Deck 15: Our Galaxy

Question 1

What is an ionization nebula?

A)a region of very hot, low- density gas surrounding a recent supernova
B)a clump of gas that will soon give birth to a new star
C)a name sometimes used to describe spiral galaxies besides the Milky Way
D)a colorful cloud of gas that glows because it is heated by light from nearby hot stars

Answer 1

D

Question 2

Where does most star formation occur in the Milky Way Galaxy?

A)in the spiral arms
B)everywhere throughout the galactic disk
C)within the halo
D)in the central bulge

Answer 2

A

Question 3

How can we see through the interstellar medium?

A)by observing with telescopes above the Earth's atmosphere, such as the Hubble Space Telescope
B)by observing at wavelengths (high- energy X- rays and long wavelength radio waves)that are not absorbed by interstellar dust
C)by observing with only the biggest telescopes
D)by observing only the brightest stars in the galaxy
E)We cannot see through the interstellar medium.

Answer 3

B

Question 4

What kinds of objects lie in the disk of our galaxy?

A)gas and dust
B)old K and M stars
C)open clusters
D)O and B stars
E)all of the above

Question 5

How do high- mass stars make it more difficult for a molecular cloud to form more stars?

A)Ultraviolet photons heat the gas.
B)Radiation pressure pushes ionized gas away.
C)Ultraviolet photons ionize the gas.
D)Strong winds push gas away.
E)All of the above

Question 6

The galactic center lies in the direction of which constellation?

A)the Big Dipper
B)Sagittarius
C)Taurus
D)Leo
E)Orion

Question 7

What evidence suggests that the protogalactic cloud that formed the Milky Way resulted from several collisions among smaller clouds?

A)The Milky Way is the central galaxy of a dense cluster of galaxies.
B)Halo stars differ in age and heavy- element content, but these variations do not seem to depend on the stars' distance from the galactic center.
C)The stars in the halo of the Milky Way are organized into several dense clusters arranged throughout the halo.
D)The Milky Way resembles an elliptical galaxy more than other spirals do.
E)The bulge of the Milky Way is surrounded by many globular clusters, just as elliptical galaxies are.

Question 8

What kinds of objects lie in the halo of our galaxy?

A)open clusters
B)globular clusters
C)O and B stars
D)gas and dust
E)all of the above

Question 9

Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?

A)It is a very high mass star.
B)It is a very young star, recently formed.
C)It is probably a halo star that is currently passing through the disk.
D)It has been pushed to high speed by the shock wave from a nearby supernova.

Question 10

How does the interstellar medium obscure our view of most of the galaxy?

A)Molecules in the interstellar medium absorb all wavelengths of light.
B)Hydrogen gas produces so much visible light that the interstellar medium is opaque, blocking our view of anything beyond it.
C)The small mixture of dust grains in the interstellar medium absorbs visible light.
D)Dust reflects most light from distant regions of the galaxy back towards the source.
E)all of the above

Question 11

Most stars in the Milky Way's halo are .

A)found inside molecular clouds
B)blue or white in color
C)very young
D)very old

Question 12

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

A)the Keck I telescope on the summit of Mauna Kea
B)the Hubble Space Telescope
C)the SOFIA airborne infrared observatory
D)the Chandra X- ray Observatory

Question 13

The image of our galaxy in radio emission from carbon monoxide maps the distribution of molecular clouds. Which of the following would give a similar picture of our galaxy?

A)visible light, showing the edges of supernova bubbles
B)visible light, which is closest to how the night sky appears from Earth
C)X- rays from hot gas bubbles in the disk
D)21- cm- line radio emission from atomic hydrogen
E)infrared emission from interstellar dust grains

Question 14

On a dark summer night in the northern hemisphere, we can see the Milky Way, but we can't see the center of the Milky Way. Why not?

A)There are no stars in the center of the Milky Way, just a supermassive black hole.
B)Interstellar dust and gas absorb and scatter visible light.
C)We are in the center of the Milky Way.
D)The center of the Milky Way does not emit enough visible light.

Question 15

Why has star formation ceased in the galactic halo?

A)All of the galaxy's cool gas settled to the galactic plane long ago.
B)All the halo gas consists of elements with more protons than helium.
C)There are no white dwarf binaries to trigger star formation via their supernovae.
D)The halo has no gas at all.

