the proportion of elements heavier than hydrogen and helium.
Orbital speed is determined by....
a stars distance from the galactic center.
Why are collisions between galaxies more likely than collisions between stars within a galaxy?
Relative to their sizes, galaxies are closer together than stars.
Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies.
can a central dominant galaxy ever be a spiral galaxy?
Why should galaxy collisions have been more common in the past than they are today?
Galaxies were closer together in the past because the universe was smaller.
The distinguishing feature of a starburst galaxy is:
a rate of star formation that may be 100 or more times greater than that in the Milky Way.
According to current understanding, what is a quasar?
an active galactic nucleus that is particularly bright.
The mass of the supermassive black hole thought to power a typical active galactic nucleus would be around ______.
1 billion solar masses.
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
Correct Answer:

light emitted by hot gas in an accretion disk that swirls around the black hole.
According to the theory that active galactic nuclei are powered by supermassive black holes, the source of energy that supplies the power is _____.
gravitational potential energy released by matter that is falling toward the black hole.
Intergalactic hydrogen clouds are easiest to study by looking at ______.
Correct Answer:

absorption lines in quasar spectra.

Only the nearest intergalactic hydrogen clouds are visible in 21 cm emission.
We can study how galaxies evolve because ______. [Hint]

-We are really smart astronomers.
-The farther away we look, the further back in time we see.
-We can watch as they interact in real time.
-Galaxies are transparent to optical light.
The light from distant galaxies take a long time to get to us, so we see them as they were millions of years ago.
The differences in the ages of stars in different parts of spiral galaxies tell us that ______.
Correct Answer:

halo stars formed before the galaxy got its gaseous disk.
Variations in age and heavy element content among the Milky Way's globular clusters halo suggest that ______.
Correct Answer:

not all of the globular clusters originated from the same protogalactic cloud.
One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed due to other interactions, we'd expect this galaxy to be ______.
a spiral galaxy.
Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either:
relatively little angular momentum or relatively high density. Low angular momentum will tend to prevent gas from collecting into a spinning disk, and high density will tend to allow the gas to form stars before it finishes collapsing to make a disk.
A quasar's spectrum is hugely redshifted. What do most astronomers think this large redshift tells us about the quasar?
Correct Answer:

The distance to the quasar.
A few decades ago, there was great controversy among astronomers over the question of quasar distances, with some arguing that quasars are much nearer than application of Hubble's law would seem to imply. Why do nearly all astronomers now agree that quasars really are quite far away?
We now have images and spectra that clearly show quasars to be embedded at the centers of distant galaxies and within distant galaxy clusters.
Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
Active galactic nuclei exist tend to become less active as they age.

Remember that at large distances we are seeing into the past. The fact that active galactic nuclei are far away means that they are young. The galaxies we see nearby are older. Thus we conclude that activity is more common among young galaxies than older ones. The emission is assumed to come from the accretion disk around a massive black hole. Since the black hole cannot disappear, some other mechanism must cause the accretion disk to become less bright.
Suppose that we observe a source of X-rays that varies substantially in brightness over a period of a few days. What can we conclude?
Correct Answer:

The X-ray source is no more than a few light-days in diameter.
Central black holes can be very efficient for converting the mass-energy of infalling matter to thermal energy in the accretion disk. Roughly what percentage of the mass-energy can be converted to other forms of energy as matter falls into a black hole?
Correct Answer:

Quasar spectra often show many absorption lines that all appear to be due to the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. Why do we think this is the case?
Correct Answer:

