Which part of the galaxy has gas with the hottest average temperature?
The Halo
What is the typical percentage (by mass) of elements other than hydrogen and helium in stars that are forming right now in the vicinity of the Sun?
2%
Where are most of the Milky Way's globular clusters found?
in the halo
How do we determine the Milky Way's mass outside the Sun's orbit?
from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun
How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
The total amount of gas will be much less than it is today.
If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles
What are cosmic rays?
subatomic particles that travel close the speed of light
Harlow Shapley concluded that the Sun was not located at the center of the Milky Way Galaxy by
mapping the distribution of globular clusters in the galaxy.
Approximately how far is the Sun from the center of the galaxy?
27,000 light-years
Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?
200 million years
What elements do astronomers consider heavy elements?
all elements besides hydrogen and helium
How do high mass stars transport newly formed elements into the interstellar medium?
stellar winds and supernova explosions
How do low mass stars transport newly formed elements into the interstellar medium?
stellar winds and formation of planetary nebula
How do high-mass stars make it more difficult for a molecular cloud to form more stars?
1. Radiation pressure pushes ionized gas away.
2. Ultraviolet photons heat the gas.
3. Strong winds push gas away.
4. Ultraviolet photons ionize the gas.
What evidence suggests that the protogalactic cloud that formed the Milky Way resulted from several collisions among smaller clouds?
Halo stars differ in age and heavy-element content, but these variations do not seem to depend on the stars' distance from the galactic center.
What evidence supports the existence of a black hole at the center of our galaxy?
The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region only 3 light-years across.
T/F: Most of the mass of the galaxy is located at the galactic center in the form of a massive black hole.
FALSE
Elliptical Galaxy Formation: (4)
#NAME?
Spiral Galaxy Formation: (2)
#NAME?
Our Sun belongs to the ____ of the Milky Way Galaxy
disk component
you know that from afar we'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?
Its light will become so redshifted that it will be undetectable.
If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________.
at a constant, normal rate as you approach the event horizon
If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would __________.
accelerate as you fall and cross the event horizon completely unhindered
Which statement concerning black hole masses and Schwarzschild radii is not true?
In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.
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.
Which of the following observatories is most likely to discover a black hole in a binary system?
the Chandra X-Ray Observatory
Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion d
the red giant will probably supernova within the next million years
Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) One foolhardy da
X-rays from the accretion disk
Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) Through a bizarr
They evolved on a different planet in a different star system, and moved to their current location.
What is the basic definition of a black hole?
an object with gravity so strong that not even light can escape
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly ________.
3.0 solar masses
What do we mean by the event horizon of a black hole?
It is the point beyond which neither light nor anything else can escape.
What do we mean by the singularity of a black hole?
It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
The Schwarzschild radius of a black hole depends on ________.
only the mass of the black hole
Degeneracy pressure stops the crush of gravity in all the following except
a very massive main-sequence star.
Which of the following correctly describes how light will be affected as it tries to escape from a massive object?
Light escaping from a compact massive object, such as a neutron star, will be redshifted.
Consider an X-ray binary system in which a compact object, surrounded by an accretion disk, is in a binary orbit with another star. All of the following statements about such accretion disks are true except:
Several examples of flattened accretion disks being "fed" by a large companion star can be seen clearly in photos from the Hubble Space Telescope.
Prior to 1991, most astronomers assumed that gamma-ray bursts came from neutron stars (with accretion disks) within the Milky Way Galaxy. How do we now know that this hypothesis was wrong?
Observations from the Compton Gamma-Ray Observatory showed that gamma-ray bursts come randomly from all directions in the sky.
Imagine what would happen if Jupiter were suddenly replaced by a black hole with the same mass as Jupiter.
The orbits of the solar system would be unaffected (including Jupiter's).
Rank the following 4 items in order of increasing density (low to high).
black hole singularity, main-sequence star, white dwarf, neutron star
main-sequence star, white dwarf, neutron star, black hole singularity
True or False:
No visible light can escape a black hole, but things such as gamma rays, X-rays, and neutrinos can.
FALSE
T/F: Light escaping from white dwarfs will show a gravitational redshift.
