In a photo like the Hubble eXtreme 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?
answer
the galaxies that are farthest away
question
Which of the following statements about types of galaxies is not true?
answer
Elliptical galaxies are bluer and contain more dust than spiral galaxies.
question
The most basic difference between elliptical galaxies and spiral galaxies is that ______.
answer
elliptical galaxies lack anything resembling the disk of a spiral galaxy
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Hubble's galaxy classification diagram (the "tuning fork") ______.
answer
relates galaxies according to their shapes but not according to any evolutionary status.
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Although it is highly unlikely to happen, 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?
answer
It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought.
question
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?
answer
We can learn the distance to the galaxy.
question
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?
answer
By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation.
question
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 only as a result of Hubble's law.)
answer
away from us at 2,200 km/s
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Does Hubble's law work well for galaxies in the Local Group? Why or why not?
answer
No, because galaxies in the Local Group are gravitationally bound together.
question
Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
answer
White dwarf supernovae all have roughly the same true peak luminosity, whereas massive supernovae come in a wide range of peak luminosities.
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Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually. Which of the following techniques might allow us to measure its distance?
answer
We could use a white dwarf supernova as a standard candle.
question
You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances?
answer
Supernova A is twice as far away as Supernova B.
question
What is the best way to determine a galaxy's redshift?
answer
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.
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Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy B. According to Hubble's law, what can you say about their approximate relative distances?
answer
Galaxy B is twice as far as Galaxy A.
question
Which statement below correctly describes the relationship between expansion rate and age for the universe?
answer
The faster the rate of expansion, the younger the age of the universe.
question
What does cosmological redshift do to light?
answer
stretches its wavelength
question
The lookback time of the cosmological horizon is ______.
answer
the age of the universe
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Why can't we see past the cosmological horizon?
answer
Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
question
Given that the universe is about 14 billion years old, which of the following statements is logically valid?
answer
The oldest galaxies we see at great distances are younger than the oldest galaxies we see nearby.
question
Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 20.1 in your text), the estimated number of galaxies in our observable universe is about ______.
answer
100 billion
question
Which of the following is not one of the three major categories of galaxies?
answer
globular galaxies
question
Galaxies with disks but no evident spiral arms are called ______.
answer
lenticular galaxies
question
On a graph plotting galaxy luminosities against galaxy colors, the red sequence represents galaxies that are _________ than galaxies of the blue cloud.
answer
larger and more elliptical
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Which of the following best describes the status of the Milky Way in our Local Group of galaxies?
answer
It is one of the two largest galaxies in the group.
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A standard candle is _________.
answer
a light source of known luminosity
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What is a Cepheid variable?
answer
A type of luminous star that makes an excellent standard candle.
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What two observable properties of a Cepheid variable are directly related to one another?
answer
the period between its peaks of brightness and its luminosity
question
What does Hubble's law tell us?
answer
The more distant a galaxy, the faster it is moving away from us.
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Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
answer
They are rare events, so we have observed them in only a tiny fraction of all galaxies.
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Overall, what is our most accurate technique for measuring the distance to a nearby star?
answer
stellar parallax
question
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?
answer
The inside of the balloon does not represent any part of our universe.
question
If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.
answer
its light traveled through space for 1 billion years to reach us
question
Cosmological redshift is the result of ______.
answer
the expansion of the universe
question
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 ______.
answer
the cosmological horizon
question
Current estimates place the age of the universe at about ______.
answer
14 billion years
question
The fact that the universe is expanding means that space itself is growing within ___________.
answer
the observable universe
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