Astro Final, 16, Pre-Lecture Chapter 17 + Visual Quiz, ASTR FINAL, Astronomy Ch. 17, ASTR=Chapter 18 Visual Quiz, Module 13: Exploring Our Galaxy: The Milky Way, Chapter 18 : Astronomy Reading Quiz
The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
(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 primeval universe.
Which of the following represent observations or measurements that have confirmed key predictions of the Big Bang model?
-Characteristics of the cosmic microwave background. -The abundance of helium in the universe.
Complete each of the following sentences about the early universe
-Most of the helium in the universe formed during the ERA OF NUCLEOSYNTHESIS. -The GUT ERA refers to a time period in which the strong, weak, and electromagnetic forces were all unified. -The PLANCK ERA refers to a time period in which all four forces are thought to have been unified. -The COSMIC MICROWAVE BACKGROUND is our name for the photons we see that were first released when the universe was about 380,000 years old. -The BIG BANG is the name we give to the moment when the expansion of the universe began. -A very short period of extremely rapid expansion, called INFLATION, is thought to have occurred when the universe was about 10^-38 second old.
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A standard candle is _________.
A light source of known luminosity standard candles known luminosities
According to the Big Bang theory, why do we live in a universe that is made of almost entirely 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 expect the very slight excess of matter particles
Which of the following is not one of the three major categories of galaxies? Spiral galaxies Globular galaxies Elliptical galaxies Irregular galaxies
Globular Galaxies
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
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 was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
The basic chemical composition of the universe had been determined.
Which of the following statements explains why the night sky is dark?
The universe has a finite age.
According to the Big Bang theory, how many forces -- and which ones -- operated in the universe during the GUT era?
Two forces: gravity and a single force that later became the strong, weak, and electromagnetic forces.
How do we determine the conditions that existed in the very early universe?
We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.
Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.
have a greater abundance of heavy elements
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______. visible light X-rays radio waves infrared light
infared light 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.
Spiral arms appear bright because ________.
they contain more hot young stars than other parts of the disk
Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be _________________.
uniformly bright
The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is typically ______.
1 billion solar masses
Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 16.1 in your text), the estimated number of galaxies in our observable universe is at least ______. 100 million 50,000 100 billion infinity
100 billion This estimate is found by counting galaxies in this small piece of the sky, and multiplying by the number of such pieces it would take to fill the entire sky.
Current estimates place the age of the universe at about _________.
14 billion years More precisely, the current estimate is 13.7 billion years, give or take a few hundred million years.
Cepheid Variable? A bright source of variable X-ray emission, thought to harbor a supermassive black hole A type of galaxy that varies in its light output A type of very luminous star that makes an excellent standard candle A main-sequence star of spectral type B5
A Type of very luminous star that makes an excellent standard candle Cepheids are pulsating variable stars for which we can infer the luminosity from the time between peaks of brightness, which makes them valuable as standard candles.
What does this diagram represent?
A particle and antiparticle colliding and converting all their mass into photons.
This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. How long did this light travel through space before it reached the telescope?
About 14 billion years.
Question 22: According to current understanding, what is a quasar?
An active galactic nucleus that is particularly bright right answer feedback: Observations show that quasars lie in the centers of distant galaxies, indicating that they are very luminous active galactic nuclei.
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 therefore 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 had already dropped too low for helium fusion to occur.
Which of the following statements about large-scale structure is probably not true?
Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?
Earth is so big that it the part explored by an ant appears flat.
It is more difficult to determine the amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
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. As the universe expands, the average distance between galaxies increases, making collisions less likely (on average) as time passes. (Note that this does not affect the likelihood of collisions within clusters, since clusters are gravitationally bound and are not expanding with time.)
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.
How does the hypothesis 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.
Which of the following is not an observed characteristic of the cosmic microwave background?
It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.
What happens to gas as it freely expands?
It gets less dense and cools
Which of the following statements is not true of the object known as Sgr A* in the center of our galaxy?
It is by far the brightest source of visible light lying in the direction of the galactic center.
Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
It is probably a halo star that is currently passing through the disk.