Question 16

Which kind of star is highly unlikely to be found in the halo of the Milky Way?

A)G star
B)K star
C)M star
D)O star

Question 17

Over time, what is the net effect of the star- gas- star cycle in the Milky Way?

A)The total mass in the galaxy's interstellar medium is gradually increased, and the new gas is continually enriched in heavy elements.
B)The gas of the interstellar medium is continually depleted in elements heavier than helium.
C)The temperature of the interstellar medium varies between the cool 10- 50 degrees (Kelvin)required to form stars and the 2000- 30,000 degree (Kelvin)temperatures of the outer layers of the stars that form. But over time, more and more of the interstellar medium remains cool.
D)The total mass in the galaxy's interstellar medium is gradually reduced, and the remaining gas is continually enriched in heavy elements.
E)There is no net effect to the cycle. Mass in the interstellar medium lost to star formation is exactly replaced by stellar winds and supernova explosions.

Question 18

What elements do astronomers consider heavy elements?

A)elements that are heavier than iron
B)elements that are heavier than hydrogen
C)elements that are heavier than uranium
D)elements that are heavier than carbon
E)all elements besides hydrogen and helium

Question 19

Where do most dust grains form?

A)in molecular clouds
B)in planetary nebulae
C)in supernovae
D)in the winds of red giant stars
E)all of the above

Question 20

The very first stars in the universe were made of

A)hydrogen and helium, 0.1% heavy elements.
B)hydrogen and helium, 2% heavy elements.
C)hydrogen and helium, 10% heavy elements.
D)hydrogen and helium.

Question 21

The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _.

A)space- based ultraviolet telescopes
B)X- ray telescopes
C)ground- based visible- light telescopes
D)radio telescopes observing at a wavelength of 21 centimeters

Question 22

Over time, the star- gas- star cycle leads the gas in the Milky Way to .

A)become denser and hotter
B)become denser and have a greater abundance of heavy elements
C)have a lower abundance of heavy elements
D)have a greater abundance of heavy elements

Question 23

Why are we unlikely to find Earth- like planets around halo stars in the Galaxy?

A)Halo stars formed from gas containing few heavy elements, the elements needed to create terrestrial planets like the Earth.
B)Halo stars formed directly from gas in the halo without forming a planetary disk.
C)Halo stars formed from gas containing lots of heavy elements and few of the light elements needed to create terrestrial planets like the Earth.
D)Planets around stars are extremely rare in our Galaxy.
E)Halo stars are all very low mass stars that cannot hold onto planets.

Question 24

Based on observations, which of the following statements about stars in the Milky Way is generally true?

A)The older the star, the bluer its color.
B)The younger the star, the higher its mass.
C)The older the star, the faster its orbital speed.
D)The older the star, the lower its abundance of heavy elements.
E)The less massive the star, the older it is.

Question 25

What kind of object do we think lies in the center of the Milky Way Galaxy?

A)a dense cluster of young, hot stars
B)a gigantic X- ray binary system
C)an enormous collection of dark matter, explaining why we detect no light at all from the galactic center
D)a 3 to 4 million solar mass black hole

Question 26

If you could watch a time- lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

A)Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring
B)The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.
C)The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.
D)Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles

Question 27

What is the most abundant molecule in interstellar clouds besides molecular hydrogen?

A)carbon monoxide
B)water
C)ammonia
D)methane
E)molecular helium

Question 28

Which of the following patterns in the Milky Way structure is consistent with the idea that the Milky Way has not suffered a major collision with another massive galaxy?

A)The disk is composed of stars orbiting in the same plane, same direction.
B)Globular clusters are compact groups of thousands of stars.
C)The disk is composed of stars of many different ages.
D)The halo is full of old stars orbiting in many different directions.

Question 29

How does the diameter of the disk of Milky Way Galaxy compare to its thickness?

A)The diameter is about 100 times as great as the thickness.
B)The diameter and thickness are roughly equal.
C)The diameter is about 100,000 times as great as the thickness.
D)The diameter is about 10 times as great as the thickness.

Question 30

Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found .

A)only in the central bulge
B)only in or near star- forming clouds
C)in the halo
D)also evenly spread throughout the galactic disk

Question 31

Which of the following does not accurately describe what we observe toward the Galactic center?