We are seeing absorption lines from clouds of gas that lie between us and the quasar, and therefore each cloud has a different redshift.
What was the significance of the end of the era of nucleosynthesis, when the universe was about 3 minutes old?
the basic chemical composition of the universe had been determined.
According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?
During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
Correct. The excess was only about 1 extra matter particle for every billion particles of matter and antimatter, and the annihilation of the rest of the billion explains why there are so many photons in the universe.
In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?
Before that time, the gas in the universe was dense and ionized and thus did not allow light to travel freely.
In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?
By the time stable helium nuclei had formed, the temperature and density has already dropped too low for helium fusion to occur.
How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?
Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.
Based on the results from the WMAP satellite, the overall composition of the energy of the universe is ______.
4% ordinary (baryonic) matter, 23% non-baryonic dark matter, 73% dark energy
Why can't current theories describe what happened during the Planck era?
We do not yet have a theory that links quantum mechanics and general relativity.
How do we determine the conditions that existed in the very early universe?
From the current expansion rate we can work backward to estimate temperature and densities at various times in the early universe.
According to the Big Bang theory, how many forces -- and which ones --- operated in the universe during the GUT era?
Correct Answer:
gravity and a single force that later became the strong, weak, and electromagnetic forces.
Based on our current understanding of physics, it would seem that we can understand the conditions that prevailed in the early universe as far back in time as ______.
Correct Answer:

one ten-billionth of a second after the Big Bang.
What happens when a particle of matter meets its corresponding antiparticle of antimatter?
The combined mass-energy of the two particles is completely transformed into radiation-energy (two photons).
What is the significance of the Planck time?
Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.
The four fundamental forces that operate in the universe today are:
strong force, weak force, electromagnetic force, gravity
A "GUT" (grand unified theory) refers to theories that ______.
Correct Answer:

unify the strong force with the electromagnetic and weak forces.
Gravity is not expected to unify with the other forces until much higher energies than those at which the GUT force operates.
What do we mean by inflation?
We mean a sudden and rapid acceleration of the universe that went on for a tiny fraction of a second at the time the strong force froze out from the GUT force.
correctly summarize the events in the early universe according to the Big Bang theory
The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.
The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
Correct Answer:

(1) the theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the universe.
Measuring the amount of deuterium in the universe allows us to set a limit on _______.
Correct Answer:

the density of ordinary (baryonic) matter the universe.
Based on the Big Bang theory, what do observations of the cosmic microwave background and of chemical abundances in the universe suggest about dark matter?
Much of the dark matter must be made of non-ordinary (nonbaryonic) matter, such as WIMPs.
Correct. The level of variation seen in the temperature of the cosmic microwave background suggests that there was additional, weakly interacting dark matter around. The abundances of deuterium and lithium both point to baryonic matter making up a lower density than the observed density of dark matter, suggesting that much of the dark matter must be nonbaryonic.
Which of the following observations cannot be explained by the Big Bang theory UNLESS we assume that an episode of inflation occurred?
Correct Answer:

The fact that the temperature of the cosmic microwave background is almost the same everywhere.
The idea of inflation makes one clear prediction that, until quite recently, seemed to contradict the available observations. What is this prediction?
The universe should be geometrically "flat" (in the four dimensions of spacetime).
Correct. Until recently, it was assumed that the overall geometry depended on the total density of matter, which seems to be less than the critical density needed for a flat universe. However, astronomers were astonished to find that energy also contributes to making the universe flat, and recent observations suggest it is the dominant overall ingredient of the universe (the so-called "dark energy").
Olber's Paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
why is the sky dark at night?
Telescopes being planned for the study of the earliest stages in galactic lives will be optimized for observations in ______.
infrared light

Correct. We must observe very distant galaxies to see how galaxies looked when they were very young, and these galaxies have such large redshifts that any light they emitted as visible or ultraviolet has been shifted into the infrared.
Why should galaxy collisions have been more common in the past than they are today?
Galaxies were closer together in the past because the universe was smaller.
The distinguishing feature of a starburst galaxy is:
a rate of star formation that may be 100 times greater than that of the milky way.
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
light emitted by hot gas in an accretion disk that swirls around the black hole.
According to the theory that active galactic nuclei are powered by supermassive black holes, the source of energy that supplies the power is _____. [Hint]

