Deck 14: Our Galaxy

Question 1

Suppose you read somewhere that 10 percent of the matter in the Milky Way is in the form of dust grains. Should you be surprised? If so, why?

A) There is nothing surprising about 10 percent figure s because dust grains are the material from which stars are born.
B) Given how easily dust grains can form, 10 percent is a surprisingly low fraction.
C) The fraction of 10 percent is surprisingly high because dust grains can form only at low temperatures.
D) The fraction of 10 percent cannot be correct. Dust grains are solid and only 2 percent of the matter in the galaxy is made of anything besides hydrogen and helium.

Answer 1

D

Question 2

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

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

Answer 2

D

Question 3

How can we see through the interstellar medium?

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

Answer 3

A

Question 4

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

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

Question 5

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

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

Question 6

What produces the 21-cm radio line that we can be used to map the Milky Way Galaxy?

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

Question 7

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

A) 28 light-years
B) 280 light-years
C) 2,800 light-years
D) 28,000 light-years
E) 28 million light-years

Question 8

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

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

Question 9

What makes up the interstellar medium?

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

Question 10

Where do most dust grains form?

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

Question 11

How are interstellar bubbles made?

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

Question 12

Where are most heavy elements made?

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

Question 13

What is a shock wave?

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

Question 14

Sound waves in the interstellar medium

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

Question 15

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

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

Question 16

What elements do astronomers consider heavy elements?

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

Question 17

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

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

Question 18

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) mapping the distribution of stars in the galaxy.
C) mapping the distribution of globular clusters in the galaxy.
D) mapping the distribution of gas clouds in the spiral arms.
E) looking at other nearby spiral galaxies.

Question 19

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

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

Question 20

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

Question 21

The Sun is located near the edge of our galaxy, approximately 100,000 light-years from the galactic center.

Question 22

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

Question 23

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

A) 23,000 years
B) 230,000 years
C) 2.3 million years
D) 230 million years
E) 23 billion years

Question 24

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

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

Question 25

Open clusters and young stars are generally found only in the disk of the Milky Way and not in the halo.

Question 26

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

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

Question 27

How do we learn about the conditions at the center of our own galaxy, the Milky Way?

A) High-resolution photographs obtained by the Hubble Space Telescope have revealed the galactic center using visible light.
B) While we cannot see the galactic center with visible or ultraviolet light, radio and X-rays from the center can be detected and used to determine the conditions there.
C) The gas and dust in the Milky Way prevent any type of direct observation of the galactic center, but theoretical models allow us to predict what is happening there.
D) We must look at the centers of other (external) galaxies and assume that they are similar to the Milky Way.

Question 28

Shapley used the distribution of globular clusters in our galaxy to determine that the Sun was not at the center of the Milky Way.

Question 29

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 total mass in the galaxy's interstellar medium is gradually reduced, and the remaining gas is continually enriched in heavy elements.
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 gas of the interstellar medium is continually depleted in elements heavier than helium.
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 30

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

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

Question 31

Compared to our Sun, most stars in the halo are

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

Question 32

The Milky Way looks the same in X-rays as it does at infrared wavelengths.

Question 33

All heavy elements are made during supernova explosions.

Question 34

What is Sgr A∗?

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

Question 35

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

A) 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.
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) 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.
D) 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.

Question 36

Observing our galaxy at radio wavelengths allows us to see through the dust in the disk that obscures our view.

Question 37

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

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

Question 38

The galactic center lies in the direction of which constellation?

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

Question 39

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

A) We observe an extremely bright X-ray source at the center of our galaxy.
B) We can see gas falling into an accretion disk and past the event horizon of a black hole.
C) 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.
D) We observe a large, dark object that absorbs all light at the center of our galaxy.
E) all of the above

Question 40

We can see most of the Milky Way with visible light.

Question 41

Why do spiral arms have a blue color?

Question 42

Suppose you discovered a star made purely of hydrogen and helium. How old do you think it would be? Explain.

Question 43

Briefly describe how we can use the orbital characteristics of stars at many distances from the galactic center to determine the distribution of mass in the Milky Way.

Question 44

Briefly explain why stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements than stars that formed more recently.

Question 45

Most of the current star formation in the Milky Way occurs in its spiral arms.

