Chapter 20
Conceptual Quiz
Part A
In a photo like the Hubble Deep Field (Figure 20.1 in your text), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
Hint A.1
Study Section 20.1 (and review Chapter 1)
ANSWER:

The galaxies that have the most hot, young O and B stars. The galaxies that are farthest away. The galaxies that are nearest to us. The galaxies that are the reddest in color.
Part B
Which of the following statements about types of galaxies is NOT true?
ANSWER:

Among the large galaxies in the universe outside of clusters, most are spiral. Elliptical galaxies are bluer and contain more dust than spiral galaxies. Large elliptical galaxies are more common in clusters of galaxies than they are outside of clusters. Spiral galaxies have younger stars than elliptical galaxies.
Part C
The most basic difference between elliptical galaxies and spiral galaxies is that ______.
Hint C.1
Study Section 20.1
ANSWER:

elliptical galaxies have a spheroidal component (of stars distributed spherically about the galactic center), and spiral galaxies do not. elliptical galaxies lack anything resembling the halo of a spiral galaxy. elliptical galaxies lack anything resembling the disk of a spiral galaxy. elliptical galaxies are very old and spiral galaxies are very young.
Part D
Hubble's galaxy classification diagram (the "tuning fork") ______.
Hint D.1
Study Section 20.1
ANSWER:

Relates galaxies according to their shapes, but not according to any evolutionary status. shows how galaxies evolve from one form to another. explains active galactic nuclei. suggests the existence of black holes.
Part E
Using the technique of main-sequence fitting to determine the distance to a star cluster requires that _____.
Hint E.1
Study Section 20.2
ANSWER:

the cluster be near enough for us to measure the parallax of its stars. we have a well-calibrated period--luminosity relation for Cepheid variable stars. we have telescopes powerful enough to allow us to identify the spectral types of many individual main-sequence stars in the cluster. we use ultraviolet and X-ray telescopes.
Part F
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?
Hint F.1
Study Section 20.2
ANSWER:

It would mean that all the objects we've assumed are standard candles really are not good standard candles, and therefore that we have no idea of the true distance to the Andromeda Galaxy. It would not have any effect on our estimate of the distance to the Andromeda Galaxy. It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought. It would mean the distance to the Andromeda Galaxy is 10% less than we thought.
Part G
Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What good will this observation do us?
Hint G.1
Study Section 20.2
ANSWER:

Under the rules of the International Astronomical Union, we will be entitled to naming rights for the galaxy. It will allow us to calculate the rotation rate of the galaxy. We will be able to use its period to determine its luminosity and hence (if we also measure the apparent brightness) to calculate the distance to its galaxy. It will allow us to determine the mass of the galaxy.
Part H
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?
Hint H.1
Study Section 20.3
ANSWER:

He found that the universe is expanding, and therefore concluded that Andromeda must lie outside our own galaxy. He was able to measure the parallax of the Andromeda Galaxy. By observing individual Cepheid variable stars in Andromeda and applying the period--luminosity relation. He was the first person ever to look through a telescope at the object we now call the Andromeda Galaxy.
Part I
Suppose Hubble's constant were 25 km/s/Mly (kilometers per second per million light-years), then 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.)
Hint I.1
Study Section 20.3
ANSWER:

away from us at 250 km/s away from us at 250,000 km/s away from us at 25 km/s toward us at 2,500 km/s away from us at 2,500 km/s
Part J
Does Hubble's law work well for galaxies in the Local Group? Why or why not?
Hint J.1
Study Section 20.3
ANSWER:

Yes, it works every so well that we have never detected any measurable deviations from its predictions. No, because we do not know the precise value of Hubble's constant. No, because Hubble did not know the Local Group existed when he discovered his law. No, because galaxies in the Local Group are gravitationally bound together.
Part K
Why are white dwarf supernovae more useful for measuring cosmic distances than massive star supernovae?
Hint K.1
Study Section 20.2
ANSWER:

