Chapter 20

Which of the following types of galaxies are reddest in color? A) spirals B) ellipticals C) lenticulars D) irregulars

B) ellipticals

What is the primary practical difficulty that limits the use of Hubble's law for measuring distances? A) Redshifts of galaxies are difficult to measure. B) The recession velocities of distant galaxies are so great that they are hard to measure. C) We do not know Hubble's constant very accurately yet. D) Hubble's law is only useful theoretically; it is difficult to use in practice. E) The motion of Earth relative to the Milky Way is difficult to account for.

C) We do not know Hubble's constant very accurately yet.

12) Planets have been detected around a pulsar.

TRUE

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) 100 million. B) 1 billion. C) 10 billion. D) 100 billion. E) 1 trillion.

D) 100 billion.

Compared to spiral galaxies, elliptical galaxies are A) redder and rounder. B) redder and flattened. C) bluer and rounder. D) bluer and flattened. E) always much smaller.

A) redder and rounder.

Why can't we see past the cosmological horizon? A) The universe extends only to this horizon. B) Beyond the cosmological horizon, we are looking back to a time before the universe had formed. C) We do not have telescopes big enough. D) We do not have sensitive enough detectors. E) The cosmological horizon is infinitely far away, and we can't see to infinity.

B) Beyond the cosmological horizon, we are looking back to a time before the universe had formed.

Why are Cepheid variables important? A) Cepheid variables are stars that vary in brightness because they harbor a black hole. B) 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. C) Cepheids are a type of young galaxy that helps us understand how galaxies form. D) Cepheids are supermassive stars that are on the verge of becoming supernovae and therefore allow us to choose candidates to watch if we hope to observe a supernova in the near future.

B) 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.

Most large galaxies in the universe are A) elliptical. B) spiral or lenticular. C)irregular. D) abnormal.

B) spiral or lenticular.

What is the most accurate way to determine the distance to a nearby galaxy? A) radar ranging B) stellar parallax C) using Cepheid variables D) the Tully-Fisher relation E) Hubble's law

C) using Cepheid variables

Dr. Smith believes that the Hubble constant is H0 = 70 km/s/Mpc. Dr. Jones believes it is H0 = 50 km/s/Mpc. Which statement below automatically follows? A) Dr. Smith believes that the universe is expanding, but Dr. Jones does not. B) Dr. Smith believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a faster speed than Dr. Jones believes. C) Dr. Smith believes that the universe is older than Dr. Jones believes. D) Dr. Smith believes that the universe is younger than Dr. Jones believes. E) Dr. Smith believes that the universe will someday stop expanding, while Dr. Jones believes it will expand forever.

D) Dr. Smith believes that the universe is younger than Dr. Jones believes.

How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies? A) There are no Cepheids in the Milky Way, so his discovery proved that it had to be in another galaxy. B) He measured the stellar parallax of the Cepheid in Andromeda, was able to determine the distance to it, and showed that it was far outside the Milky Way Galaxy. C) He used main-sequence fitting to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy. D) 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. E) Since a Cepheid is a type of luminous galaxy, when he found it in Andromeda he was able to prove that Andromeda was a separate galaxy from the Milky Way.

D) 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.

3) Stars are continually forming in the halo of our Galaxy today.

FALSE

7) Massive-star supernovae and white-dwarf supernovae work equally well as standard candles for measuring cosmic distances.

FALSE

Stars are continually forming in the halo of our Galaxy today

FALSE

most of the current star formation in the milky way occurs in spiral arms

TRUE