A)At X- rays, we see faint emission from an accretion disk around a black hole.
B)At radio wavelengths, we see giant gas clouds threaded by powerful magnetic fields.
C)At infrared wavelengths, we see a massive star cluster.
D)At optical wavelengths, we see a cluster of old, red stars.

Question 32

What is a shock wave?

A)a pressure wave that moves slower than the speed of sound
B)a pressure wave that moves faster than the speed of sound
C)an electromagnetic wave that can create electrical shocks
D)a pressure wave that moves faster than the speed of light
E)an electromagnetic wave created when electrons recombine with protons

Question 33

Which of the following properties is more likely to be a characteristic of a halo star in the Milky Way, not a disk star?

A)It has a low ratio of iron to hydrogen.
B)It is moving very slowly relative to the Sun's motion.
C)It could be quite young.
D)It orbits the center of the Milky Way galaxy.

Question 34

How do disk stars orbit the center of the galaxy?

A)They have orbits randomly inclined and in different directions relative to the galactic center.
B)They follow spiral paths along the spiral arms.
C)They follow orbits that move up and down through the disk, typically taking them about 50,000 light- years above and below the disk on each orbit.
D)They all orbit in roughly the same plane and in the same direction.

Question 35

The Sun's location in the Milky Way Galaxy is _ .

A)very near the galactic center
B)in the halo of the galaxy, about 28,000 light- years above the galactic disk
C)at the very outer edge of the galactic disk
D)in the galactic disk, roughly halfway between the center and the outer edge of the disk

Question 36

Sound waves in the interstellar medium

A)travel so slowly that they are undetectable.
B)can travel through the gas, but the very low density of the interstellar medium makes them inaudible.
C)can travel through the halo interstellar medium but not the disk of the galaxy.
D)cannot travel through the gas.
E)travel extremely quickly and are therefore very loud.

Question 37

Which of the following observations confirms that Sgr A at the center of our galaxy is indeed a supermassive black hole?

A)measurements of occasional X- ray emission from Sgr A
B)measurements of the orbits of stars around Sgr A
C)measurements of trans- dimensional space warps in the vicinity of Sgr A
D)measurements of radio emission from gas around Sgr A

Question 38

Which of the following stars would be most likely to have the same abundance of elements heavier than helium as a red main sequence star in globular cluster M13?

A)red giant in globular cluster M13
B)a red main- sequence star in the disk of the Milky Way
C)None of these choices

Question 39

Compared to our Sun, most stars in the halo are

A)young, red, and dim and have fewer heavy elements.
B)old, red, and dim and have many more heavy elements.
C)old, red, and dim and have fewer heavy elements.
D)young, blue, and bright and have many more heavy elements.
E)old, red, and bright and have fewer heavy elements.

Question 40

The Sun lives in which component of the Milky Way?

A)the disk
B)the bulge
C)the halo
D)a globular cluster

Question 41

What makes up the interstellar medium?

A)gas and dust
B)K and M stars
C)O and B stars
D)open clusters
E)all of the above

Question 42

What do we call the bright, sphere- shaped region of stars that occupies the central few thousand light- years of the Milky Way Galaxy?

A)a globular cluster
B)the galaxy's halo
C)the galaxy's disk
D)the galaxy's bulge

Question 43

What do halo stars do differently from disk stars?

A)They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
B)They remain stationary, quite unlike disk stars that orbit the galactic center.
C)They orbit the center of the galaxy at much lower speeds than disk star.
D)Halo stars explode as supernovae much more frequently than disk stars.

Question 44

Why do we believe that most of the mass of the Milky Way is in the form of dark matter?

A)Our view of distant galaxies is sometimes obscured by dark blotches in the sky, and we believe these blotches are dark matter located in the halo.
B)Although dark matter emits no visible light, it can be seen with radio wavelengths, and such observations confirm that the halo is full of this material.
C)The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
D)Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk, suggesting that the halo is full of dark matter.

Question 45

Which star is older?

A)an A star in the globular cluster M13
B)an M star in the globular cluster M13
C)Both stars (the A and M star)are the same age.

Question 46

Harlow Shapley concluded that the Sun was not located at the center of the Milky Way Galaxy by

A)looking at the shape of the "milky band" across the sky.
B)looking at other nearby spiral galaxies.
C)mapping the distribution of globular clusters in the galaxy.
D)mapping the distribution of gas clouds in the spiral arms.
E)mapping the distribution of stars in the galaxy.