-matter-antimatter annihilation occurring just outside the event horizon of the black hole.
-gravitational potential energy released by matter that is falling toward the black hole.
-nuclear fusion in the accretion disk surrounding the black hole.
-jets emerging from the accretion disk.
gravitational potential energy released by matter that is falling toward the black hole.
The mass of the supermassive black hole thought to power a typical active galactic nucleus would be around ______.
1 billion solar masses

A black hole of this mass probably arose from the supernova of a single high-mass star.
intergalactic hydrogen clouds are easiest to study by looking at...
Correct Answer:
absorption lines in quasar spectra.
The older the star, the lower its proportion of...
of elements heavier than hydrogen and helium.
What do we call the bright, sphere-shaped region of stars that lie within a few thousand light-years of the center of the Milky Way Galaxy?
suns location in the milky way galaxy.
Correct Answer:
in the galactic disk, roughly halfway between the center and the outer edge of the disk.
What are the Magellanic Clouds?
Two small galaxies that probably orbit the Milky Way Galaxy.
What do we mean by chemical enrichment of the gas in the Milky Way Galaxy?
It is the idea that the percentage of heavy elements (heavier than hydrogen and helium) is slowly growing with time.
What are cosmic rays?
they are subatomic particles that travel close to the speed of light.
The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with ______.
radio telescopes observing at a wavelength of 21 centimeters.
Correct. Radio emission in the 21-cm line is the only significant emission from most atomic hydrogen gas.
What is an ionization nebula?
It is a colorful cloud of gas that glows because it is heated by light from nearby hot stars.
What is a rotation curve?
A graph showing how orbital velocity depends on distance from the center.
In general, which galaxies are seen in the earliest (youngest) stages of their lives?
the galaxies that are farthest away.
Hubble's galaxy classification diagram (the "tuning fork") ______.
Correct Answer:
Relates galaxies according to their shapes, but not according to any evolutionary status.
Suppose that we suddenly discovered that all these years we'd been wrong about the distance from Earth to the Sun, and it is actually 10% greater than we'd thought. How would that affect our estimate of the distance to the Andromeda Galaxy?
Correct. The Earth--Sun distance (1 AU) is the baseline for parallax, so all the distances we've measured by parallax would be 10% greater than we'd thought. Since parallax is used to help us calibrate all other standard candle techniques, all our other measurements also would have to increase by 10%.
Why are white dwarf supernovae more useful for measuring cosmic distances than massive star supernovae?
White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
tully-fisher works only for...
spiral galaxies
What prevents a white dwarf from having a mass greater than the white dwarf limit (or Chandrasekhar limit)?
Electron degeneracy pressure depends on the speeds of electrons, and as a white dwarf's mass approaches the white dwarf limit, its electron speeds are already approaching the speed of light.
How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?
The accretion disk around a neutron star is much hotter and emits higher-energy radiation than the accretion disk around a white dwarf.
Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice?
Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

Correct. These are effects predicted by general relativity.
True or false, A spaceship passing near a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main-sequence star.
True. For example, a spaceship might pass quite safely by the black hole at a distance of 10,000 kilometers, but at 10,000 kilometers from the center of the main-sequence star the spaceship would be deep inside the star. (Of course, the spaceship would be destroyed long before it got that close to the star's center.)
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly
3 solar masses
Suppose you have a 100-watt light bulb that you leave turned on for one minute. How much energy does it use?
6,000 joules
Correct. 1 watt = 1 joule/s, so a 100-watt light bulb uses 100 joules of energy per second. Thus, in 1 minute, it uses 60 x 100 = 6,000 joules of energy.
Suppose a photon has a frequency of 300 million hertz (300 megahertz). What is its wavelength?
Correct Answer:
1 meter.
Yes, it can: wavelength x frequency = speed of light
The star Sirius has a surface temperature about twice that of the Sun (12,000 Kelvin for Sirius versus 6,000 Kelvin for the Sun). Per unit surface area, how much more energy does Sirius radiate than the Sun?
16 times as much energy.
Correct: the amount of energy from thermal radiation depends on the fourth power of temperature. So an object 2x as hot produces 24 =16 times as much energy per unit surface area.
According to the universal law of gravitation, if you triple the distance between two objects, then the gravitational force between them will ______.