TRUE
The circular but relatively flat portion of the galaxy is the _____.
disk
The first portion of the galaxy to form was the _____.
halo
A tightly packed group of a few hundred thousand very old stars is a _____.
globular cluster
A _____ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust.
spiral arm
Our Milky Way galaxy is a _____.
spiral galaxy
Stars orbiting in the _____ near the galaxy's center can have orbits highly inclined to the galactic plane.
bulge
The following figures show several stars found in the disk and halo of the Milky Way Galaxy. Rank the stars based on their current age, from oldest to youngest. If two (or more) stars have approximately the same age (that is, ages within a few million yea
1. (Tied) -Red Giant in Globular Cluster M13
-Red main-sequence star in globular cluster M13
2. -The Sun
3. -Hot, blue main-sequence star in disk
Listed following are several stars found in the disk and halo of the Milky Way Galaxy. Assume that both the blue and yellow disk stars are members of the same open cluster. Rank the stars based on the abundance of elements heavier than carbon that you wou
1. (tied) -Hot, blue main-sequence star in open cluster in disk
- Yellow main-sequence star in open cluster disk
2. (tied)- -Red Giant in Globular Cluster M13
-Red main-sequence star in globular cluster M13
Disk Stars (yellow):
youngest stars, the sun, stars that all orbit in nearly the same plane, high mass stars
Halo Stars (green)
Stars whose orbits can be inclined at any angle, oldest stars, globular clusters, stars with the smallest abundance of heavy elements.
What are the key differences between the disk and the halo?
#NAME?
Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest.
- the edge of the central bulge, a cloud of gas and dust in the outskirts of th
1. a globular cluster in the outskirts of the halo
2. a cloud of gas and dust in the outskirts of the disk
3. our solar system
4. the edge of the central bulge
How does the interstellar medium affect our view of most of the galaxy?
It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
If we could see our own galaxy from 2 million light-years away, it would appear ________.
as a flattened disk with a central bulge and spiral arms
How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
It should have a higher fraction of elements heavier than hydrogen and helium.
All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except ________.
-X-ray binaries
-young stars
-high-mass, red supergiant stars
-globular clusters
globular clusters
Which of the following statements comparing halo stars to our Sun is not true?
Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.
What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?
The orbital speeds of stars far from the galactic center are surprisingly high.
Spiral arms appear bright because ________.
they contain more hot young stars than other parts of the disk
If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
What is the best evidence for an extremely massive black hole in the center of the Milky Way?
The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.
How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
The diameter is about 100 times as great as the thickness.
The Sun's location in the Milky Way Galaxy is ________.
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 orbit the Milky Way Galaxy
Fill in the blank: Elements heavier than hydrogen and helium constitute about ________ of the mass of the interstellar medium.
2%
Interstellar dust consists mostly of ________.
microscopic particles of carbon and silicon
What best explains why our galaxy has spiral arms?
The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.
Most stars in the Milky Way's halo are ________.
very old
What is the diameter of the disk of the Milky Way?
100,000 light years
What is the thickness of the disk of the Milky Way?
1,000 light years
Why do stars in the disk of a spiral galaxy orbit in roughly the same direction, in the same plane in space?
The gas that formed the stars was shaped as a giant, rotating disk.
The very first stars in the universe were made of
hydrogen and helium.
On a dark summer night in the northern hemisphere, we can see the Milky Way, but we can't see the center of the Milky Way. Why not?
Interstellar dust and gas absorb and scatter visible light.
Why are we unlikely to find Earth-like planets around halo stars in the Galaxy?
Halo stars formed from gas containing few heavy elements, the elements needed to create terrestrial planets like the Earth.
Why do disk stars bob up and down as they orbit the galaxy?
because the gravity of other disk stars always pulls them toward the disk
The globular cluster M13 belongs to the ____ of the Milky Way Galaxy.
spheroidal component
An ____ contains hot, ionized gas but very little cool gas or dust
elliptical Galaxy
The Milky Way is an
spiral galaxy
The type of galaxy known as an ____ was more common in the universe 10 billion years ago.
irregular galaxy
Scientists investigation ____ study how the distribution of galaxies changes with time.
cosmology
Spiral Galaxies Characteristics: (5)
contain abundant clouds of cool gas and dust, are rare in central regions of galaxy clusters, have significant, ongoing star formation, contain many bright hot stars, have a flattened disk of stars
Elliptical Galaxies Characteristics: (2)
are more reddish in color, contain primarily old low-mass stars
What baseline distance must we know before we can measure parallax?
the Earth-Sun distance
three-step process of how we use Cepheids as a tool to make cosmic distance measurements?
Step 1: Measure the period of the Cepheid's brightness variations.
Step 2: Use the period-luminosity relation to determine the Cepheid's luminosity.
Step 3: Calculate the Cepheid's distance from its luminosity and apparent brightness.
