Deck 20: The Milky Waya Normal Spiral Galaxy

Question 1

Stars in the halo are most easily identified passing by the Sun because of their distinctively redder colors.

Answer 1

False

Question 2

Dark matter is very unlikely to be made up of planets,white dwarfs,or black holes because otherwise we would see stars being gravitationally lensed much more often.

Answer 2

True

Question 3

The Milky Way has two major spiral arms that emanate from the ends of its bar.

Answer 3

True

Question 4

Open clusters are only found in the disk of the Milky Way;globular clusters are only found in the halo of the Milky Way.

Question 5

There is a black hole at the center of the Milky Way that is rapidly accreting stars and gas.

Question 6

There are many more individual stars residing in the Milky Way stellar halo than inside globular clusters.

Question 7

Cosmic rays come mostly from extragalactic sources and contribute little to the overall energy balance in the Milky Way disk.

Question 8

The oldest open cluster known in the Milky Way is about 6 billion years younger than the average globular cluster.

Question 9

Stars in the Milky Way that are 12 to 13 billion years old are only found in globular clusters.

Question 10

A dwarf galaxy is currently being ripped apart and accreted onto the Milky Way.

Question 11

Every open star cluster in the Milky Way is younger than even the youngest globular cluster.

Question 12

The chemical abundance of a main-sequence star usually reflects how much nuclear burning has gone on inside it.

Question 13

The Milky Way is classified as an __________ galaxy.

A) Sa
B) SBbc
C) Sd
D) SBc

Question 14

Which stars in our Milky Way galaxy have orbits that are NOT similar to stars in elliptical galaxies?

A) Old disk stars
B) Halo stars
C) Bulge stars
D) Globular clusters

Question 15

Globular clusters have much higher stellar densities than the local solar neighborhood.

Question 16

Our galaxy is a typical barred spiral galaxy.

Question 17

Our galaxy is classified as an Sd galaxy in the Hubble sequence.

Question 18

Halo stars formed before gas in the Milky Way collapsed to form the disk.

Question 19

The Milky Way Galaxy is a(n)__________ galaxy.

A) irregular
B) elliptical
C) spiral
D) lenticular

Question 20

The Milky Way appears as __________ in the night sky because __________.

A) randomly distributed stars;the Sun lies near the center of the Milky Way
B) a faint band of light sprinkled with dark clouds;the Sun lies in the disk
C) a faint band of light;the Sun lies in the halo
D) an ellipse of light in the Southern sky;the Sun lies midway along the disk

Question 21

Where are the most metal-rich stars found in the Milky Way?

A) In the disk near the Sun
B) At the center
C) In the halo
D) In globular clusters

Question 22

What type of standard candle is used to determine distances to globular clusters?

A) RR Lyrae stars
B) Cepheid variable stars
C) T Tauri stars
D) Type I supernovae

Question 23

Studying standard candles in globular clusters offered the first conclusive proof that our galaxy was much __________ than originally believed.

A) smaller
B) rounder
C) larger
D) flatter

Question 24

Which of the following are NOT found in our galaxy's halo?

A) Globular clusters
B) Planetary nebulae
C) RR Lyrae stars
D) Clouds emitting 21-cm emission

Question 25

In which part of the Milky Way would you find little or no neutral hydrogen,no current star formation,and stars that are older than 10 billion years?

A) The halo
B) The disk
C) The galactic center
D) The solar neighborhood

Question 26

How many years does it take the Sun to complete one orbit around the Milky Way? Note that the Sun is traveling at approximately 220 km/s and is 26,000 light-years from the center of the Milky Way.

A) 180 thousand years
B) 7 million years
C) 35 million years
D) 220 million years

Question 27

The Sun is located approximately

A) halfway out in the disk.
B) one-third of the way out in the halo.
C) one-quarter of the way out in the bulge.
D) three-quarters of the way out in the disk.

Question 28

Most of the mass in our galaxy is in the form of

A) stars.
B) gas.
C) dust.
D) dark matter.

Question 29

Do we know whether or not the dark matter in the Milky Way is made up of planets,white dwarfs,neutron stars,and black holes?

A) We are not sure;they could make up much of the dark matter.
B) We observe no emission from these objects;therefore they cannot make up much of the dark matter.
C) Searches for gravitational lensing that such objects would produce found very few examples;therefore they cannot make up much of the dark matter.
D) Calculations of the chemical enrichment such objects would produce suggest the halo could be enriched by them;therefore they could make up most of the dark matter.

Question 30

By comparing globular clusters,you find that Cluster A's RR Lyrae stars are 100 times fainter than Cluster B's RR Lyrae stars.You know that both clusters have approximately the same chemical composition and age,and thus their RR Lyrae stars should have the same luminosities.Which is TRUE about the clusters' distances?

