Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your textbook), the estimated number of galaxies in our observable universe is about ____________.
Which of the following is not one of the three major categories of galaxies?
A standard candles is ______________.
a light source of known luminosity
What is main sequence fitting?
a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
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
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 _________
Which of the following is an important starting assumption in models of galaxy formation?
Some regions in the universe start out denser than others
Current understanding holds that a galaxy’s type (spiral, irregular, or elliptical) ____________.
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies
Collisions between galaxies typically unfold over a period of _________.
hundreds of millions of years
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 theory that 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 life?
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
Hubble’s galaxy classification diagram (the “turning fork”) ____________.
relates galaxies according to their shapes, but not according to any evolutionary status.
Using the technique of main sequence fitting to determine the distance to a star cluster requires that ______________.
we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster
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?
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 these same lines as measured in the labratory.
Which 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
What 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 ______________.
we are really smart astronomers.. the farther we look, the further back in time we see
Which of the following statements is not an assumption used in models of galaxy formation?
Gas contracted to form disks of galaxies before any stars were born
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 of the following phenomena are NOT thought to be results of collisions or other interactions between galaxies?
The fact that spiral galaxies have both disk and halo components
Interactions among galaxies also are thought to influence a galaxy’s type in at least some cases. Which of the following does NOT support the idea that interactions can shape galaxies?
The fact that more distant galaxies have larger redshifts
Which characteristic is NOT generally true of a starburst galaxy?
The observed features of the starburst are thought to be caused by the presence of a supermassive black hole in the galaxy’s center.
A quasar’s spectrum is hugely redshifted. What does this large redshift tell us about the quasar?
the distance to the quasar
What evidence tells us that quasars are the centers of distant galaxies?
Images and spectra show quasars to be embedded at the centers of distant galaxies.
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
All of the following observations are real.Which one does NOT support the theory that active galactic nuclei are powered by accretion disks around massive black holes?
The most luminous active galactic nuclei have huge redshifts.
According to the theory that 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.