Which cosmic distance measurement techniques are considered standard candle techniques?
white dwarf supernovae (distant standards), main-sequence fitting, Cepheids
Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?
We have not observed white dwarf supernovae in most galaxies.
If that method led us to underestimate the Earth-Sun distance by 10%, how would it affect other measurements in the distance chain?
They would all be off by the same 10%.
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
Cepheids
Hubble's law expresses a relationship between __________.
the distance of a galaxy and the speed at which it is moving away from us
We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________.
measuring the distances to many distant galaxies with a standard candle technique
Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 (3�104) kilometers per second. According to Hubble's law, approximately how far away is the galaxy?
1.4 billion light-years
Based on what you have learned, which of the following best describes the meaning of Hubble's constant (H0 )?
It describes the expansion rate of the universe, with higher values meaning more rapid expansion.
Where are large dust clouds predominantly located in the galaxy M51?
within or on the edges of the spiral arms
Where are the ionization nebulae predominantly located in the galaxy M51?
within or on the edges of the spiral arms of the galaxy
Why are the ionization nebulae so bright?
They are regions where gas is ionized by hot, young stars.
In a photo like the Hubble Deep Field (the figure above), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
the galaxies that are farthest away
Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
We can learn the distance to the galaxy.
In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?
By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation
Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble's law.)
away from us at 2,000 km/s
Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.
Does Hubble's law work well for galaxies in the Local Group? Why or why not?
No, because galaxies in the Local Group are gravitationally bound together.
What is the best way to determine a galaxy's redshift?
Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory.
Why can't we see past the cosmological horizon?
Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
A standard candle is ________.
a light source of known luminosity
What is a Cepheid variable?
a type of very luminous star that makes an excellent standard candle
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
They are rare events, so we have observed them in only a tiny fraction of all galaxies.
When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?
The inside of the balloon does not represent any part of our universe.
If we say that a galaxy has a lookback time of 1 billion years, we mean that ________.
its light traveled through space for 1 billion years to reach us
Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called ________.
the cosmological horizon
Current estimates place the age of the universe at about ________.
14 billion years
Collisions between galaxies typically unfold over a period of ________.
hundreds of millions of years
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.
Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately
100 billion.
Which of the following types of galaxies appear reddest in color?
ellipticals
Which types of galaxies have a clearly defined disk component?
spirals and lenticulars
Most large galaxies in the universe are
Spiral or lenticular
Why do elliptical galaxies appear yellow or red?
They have very little dust or cool gas, and thus have little ongoing star formation and contain no hot, young blue stars
Which of the following is a consequence of Hubble's Law?
The more distant a galaxy is from us, the faster it moves away from us.
What makes white-dwarf supernovae very good standard candles for distance measurements?
They are very bright, so they can be used to determine the distances to galaxies billions of light-years away and they should all have approximately the same luminosity.
What is the most accurate way to determine the distance to a very distant galaxy?
Cepheid variables
Which of the following cannot be true of the very first stars formed in the universe?
Some may have had rocky planets around them.
Which of the following statements is an explanation, not an observation?
Stars shine with energy generated by nuclear fusion in their cores.
The speed of the star's orbit depends on the star's distance from the galactic center and __________.
the mass of the galaxy that resides within the star's orbit
According to the law of gravity as we understand it today, the only explanation for these this flatness is that __________
substantial amounts of mass must reside at great distances from the galactic center
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
Based on observations of the cosmic microwave background, the overall composition of the universe is approximately ________.
4% ordinary (baryonic) matter, 23% non-ordinary (nonbaryonic) dark matter, 73% dark energy
Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?
The orbital speeds would fall off sharply with increasing distance from the galactic center.
Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
How does gravitational lensing tell us about the mass of a galaxy cluster?
Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
They travel at speeds close to the speed of light.
Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
The actual density, even with dark matter included, is less than about a third of the critical density.
Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
Dark energy does not exist and there is much more dark matter than we are aware of to date.
Which of the following best summarizes what we mean by dark energy?
It is a name given to whatever is causing the expansion of the universe to accelerate with time.
Which of the following best summarizes what we mean by dark matter?
matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
What is a rotation curve?
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. What is this prediction?
The universe should be geometrically "flat" (in the four dimensions of spacetime).
Which of the following best sums up current scientific thinking about the nature of dark energy?
Dark energy probably exists, but we have little (if any) idea what it is.
The distribution of the dark matter in a spiral galaxy is
approximately spherical and about ten times the size of the galaxy halo.
