Chapter 20
Why are Cepheid variables important?
Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements
How does a lenticular galaxy differ from a normal spiral galaxy?
It has no spiral arms.
Which of the following statements about galaxies is true?
Small galaxies outnumber large galaxies but large galaxies produce most of the light in the universe.
Suppose that we look at a photograph of many galaxies. Assuming that all galaxies formed at about the same time, which galaxy in the picture is the youngest?
the one that is farthest away
What is the major difference between an elliptical galaxy and a spiral galaxy?
An elliptical galaxy lacks a disk component.
Why can't we see past the cosmological horizon?
Beyond the cosmological horizon, we are looking back to a time before the universe had formed.
Dr. X believes that the Hubble constant is H0 = 55 km/s/Mpc. Dr. Y believes it is H0 = 80 km/s/Mpc. Which statement below automatically follows?
Dr. X believes that the universe is older than Dr. Y believes.
How did Edwin Hubble measure the distance to the Andromeda Galaxy?
He applied the period-luminosity relation to Cepheid variables.
What is Hubble's law?
The recession velocity of a galaxy is directly proportional to its distance from us.
Which of the following is true about irregular galaxies?
They were more common when the universe was younger.
If we represent the Milky Way Galaxy as the size of a grapefruit (14-cm diameter), the distance to the Andromeda Galaxy would be about
a few meters
Which of the following sequences lists the methods for determining distance in the correct order from nearest to farthest?
parallax, main-sequence fitting, Cepheid variables, Hubble's law
Compared to spiral galaxies, elliptical galaxies are
redder and rounder.
What does the equivalent of an H-R diagram for galaxies, plotting luminosity versus color, show?
two clumps, one blue with relatively low luminosity, one red with relatively high luminosity, and a valley in between with few galaxies
What is the most accurate way to determine the distance to a nearby galaxy
using Cepheid variables
What is the most accurate way to determine the distance to a very distant irregular galaxy?
using a Type Ia supernova as a standard cand
What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe?
velocity and distance
Recall that Hubble's law is written v = H0d, where v is the recession velocity of a galaxy located a distance d away from us, and H0 is Hubble's constant. Suppose H0 = 65 km/s/Mpc. How fast would a galaxy located 500 megaparsecs distant be receding from us?
32,500 km/s
How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?
From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
Why is the Hyades Cluster important for building up a catalog of the true luminosities of main-sequence stars?
It is close enough to us that the distance to the cluster stars can be found by stellar parallax.
What makes Type Ia supernova good standard candles? (mark all that apply) They are very bright, so they can be used to determine the distances to galaxies billions of light-years away. They should all have approximately the same luminosity. They occur so frequently that we can use them to measure the distances to virtually all galaxies. We have had several occur close to us in the Milky Way, so we have been able to determine their luminosities very accurately.
They are very bright, so they can be used to determine the distances to galaxies billions of light-years away. They should all have approximately the same luminosity.
What is the primary practical difficulty that limits the use of Hubble's law for measuring distances?
We do not know Hubble's constant very accurately yet.
Type Ia supernovae are good standard candles for distance measurements for all the following reasons except which?
White-dwarf supernovae occur only among young and extremely bright stars.
Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately
a few hundreds of billions.
Which types of galaxies have a clearly defined spheroidal component? spirals only ellipticals only lenticulars only irregulars only all but irregulars
all but irregulars
What is a standard candle?
an object for which we are likely to know the true luminosity
Based on current estimates of the value of Hubble's constant, how old is the universe?
between 12 and 16 billion years old
Which of the following types of galaxies are most commonly found in large clusters?
ellipticals
Which of the following types of galaxies are most spherical in shape?
ellipticals
Which of the following types of galaxies are reddest in color?
ellipticals
The disk component of a spiral galaxy includes which of the following parts?
spiral arms
Most large galaxies in the universe are
spiral or lenticular.
Which types of galaxies have a clearly defined disk component?
spirals and lenticulars
What is the most accurate way to determine the distance to a nearby star?
stellar parallax
Which of the following is a consequence of Hubble's Law?
the more distant a galaxy is from us, the faster it moves away from us