Which statement about the cosmic microwave background is NOT true?
It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
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.
This diagram represents what we call a saddle-shaped geometry. In the context of this chapter, what is its significance?.
It represents a two-dimensional analogy to one possible geometry for our universe.
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.
What does this graph show?
It shows calculations indicating that the temperature of the universe began quite high but is now quite low.
The data points on this graph represent the measured spectrum of the cosmic microwave background, while the solid curve represents a theoretically calculated thermal radiation spectrum for a temperature of 2.73 K. What is the significance of the near-perfect match between the data and the theoretical spectrum?
It shows that the cosmic microwave background has a thermal radiation spectrum, just as the Big Bang theory predicts it should.
According to the model in which 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 radio waves emitted from radio lobes found on either side of the galaxy we see in visible light intense radiation emitted by the black hole itself the combined light of thousands of young, high-mass stars that orbit the black hole
Light emitted by hot gas in an accretion disk that swirls around the black hole
Which of the following offers at least some evidence that inflation really occurred in the early universe?
Measurements of brightness variations in the cosmic microwave background.
This graph is made from data measurements of the cosmic microwave background (dots) and a model based on the hypothesis of inflation. What does it show?
Measurements of the separation between regions of different temperature (in the cosmic microwave background) agree with predictions made by the hypothesis of inflation
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.
Is space expanding within clusters of galaxies?
No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
Which of the following statements can NOT be tested by science today?
Prior to the Planck time, our universe emerged from another universe.
This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. What do the dark and bright variations represent?
Regions of the sky with very slightly different density at the time the radiation was emitted.
Which of the following is an important starting assumption in models of galaxy formation? Black holes form first, seeding the formation of galaxies. Some regions in the universe start out denser than others. All galaxies start out as spiral galaxies. Galaxies form first, then black holes.
Some regions in the universe start out denser than others. These small density enhancements are the seeds around which galaxies form; without these "seeds," models indicate that galaxies could not yet have formed in a 14-billion-year-old universe.
Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
The 10% figure is too high because there are not enough heavy elements to make that much dust.
Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
The Chandra X-ray Observatory
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 Big Bang the cosmological horizon the Hubble Deep Field the lookback time
The Cosmological Horizon It is a horizon in time, not in space. That is, there is no physical boundary at the cosmological horizon, but we cannot see beyond it because we'd be trying to look to a time before the universe was born. (See Figure 1.3 of The Essential Cosmic Perspective.)
Based on current evidence, how does the actual density of matter in the universe compare to the critical density?
The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.
What happens when a particle of matter meets its corresponding antiparticle of antimatter?
The combined mass of the two particles is completely transformed into energy (photons).
The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation that follows (these are not real observations) would call our current theory into question?
The discovery of a galaxy with a helium abundance of only 10% by mass.
Study this diagram that summarizes the eras of the universe. Which of the following statements is not true?
The electroweak era was the longest lasting of all the eras in the history of the universe.
Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur?
The galaxies in clusters would begin to fly apart.
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. right answer feedback: The surface of the balloon represents all three dimensions of space in our universe.
What does Hubble's law tell us? he faster a spiral galaxy's rotation speed, the more luminous it is. The longer the period of a Cepheid variable, the greater its luminosity. For every force, there is an equal and opposite reaction force. The more distant a galaxy, the faster it is moving away from us.
The more distant a galaxy, the faster it is moving away from us. right answer feedback: From this fact, we infer that we live in an expanding universe. Question
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 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.
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 an approximately 4-million-solar-mass object in a region no larger than our solar system.
What two observable properties of a Cepheid variable are directly related to one another? Its mass and its distance Its luminosity and its mass The period between its peaks of brightness and its distance The period between its peaks of brightness and its luminosity
The period between its peaks of brightness and its luminosity right answer feedback: The period-luminosity relation allows us to determine its luminosity from the period between its peaks of brightness.
Which of the following phenomena is probably not related to the presence of a supermassive black hole? Quasars The presence of globular clusters in the halos of galaxies The radio emission from radio galaxies The huge jets seen emerging from the centers of some galaxies
The presence of globular clusters in the halos of galaxies
How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
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 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.