increase by a factor of 9.

decrease by a factor of 9.
decrease by a factor of 3.
increase by a factor of 3.
decreases by a factor of 9
A 1.3 MSun white dwarf is:

A) larger in size than a 0.7 MSun white dwarf
B) smaller in size than a 0.7 MSun white dwarf
C) the same size as a 0.7 MSun white dwarf
D) It is impossible to tell since the sizes of white dwarfs are not correlated to their masses.
smaller in size than a 0.7 M sun white dwarf

White dwarfs gets smaller with increasing mass, since the added mass causes the star to compress (but it does not totally collapse because of electron degeneracy pressure).
Which of the following represents a successive decrease in size?

A) 1.3 MSun white dwarf, 0.7 MSun white dwarf, 3 MSun neutron star, 10 MSun black hole
B) 0.7 MSun white dwarf, 1.3 MSun white dwarf, 3 MSun neutron star, 10 MSun black hole
C) 1.3 MSun white dwarf, 0.7 MSun white dwarf, 3 MSun neutron star, 1 MSun black hole
D) 0.7 MSun white dwarf, 1.3 MSun white dwarf, 3 MSun neutron star, 1 MSun black hole
D) 0.7 MSun white dwarf, 1.3 MSun white dwarf, 3 MSun neutron star, 1 MSun black hole

Less massive white dwarfs are larger than more massive white dwarfs, neutron stars are about 10 km in radius, and the size of the event horizon of a black hole scales as 3 km * (M/MSun). So a 10 MSun would have a radius of 30 km and a 1 MSun black hole would have a radius of 3 km.
7) Which two quantities can we directly observe to determine the mass of the Milky Way?
B) velocity and distance of the Sun about the Galactic center

Since the Sun’s orbit around the Galactic Center takes 230 million years, we cannot hope to measure the period directly. We can, however, measure the distance and velocity to get the mass of the Milky Way.
12) Briefly explain how we know pulsars could not possibly be white dwarfs.
Pulsars spin too fast for a white dwarf to remain intact. Since the rotational velocity of the white dwarf would exceed the escape velocity from the white dwarf at these high rotation rates, the white dwarf would literally fly apart. Therefore, pulsars must be neutron stars.
The lower limit on stellar mass of 0.08 MSun is a result of the onset of ____________ pressure, while the upper limit of 150 MSun is a result of ____________ pressure blowing apart the protostar. (The order matters here).
D) electron degeneracy, radiation
Give two examples of phenomena governed by a 1/(distance)2 law.
propagation of light
propagation of sound
electrostatic force
How many helium nuclei need to be fused together to make carbon?
Three – it takes three 4He to make one 12C
If the velocity of a planet in orbit around a star decreases by a factor of two, what must happen to its distance from the star assuming no other force is acting on it?
From the conservation of angular momentum, mass * velocity * radius, must be constant if no external force acts on the system, so if velocity goes down by two, the distance must increase by a factor of two.
What contradiction that we discussed in class is solved by understanding the Malmquist
The fact that most stars we see naked eye in the night sky are more luminous than the Sun, while studies have shown that the Sun is more massive (and therefore more luminous than) most stars. The Malmquist bias comes about because we can preferentially see intrinsically more luminous stars to greater distances than low luminosity stars, thus biasing our result.
Briefly explain why we need to observe what’s in and behind molecular clouds with infrared telescopes and not optical telescopes.
The gas and dust in molecular clouds absorb and scatter optical light, but allow longer wavelength light such as infrared light to pass through. Therefore, one wants to use an infrared telescope to see what is in and behind a molecular cloud.
Suppose a star much hotter than our Sun has a measured wavelength of peak flux in the visible band just like our Sun. This seems to contradict Wien’s Law. What is the most logical explanation for such an observation?
If the star is moving away from us at a high enough velocity, its peak wavelength in the ultraviolet will be redshifted into the optical band.