Question 47

About what percentage of the mass of a molecular cloud is in the form of dust?

A)10%
B)1%
C)50%
D)98%

Question 48

If you were to take a voyage through the entire disk of the Milky Way, what kind of material would you spend most of your time in?

A)hot bubbles of ionized hydrogen
B)star clusters
C)rarefied clouds of atomic hydrogen
D)cool, dense clouds of molecular hydrogen
E)empty space- a pure vacuum

Question 49

How are interstellar bubbles made?

A)by the winds of massive stars and supernovae explosions
B)by the collapse of a gas cloud to form stars
C)by collisions between the Milky Way and satellite dwarf galaxies
D)by the rapidly rotating magnetic fields of pulsars
E)by the ejection of planetary nebulae from low- mass stars

Question 50

All the following types of objects are found almost exclusively in the disk (rather than the halo)of the Milky Way except .

A)globular clusters
B)X- ray binaries
C)high- mass, red supergiant stars
D)young stars

Question 51

What is the best evidence for an extremely massive black hole in the center of the Milky Way?

A)We observe stars vanishing in the center of the Galaxy as they are sucked into the black hole.
B)The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.
C)The center of our galaxy hosts a pulsar that is spinning so fast that it could only be a black hole.
D)Huge amounts of X- rays are pouring out of the center of the galaxy.

Question 52

What is Sgr A×?

A)the brightest star in the constellation Sagittarius
B)the bulge at the center of our galaxy
C)a source of bright radio emission in the center of our galaxy
D)a source of bright X- ray emission coming from the entire constellation of Sagittarius
E)a source of bright ultraviolet light near the center of our galaxy

Question 53

Which of the following molecules is the most abundant in molecular clouds?

A)H2O
B)H2
C)CO
D)NH3

Question 54

What do we mean by the star- gas- star cycle?

A)It is the idea that stars in close binary systems can exchange gas with one another.
B)It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.
C)It describes the orbits of the stars and interstellar medium around the center of the galaxy.
D)It is the set of nuclear reactions by which heavy elements are produced in the cores of massive stars.

Question 55

Which of the following models best explains why our galaxy has spiral arms?

A)The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.
B)No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy.
C)The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation.
D)The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates.

Question 56

Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?

A)The 10% figure is too high because there are not enough heavy elements to make that much dust.
B)The 10% figure is too low because most of the mass of the galaxy is in the form of interstellar dust.
C)It is reasonable because we already know that interstellar dust obscures our view through the disk of the galaxy.
D)It seems reasonable as long as we assume that red giant stars-which produce dust grains in their stellar winds-are more common than we thought.

Question 57

Where does most star formation occur in the Milky Way today?

A)in the bulge
B)in the spiral arms
C)uniformly throughout the Galaxy
D)in the Galactic center
E)in the halo

Question 58

Spectra for the Cygnus loop and the Orion nebula are shown above. Which of these is seen in the Orion nebula, but is not clearly visible in the Cygnus loop spectrum?

A)atomic hydrogen
B)atomic helium
C)ionized oxygen
D)ionized sulfur

Question 59

What is the thickness of the disk of the Milky Way?

A)100 light years
B)1,000,000 light years
C)1,000 light years
D)10,000 light years
E)100,000 light years

Question 60

How do high mass stars transport newly formed elements into the interstellar medium?

A)supernova explosions
B)stellar winds
C)formation of planetary nebula
D)A and B
E)A and C

Question 61

What do we mean by the interstellar medium?

A)the gas and dust that lie in between the stars in the Milky Way galaxy
B)the middle section of the Milky Way galaxy
C)the name of an oracle who can channel messages from beings that live near the star called Vega
D)the dust that fills the halo of the Milky Way galaxy

Question 62

How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

A)It should be higher in mass.
B)It should have a higher fraction of elements heavier than hydrogen and helium.
C)It should orbit the galactic center at a much higher rate of speed.
D)It should be much brighter.
E)All of the above would be expected.

Question 63

What is the best evidence for a supermassive black hole in the center of the Milky Way Galaxy?