Which of the following is not implied by this diagram?
There are six distinct forces known to operate in the universe, and a seventh force called the "super force" might also exist.
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.
Which of the following statements best explains what we mean when we say that the electroweak and strong forces 'froze out' at 10^-38 second after the Big Bang?
These two forces first became distinct at this time.
Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies? We would, but we don't have enough telescopes. They are rare events, so we have observed them in only a tiny fraction of all galaxies. They can occur only in spiral galaxies, not elliptical galaxies. We cannot see them beyond a distance of about 100 million light-years
They are rare events, so we have observed them in only a tiny fraction of all galaxies. right answer feedback: A white dwarf supernova may occur on average only every few centuries in a particular galaxy, so we have to be "lucky" to have caught one in action in that galaxy during the time we have been monitoring galaxies with large telescopes. Of course, by monitoring thousands or millions of galaxies, we'll catch a number of them every year.
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.
True or False these following features are apart of centeal dominant galaxies They're found in clusters of galaxies. They are thought to form by the merger of several smaller galaxies. They often have multiple galactic nuclei near their centers.
True
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.
How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
We detect this gas with X-ray telescopes.
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.
The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?
Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances form the galactic center.
The distinguishing feature of a starburst galaxy is _________ a very large luminosity compared to the total luminosity of the Milky Way a rate of star formation that may be 100 or more times greater than that in the Milky Way the presence of an unusually large number of binary star systems containing X-ray bursters strong radio emission from "lobes" of material well outside the visible boundaries of the galaxy
a rate of star formation that may be 100 or more times greater than that in the Milky Way The term starburst refers to a burst of star formation
What do we mean by inflation?
a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence
The unusually bright centers found in some galaxies are called _________.
active galactic nuclei The power source for active galactic nuclei is thought to be accretion onto a supermassive black hole
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, spanning a few degrees across the sky
The most common form of gas in the disk of the Milky Way galaxy is _________.
atomic hydrogen gas
All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.
globular clusters
In the past, the temperature of the universe was _____
hotter than it is today
If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for _____________________.
how galaxies came to exist.
Collisions between galaxies typically unfold over a period of _________.
hundreds of millions of years Remember that typical galaxies are 100,000 light-years in diameter, so even at the speed of light a collision would take hundreds of thousands of years. Galaxies actually collide at relative speeds less than 1/1000 of the speed of light, so the collisions unfold over hundreds of millions of years
If we say that a galaxy has a lookback time of 1 billion years, we mean that it is now 1 billion light-years away it is 400 million years old its light traveled through space for 1 billion years to reach us it was 1 billion light-years away when the light left the galaxy
its light traveled through space for 1 billion years to reach us Lookback time is more meaningful than distance, because the galaxy's current distance is different from what it was at the time it emitted the light we now see (because the universe is expanding).
Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.
only in or near star-forming clouds
Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered _________________.
surprising, because those locations would never have been close enough to have any light or matter exchanged between them.
On this graph, the time corresponding to the point at the far lower right of the diagonal line represents __________.
the age of the universe today, in seconds
Based on current evidence, a supercluster is most likely to have formed in regions of space where ___________________.
the density of dark matter was slightly higher than average when the universe was very young.
Laboratory experiments conducted with a particle accelerators confirm predictions made by the theory that unifies ________________
the electromagnetic and weak forces into the electroweak force
Cosmological redshift is the result of ______. very old, red stars in distant galaxies the high speeds at which galaxies move within clusters supermassive black holes the expansion of the universe
the expansion of the universe right answer feedback: It differs from what we usually think of as a Doppler shift because the galaxies are not really moving through the universe; rather, they only appear to be moving away from us because space itself is growing between the galaxies.
Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?
the fact that the temperature of the cosmic microwave background is almost the same everywhere
Applying the Newton's version of Kepler's third law (or the orbital velocity formula) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.
the mass of the Milky Way galaxy that lies within 40,000 light-years of the galactic center
A "GUT" (grand unified theory) refers to theories that _________.
unify the strong force with the electromagnetic and weak nuclear forces