A mass-to-light ratio for a galaxy of much greater than one indicates that
on average, each solar mass of matter in the galaxy emits much less light than our Sun.
Which of the following is not evidence for dark matter?
the expansion of the universe
Which of the following are candidates for galactic dark matter?
Jupiter-size objects
WIMPs
brown dwarfs
faint red stars
Measuring the amount of deuterium in the universe allows us to set a limit on
the density of ordinary (baryonic) matter in the universe.
What do we mean when we say that a particle is weakly interacting?
It interacts with other particles only through the weak force and the force of gravity.
If all the "dark matter" in our universe were to be instantaneously removed, which of the following would not happen?
-The universe would expand forever.
-The Milky Way would fly apart.
-The Solar System would fly apart.
-Clusters of galaxies would fly ap
The Solar System would fly apart.
Which of the following items would not be considered "dark matter" by astronomers?
-weakly interacting massive particles
-black holes
-basketballs
-people
-red giants
Red giants
As the mass of the central star increases, the distance to the habitable zone __________ and the size (width) of the habitable zone __________.
increases / increases
Suppose that our Sun was cool enough to include Mercury in its habitable zone. Which of the following would be true in that case?
-Only Mercury would be in the Sun's habitable zone.
-Mercury and Venus would be in the Sun's habitable zone, but Earth and Ma
Only Mercury would be in the Sun's habitable zone.
Scientists think it is very unlikely that complex and large forms of life could evolve on planets that orbit stars that are much more massive than the Sun. Why?
The expected lifetime of a massive star is too short to allow for the evolution of complex life
High-mass main-sequence stars have a more distant and wider _____ than low-mass stars.
habitable zone
Sky surveys looking for radio signals generated by technology as part of the _____.
search for extraterrestrial intelligence.
The _____ is designed as a way of estimating the number of intelligent civilizations in the Milky Way.
Drake equation
The notion that living organisms with advantages that give them greater reproductive success (in some local environment) will survive while others perish is as example of what Darwin called _____.
natural selection.
One of the key premises for the _____ is that living organisms are able to produce far more offspring than their environment can support.
theory of evolution
The different time periods of the _____ are defined by changes in fossils found from those time periods.
geological time scale
Incoming ultraviolet light from the Sun can cause a _____ in a living organism's DNA that can affect its ability to survive and reproduce.
mutation
Which of the following best describes what we mean by a habitable world?
-a planet or moon that lies within its star's habitable zone
-a planet or moon on which humans could survive if we happened to go there
-a planet or moon that could support life, if
a planet or moon that could support life, if any life happened to be on it
The Sun's habitable zone ________.
extends from some place a little beyond the orbit of Venus to some place near the orbit of Mars
Why don't we expect to find life on planets orbiting high-mass stars?
The lifetime of a high-mass star is too short.
The "rare Earth hypothesis" holds that Earth-like planets will prove to be quite rare. Which of the following statements best sums up the current status of the debate over this hypothesis?
-We do not have enough data to settle the debate, because countera
We do not have enough data to settle the debate, because counterarguments can be made for each argument suggesting Earth-like planets may be rare.
At present, what is the primary way that the search for extraterrestrial intelligence (SETI) is carried out?
by using radio telescopes to search for signals from extraterrestrial civilizations
How does the Kepler mission plan to detect Earth-like planets around other stars?
by observing the slight dip in brightness of the central star as the planet transits
Which of the following are the best candidates to search for planets that might harbor extraterrestrial life?
-halo stars, because planets can form in isolation from disruptive stellar encounters
-low mass stars (less than one-tenth of the mass of the Sun
solar-mass stars, because they have both a large habitable zone and a long stellar lifetime.
At present, what is the primary way that astronomers carry out SETI programs (that is, search for extraterrestrial intelligence)?
by using radio telescopes to search for signals from extraterrestrial civilizations
In 1974, a radio message was sent out from the Arecibo observatory in Puerto Rico towards a globular cluster, 21,000 light-years away. Approximately how far has this message gotten as of today?
It's just passing stars that are close neighbors to the Sun in the Milky Way.
What is the Fermi Paradox?
Reasonable assumptions predict that a galactic civilization should have already arisen in the Milky Way. Yet, we have absolutely no evidence for it.
Which of the following is not a possible solution to the Fermi Paradox?
-Given the current age of our galaxy, there has not been enough time for a galactic civilization to develop.
-There is a galactic civilization, but we are currently unable to detect o
Given the current age of our galaxy, there has not been enough time for a galactic civilization to develop.