A)amazing filaments of radio emission swirling around the center of the galaxy
B)the gigantic bursts of X- ray energy from a mysterious dark object
C)The existence of the Milky Way itself: The mass of the black hole is essential to gravitationally bind the galaxy together, so it doesn't fly apart.
D)the motions of stars around a mysterious dark object

Question 64

Fill in the blank: Elements heavier than hydrogen and helium constitute about of the mass of the interstellar medium.

A)2%
B)0)002%
C)98%
D)70%

Question 65

Approximately how far is the Sun from the center of the galaxy?

A)2,700 light- years
B)27,000 light- years
C)27 million light- years
D)270 light- years
E)27 light- years

Question 66

What are the Magellanic Clouds?

A)star- forming clouds found in the constellation Orion
B)the clouds of dust and gas found interspersed in many places throughout the Milky Way Galaxy
C)two nebulae located in the disk of the Milky Way galaxy and visible only from the Southern Hemisphere
D)two small galaxies that orbit the Milky Way Galaxy

Question 67

Why do stars in the disk of a spiral galaxy orbit in roughly the same direction, in the same plane in space?

A)The gas that formed the stars was shaped as a giant, rotating disk.
B)Collisions between the stars caused their motions to organize into a general, spinning disk direction.
C)After the stars formed, gravity pulled the stars down into a plane in space, all orbiting a supermassive black hole.
D)It is pure coincidence; the stars just happen to be going in the same direction.

Question 68

What is the most common form of gas in the interstellar medium?

A)atomic hydrogen
B)molecular hydrogen
C)molecular helium
D)ionized hydrogen
E)atomic helium

Question 69

The most common form of gas in the disk of the Milky Way galaxy is .

A)molecular hydrogen
B)gas in stellar winds
C)atomic hydrogen gas
D)gas in hot bubbles

Question 70

Approximately how long does it take the sun to go through a full cycle of passing above and below the plane of the galaxy?

A)a few thousand years
B)a few hundred years
C)a few million years
D)a few tens of millions of years

Question 71

Interstellar dust consists mostly of .

A)microscopic particles of carbon and silicon
B)ozone "smog"
C)hydrogen and helium atoms
D)the same tiny particles found in household dust
E)tiny grains of water ice

Question 72

What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?

A)Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk.
B)Although dark matter emits no visible light, we have detected its radio emissions.
C)The orbital speeds of stars far from the galactic center are surprisingly high.
D)Our view of distant galaxies is often obscured by dark blotches, which are presumably made of dark matter.

Question 73

Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?

A)It is by far the brightest source of visible light lying in the direction of the galactic center.
B)It is a source of X- ray emission that we have observed with telescopes in space.
C)It is a source of bright radio emission.
D)It is thought to harbor a black hole of more than 3 million solar masses.

Question 74

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?

A)2 billion years
B)200 million years
C)2 million years
D)200,000 years
E)20,000 years

Question 75

Where are most heavy elements made?

A)in stars and supernovae
B)in the interstellar medium
C)All were made in the Big Bang, when the universe first began.
D)none of the above
E)all of the above

Question 76

Compared with stars in the disk, orbits of stars in the halo

A)do not have to be around the galactic center.
B)are confined to a relatively thin plane.
C)are elliptical, orbiting in the same direction.
D)are elliptical, with random orientations.
E)do not have to pass through the plane of the galaxy.

Question 77

How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

A)The total amount of gas will be much less than it is today.
B)The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.
C)Thanks to the recycling of the star- gas- star cycle, the interstellar medium should look about the same in 50 billion years as it does today.
D)The total amount of gas will be about the same, but the percentage of elements heavier than hydrogen and helium will have risen from the current 2% to more than 50%.

Question 78

What is the diameter of the disk of the Milky Way?

A)10,000 light years
B)1,000 light years
C)100,000 light years
D)100 light years
E)1,000,000 light years

Question 79

What evidence supports the existence of a black hole at the center of our galaxy?

A)The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region only 3 light- years across.
B)We can see gas falling into an accretion disk and past the event horizon of a black hole.
C)We observe a large, dark object that absorbs all light at the center of our galaxy.
D)We observe an extremely bright X- ray source at the center of our galaxy.
E)All of the above

Question 80

If we could see our own galaxy from 2 million light- years away, it would appear _ .

A)as a faintly glowing band of light stretching all the way around the sky
B)as a flattened disk with a central bulge and spiral arms
C)to fill the sky with widely spaced stars
D)like a single, dim star