Test Answers on Chapter 16 Review – Flashcards
Flashcard maker : Kayden Hussain
Based on the number of galaxies visible in the Hubble Deep Field, the estimated number of galaxies in our observable universe is about ________.
100 billion
What are the three major categories of galaxies?
irregular galaxies, spiral galaxies, elliptical galaxies
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 two observable properties of a Cepheid variable are directly related to one another?
the period between its peaks of brightness and its luminosity
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
Cosmological redshift is the result of ________.
the expansion of the universe
Current estimates place the age of the universe at about __________.
14 billion years
Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ___________.
infrared light
Which of the following is an important starting assumption in models of galaxy formation?
Some regions in the universe start out denser than others.
Collisions between galaxies typically unfold over a period of _________.
hundreds of millions of years
Current understanding holds that a galaxy’s type (spiral, elliptical, or irregular) _______.
may either be the result of conditions in the protogalatic cloud that formed it or the result of later interactions with other galaxies
Why are collisions between galaxies more likely than collisions between stars within a galaxy?
Relative to their sizes, galaxies are closer together than stars.
Which of the following features is not a feature of central dominant galaxies?
They are often 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.
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
The unusually bright centers found in some galaxies are called ________.
active galactic nuclei
According to current understanding, what is a quasar?
an active galactic nucleus that is particularly bright
Which of the following phenomena is probably not related to the presence of a supermassive black hole?
the presence of globular clusters in the halos of galaxies
The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly _______.
1 billion solar masses
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
In a photo like the Hubble Deep Field, 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
The most basic difference between elliptical galaxies and spiral galaxies is that _________.
elliptical galaxies lack anything resembling the disk of a spiral galaxy
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 igt 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
Assume that Hubble’s constant is 22 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 2,200 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.
What statement below correctly describes the relationship between expansion rate and age for the universe?
The faster the rate of expansion, the younger the age of the universe.
What does cosmological redshift do to light?
stretches its wavelength
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.
We can study how galaxies evolve because ________.
the farther away we look, the further back in time we see
What are the assumptions used in models of galaxy formation?
The universe started out filled almost uniformly with hydrogen and helium, the universe is expanding, some regions in the universe were slightly more dense than others
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
Which phenomena are thought to be results of collisions or interactions between galaxies?
The presence of very large, central dominant galaxies in clusters of galaxies, starbursts, the fact that elliptical galaxies are more common in clusters of galaxies than outside clusters
Interactions among galaxies also are thought to influence a galaxy’s type in at least some cases. What supports that interactions can shape galaxies?
The presence of features such as “tails” extending out of galaxies, bridges between galaxies, and rings of stars around galaxies, the fact that galaxies with distorted appearances are more common at great distances than nearby, computer modeling of collisions between galaxies
What characteristics are generally true of a starburst galaxy?
The observed features that cause us to classify it as a “starburst” must be only temporary phenomena in the galaxy’s history, its rate of star formation is many times higher than the rate of star formation in the Milky Way, Supernovae occur so frequently that their effects combine to drive a galactic wind that blows material into intergalactic space
Observations indicated that over billions of years, galaxies in general tend to change from ________.
smaller and bluer to larger and redder
A quasar’s spectrum is hugely redshifted. What does this large redshift tell us about the quasar?
the distance to the quasar
Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
Active galactic nuclei tend to become less active as they age.
Suppose we observe a source of X rays that varies substantially in brightness over a period of a few days. What can we conclude?
The X-ray source is no more than a few light-days in diameter.
What observations model active galactic nuclei powered by accretion disks around massive black holes?
Spectra of active galactic nuclei show that clouds of gas are orbiting a central object at very high speed, X-ray emission from active galactic nuclei can vary significantly in times as short as a few days, the total luminosity of an active galactic nucleus can be as high as about 10 billion times that of the Sun
According to the model in which active galactic nuclei are powered by supermassive black holes, the energy released as light comes from __________.
gravitational potential energy released by matter that is falling toward the black hole
The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that:
Galaxy formation and supermassive black hole formation must be related somehow
Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy?
A galaxy cannot have a disk if it does not have gas
High density tends to lead to more rapid star formation in a protogalactic cloud. Why does this rapid star formation tend to lead to an elliptical galaxy, rather than a spiral galaxy?
Rapid star formation means that there may not be enough gas left to make a disk
High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy?
Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud.
Our Sun belongs to the __________ of the Milky Way Galaxy.
disk component
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 a _________.
spiral galaxy
The type of galaxy known as an ____________ was more common in the universe 10 billion years ago.
irregular galaxy
Scientists investigating _________ study how the distribution of galaxies changes with time.
cosmology
_________________ have the highest rates of star formation.
Starburst galaxies
The largest individual galaxies in the universe are known as ______________.
central dominant galaxies
The collective activity of many supernova events in a relatively small volume of a galaxy can create ___________.
galactic winds
The most luminous objects in the universe are ____________.
quasars
The energy for all active galactic nuclei is thought to comes from in-fall of matter into _______________.
supermassive black holes
____________ are often characterized by sources of immense energy located hundreds of thousands of light-years away on either side of their centers.
Radio galaxies