A) Cluster A is 10 times farther away than Cluster B.
B) Cluster A is 10 times closer to us than Cluster B.
C) Clusters A and B are approximately the same distances from us.
D) It is impossible to determine the clusters' relative distances with only this information.

Question 31

Open star clusters primarily inhabit which part of spiral galaxies?

A) Disk
B) Halo
C) Bulge
D) Nucleus

Question 32

Globular clusters,when compared to open clusters,generally

A) are located closer to the center of the Milky Way.
B) are younger in age.
C) have fewer amounts of heavy elements.
D) All of the above

Question 33

If we found gas at the edge of the disk of the Milky Way at a distance of 50,000 light-years from the center,which rotated in a circular orbit with a velocity of 300 km/s,what would be the total mass enclosed within this radius?

A)
B)
C)
D)

Question 34

How have astronomers searched for evidence of MACHOs in the halo of the Milky Way?

A) They search for stars that are binary companions of MACHOs.
B) They search for rapidly moving,massive gas clouds.
C) They search for disturbances in the background of gravitational waves.
D) They search for stars whose light is briefly amplified by gravitational lensing.

Question 35

Why is 21-cm radiation the best way to map the spiral arms in the Milky Way?

A) The molecular hydrogen gas that produces this emission is concentrated in the spiral arms.
B) These photons,which are produced by neutral hydrogen,penetrate the dense clouds of gas and dust in the disk.
C) This synchrotron emission is produced by supernovae,which are concentrated in the spiral arms.
D) Radio telescopes are easier to operate than optical telescopes and observations can be made even during the daytime.

Question 36

By comparing globular clusters,you find that Cluster A's RR Lyrae stars are 100 times fainter than Cluster B's RR Lyrae stars.You know that Cluster A's stars are 1.5 times more luminous than Cluster B's stars,because Cluster A is more metal-rich and somewhat younger.How do the clusters' distances compare?

A) Cluster A is 10 times farther away than Cluster B.
B) Cluster A is 12 times farther away than Cluster B.
C) Cluster A is 7 times farther away than Cluster B.
D) There is not enough information given to determine the clusters' relative distances.

Question 37

The chemical composition of a star's atmosphere tells us

A) how much nuclear burning has gone on in the star.
B) the star's evolutionary stage.
C) the chemical composition of the cloud from which the star formed.
D) All of the above

Question 38

If RR Lyrae stars in a globular cluster had an apparent magnitude of 12 and an absolute magnitude of 0.5,what would the globular cluster's distance be?

A) 6,500 light-years
B) 7,200 light-years
C) 8,300 light-years
D) 9,400 light-years

Question 39

If a globular cluster were 1,000 light-years away from us and an RR Lyrae star in it had an absolute magnitude of 0.6,what would its apparent magnitude be?

A) 8
B) 10
C) 12
D) 14

Question 40

What observed property of the Milky Way suggests that it contains a large amount of matter not in the form of stars?

A) The rotation curve
B) The velocities of the open star clusters
C) The number and shape of the spiral arms
D) The thickness of the disk

Question 41

What is ripping apart the Sagittarius dwarf spheroidal galaxy?

A) A supermassive black hole at its center
B) The energetic decay of a WIMP
C) A violent episode of star formation
D) The gravitational tidal force of the Milky Way

Question 42

In a star that has a chemical abundance similar to the Sun,what percentage of its mass is made of heavy elements?

A) 0.02 percent
B) 0.2 percent
C) 2 percent
D) 20 percent

Question 43

In the Milky Way halo,the stars orbit

A) mostly in the prograde direction.
B) mostly in the retrograde direction.
C) equally in the prograde and retrograde directions.
D) neither in the prograde nor retrograde directions.

Question 44

Sagittarius A*,the radio source located at the center of our galaxy,is believed to be

A) a massive star cluster.
B) a supernova remnant.
C) a quasar.
D) a supermassive black hole.

Question 45

Describe the population of globular star clusters in the Milky Way: how many are there,where do they reside in the Milky Way,and how do their ages and chemical abundances compare to the Sun?

Question 46

Cosmic rays in the Milky Way are important because

A) these energetic photons easily penetrate the Earth's atmosphere.
B) they can be collected and used to generate electricity.
C) they have about the same energy as that contained in the radiation and the kinetic energy of gas clouds.
D) they carry information about AGN in external galaxies.

Question 47

Explain how a standard candle allows you to determine the distance to an object.What is it that you have to measure or know about the standard candle to derive its distance?

Question 48

The stars in the disk of the Milky Way near the Sun have

A) chemical abundances similar to the Sun.
B) much higher velocities than the Sun.
C) chemical abundances that are on average ten times higher than the Sun.
D) much lower velocities than the Sun.