We can see only white dwarf supernovae in distant galaxies, not massive star supernovae. White dwarf supernovae are much more common 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. White dwarf supernovae follow a period-luminosity relation, while massive supernovae do not.
Part L
Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually. Which of the following techniques could allow us to measure its distance?
Hint L.1
Study Section 20.2
ANSWER:

We could apply the Tully-Fisher relation. We could use Cepheid variables as standard candles. We could use a white dwarf supernova as a standard candle. We could use radar ranging.
Part M
Which statement below correctly describes the relationship between expansion rate and age for the universe?
Hint M.1
Study Section 20.4
ANSWER:

The faster the rate of expansion, the younger the age of the universe. The faster the rate of expansion, the older the age of the universe. Age is independent of the expansion rate.
Part N
What does cosmological redshift do to light?
Hint N.1
Study Section 20.4
ANSWER:

Makes it slow down. Stretches its wavelength. Makes all light infrared. Makes it brighter.
Part O
The lookback time of the cosmological horizon is ______.
Hint O.1
Study Section 20.4
ANSWER:

the age of the universe. 1 billion years. 10 billion years. the Big Bang.
Part P
Why can't we see past the cosmological horizon?
Hint P.1
Study Section 20.4
ANSWER:

Beyond the cosmological horizon, we would be looking back to a time before the universe was born. Every galaxy in the entire universe (not just the observable universe) exists within the cosmological horizon, so there's nothing to see beyond it. We do not have telescopes big enough. The cosmological horizon is infinitely far away, and we can't see to infinity.
Part Q
If the Hubble constant were 11 km/sec/million light years instead of 22 km/sec/million light years, the age of the universe would be _______.
Hint Q.1
Study Section 20.3
ANSWER:

7 billion years. 28 billion years. 14 trillion years. 14 billion years.
Part R
The Hubble's law plot in Figure 20.21 (shown below) plots the recessional velocity (km/sec) on the y-axis and apparent distance on the x-axis. If the Hubble's constant were 11 km/s/million light years instead of 22 km/s/million light years, the data points on this plot would ______.
Hint R.1
Study Section 20.3 and Figure 20.21.
ANSWER:

Form a more shallow line, closer to horizontal than the current line. Forms a flat line, parallel to the x-axis. The data would cluster around a line which is very similar to the line on the current plot, at 22 km/sec/Million light years. Form a steep, almost vertical line on the plot.
Part S
If you wanted to measure a galaxies' redshift, what is the best way to do it?
Hint S.1
Study Section 20.3
ANSWER:

Find the color of the galaxy, and estimate its distance based on how red the galaxy is. Measure the magnitude of the galaxy, estimate its distance, and calculate its redshift using Hubble's Law. Take a spectrum of the galaxy, and measure the difference in wavelength of features from the wavelengths of those same features as measured in the laboratory. Find the galaxies' apparent distance, and look up the redshift based on Hubble's Law.
Part T
You observe two edge-on spiral galaxies and note that their rotation speeds are nearly identical. Galaxy A, however, is 9 times brighter than Galaxy B. What do you infer about their relative distances?
Hint T.1
Study Section 20.2
ANSWER:

Galaxy A is 9 times closer than Galaxy B. Galaxy B is 9 times closer than Galaxy A. Not enough information to say. Galaxy B is 3 times closer than Galaxy A. Galaxy A is 3 times closer than Galaxy B.
Part U
Given that the universe is about 14 billion years old, which of the following statements is logically valid?
Hint U.1
Study Section 20.3
ANSWER:

The oldest galaxies we see at high redshift are younger than the oldest galaxies we see nearby. All galaxies that we can see are about 14 billion years old. All galaxies nearby us are about 14 billion years old. All galaxies that we see have an age that is approximately equal to the age of the universe today, minus the lookback time (or light travel time) corresponding to the redshift of that galaxy.