Question 49

Based on the chemical abundances and velocities of stars in the Milky Way's disk and halo,we can conclude that the protogalactic fragments that came together to form the galaxy merged when the fragments had

A) not yet formed stars.
B) converted most of their gas into stars.
C) begun forming stars.
D) each formed a supermassive black hole at their center.

Question 50

What is the radius of the event horizon for the black hole at the center of the Milky Way?

A) 0.02 AU
B) 0.08 AU
C) 0.17 AU
D) 0.35 AU

Question 51

How have astronomers measured the mass of the black hole at the center of our galaxy?

A) Using the rotation curve derived from 21-cm emission
B) By observing the motions of stars near the center of the galaxy
C) By measuring the brightness of the quasar
D) By measuring the Doppler shift of Sagittarius A*

Question 52

What is the main observational difficulty in observing the shape of the Milky Way?

Question 53

Describe the differences between the ages and chemical abundances of globular and open star clusters.

Question 54

By comparing globular clusters,you find that Cluster A's RR Lyrae stars are 225 times fainter than Cluster B's RR Lyrae stars.You know that both clusters have approximately the same chemical composition and age,and thus their RR Lyrae stars should have the same luminosities.Which cluster has the larger distance,and what is the ratio of the clusters' distances?

Question 55

If the Large Magellanic Cloud is orbiting the Milky Way in a circular orbit with a speed of 175 km/s and a distance of 165,000 light-years from the center,how long would it take for the Large Magellanic Cloud to complete one orbit around the Milky Way?

A) 600 million years
B) 1.8 billion years
C) 3.5 billion years
D) 17 billion years

Question 56

Cosmic rays are

A) photons with even higher energy than gamma rays.
B) high-velocity particles produced in novae.
C) primarily protons with very high energies.
D) synchrotron emission from strong magnetic fields.

Question 57

Why do we know that at least one generation of stars formed and died before the Milky Way's globular clusters formed?

A) All globular clusters reside in the disk of the Milky Way.
B) No globular cluster has zero heavy elements.
C) Globular clusters are 9-10 billion years old.
D) No globular cluster is older than 12 billion years.

Question 58

Because of the ages of globular clusters in the Milky Way,we think our galaxy's early formation history was characterized by

A) one single cloud of gas gently collapsing and star formation proceeding slowly within it.
B) one single cloud of gas that rapidly collapsed and turned most of its gas into stars.
C) violent merging of protogalactic fragments that stimulated a high rate of star formation.
D) slow merging of protogalactic fragments after they had already turned most of their gas into stars.

Question 59

A typical globular cluster is composed of 500,000 stars in a sphere whose radius is approximately 15 light-years.The distance between stars in a globular cluster is given approximately by ,where r is the radius and N is the number of stars in the cluster.Calculate this distance,and determine how many times smaller it is than the typical distance between stars in the solar neighborhood,which is 3 light-years.

Question 60

Magnetic fields in the Milky Way are concentrated in the disk because

A) halo stars are incapable of producing strong magnetic fields.
B) the fields are tied to the charged particles in dense molecular clouds.
C) gravity forces them to sink to the center of the disk.
D) supernovae explosions continually force them toward the middle of the disk.

Question 61

How do we know that globular cluster stars were not the first stars that formed in our galaxy?

Question 62

The energy of a cosmic ray is given by ,where m is the mass of the particle,c is the speed of light,and $\gamma$ is the Lorentz factor and .If a cosmic ray proton is traveling at a speed of ,what is its energy? How does that compare to the kinetic energy of a grape thrown up in the air and caught in your mouth? Assume the grape has a mass of about 5 grams and a speed of 0.5 m/s when you catch it.The mass of a proton is .

Question 63

Describe a scale model of the disk of the Milky Way using a scale in which light-years.Note that the diameter of the disk is approximately 100,000 light-years,the thickness of young stars is 500 light-years,and the thickness of the older stars is 4,000 light-years.

Question 64

The Sun is rotating about the center of the galaxy at roughly 220 km/s at a distance of about 27,000 light-years.How long will it take to complete one revolution?

Question 65

Where would we find the stars with the richest abundance of massive elements? Where would we find those with the smallest amounts of massive elements? Why?

Question 66

If you wanted to study the properties of nearby stars that were some of the first stars to form in the Milky Way,but were NOT members of a globular cluster,how could you go about finding them?

Question 67

Calculate the size of the event horizon of the supermassive black hole at the center of the Milky Way,the mass of which is .Give your answer in units of AU.

Question 68

Why can the observed chemical abundance of a star tell you something about its age?

Question 69

Describe the basic scenario that describes the formation and evolution of the Milky Way,including a description of how the halo and disk formed.

Question 70

Why is the supermassive black hole in the center of our galaxy not an AGN right now? Could it be active in the future? Why or why not?