Question 46

Observational Bias: The history of human understanding of the structure of the Milky Way starts with the bias that humans use visible light to see. Imagine you were a creature on another planet in the disk of the Milky Way who used radio waves to "see" the universe. What would have been your starting point for the structure of the Milky Way? What components would you have found easier to detect? What components would have been harder?

Question 47

Explain why in space, "no one can hear you scream."

Question 48

Modified Newtonian Dynamics? The case for dark matter in the Milky Way can be viewed as a failure of stars and gas far from the galactic center to obey Newton's law of gravitational attraction-- that is, the stars and gas seem to orbit too quickly for the visible matter their orbits enclose. Why do you think that astronomers choose to "save" Newton's form of gravitational attraction by postulating unseen dark matter, as opposed to regarding the observed orbital motion as a falsification of Newtonian gravitation on the scale of the Milky Way? [Note: At the distance scales involved, the gravitational forces in the galaxy are all fairly weak, much weaker than those encountered in a neutron star or black hole, and hence Einstein's theory of gravity (General Relativity) gives the same predictions as Newton's gravitational law].

Question 49

The average speed of stars relative to the Sun in the solar neighborhood is about 20 km/s. Suppose you discover a star in the solar neighborhood that is moving relative to the Sun at a much higher speed, say 220 km/s. What kind of orbit does this star probably have around the Milky Way? In what part of the galaxy does it spend most of its time? Explain.

Question 50

Most of the mass of the Milky Way is located in the halo of the galaxy in the form of dark matter.

Question 51

Astronomers observe huge bubbles of hot gas, some over a thousand light years across, in the Milky Way. What is their cause?

Question 52

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

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

Question 53

Almost all elements heavier than hydrogen and helium were made inside stars.

Question 54

A History of Water: One of the triumphs of 20ᵗʰ century astronomy is a convincing explanation for the origin of the chemical elements. Consider a single water molecule in your body, consisting of two hydrogen atoms and a single oxygen atom. Trace, from the earliest point in time that you can, the history of these atoms. List all of the possible environments in which these particular atoms could have spent time.

Question 55

Reflection nebula are bluer than the stars that illuminate them. The Earth's sky is bluer than the Sun. Is this a coincidence? Explain why or why not.

Question 56

Most of the mass of the galaxy is located at the galactic center in the form of a massive black hole.

Question 57

What produces the striking red, blue, and black colors of ionization nebulae?

Question 58

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) The center of the Milky Way does not emit enough visible light.
C) We are in the center of the Milky Way.
D) Interstellar dust and gas absorb and scatter visible light.

Question 59

Briefly describe the star-gas-star cycle.

Question 60

The star-gas-star cycle will continue forever because stars are continually recycling gas.

Question 61

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

Question 62

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 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.
C) The gigantic bursts of X-ray energy from a mysterious dark object
D) The motions of stars around a mysterious dark object

Question 63

The Sun lives in which component of the Milky Way?

A) The bulge
B) The disk
C) A globular cluster
D) The halo

Question 64

What affects the average orbital speed of a star in our galaxy?

A) The mass of the galaxy inside its orbit and size of its orbit
B) The mass and age of the star
C) The star's mass and the mass of the galaxy
D) The mass and age of the galaxy

Question 65

The very first stars in the universe were made of:

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

Question 66

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

A) 0.002%
B) 2%
C) 70%
D) 98%

Question 67

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

Question 68

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

Question 69

What happens after many generations of the star-gas-star cycle?

A) The heavy element abundance decreases, the amount of gas increases.
B) The heavy element abundance increases, the amount of gas increases.
C) The amount of gas and heavy elements stay the same: it's a cycle.
D) The heavy element abundance increases, the amount of gas stays the same.
E) The heavy element abundance stays the same, the amount of gas decreases. F) The heavy element abundance increases, the amount of gas decreases.

Question 70

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 10 times as great as the thickness.
D) The diameter is about 100,000 times as great as the thickness.

Question 71

Which star is older?

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

Question 72

How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

A) By applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the Galaxy
B) By counting the number of stars visible in this region of the galaxy
C) By estimating the amount of gas and dust in between the stars
D) By using the law of conservation of angular momentum to calculate the orbital speeds of nearby stars

Question 73

What are the Magellanic Clouds?

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

Question 74

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 trans-dimensional space warps in the vicinity of Sgr A
C) Measurements of radio emission from gas around Sgr A
D) Measurements of the orbits of stars around Sgr A

Question 75

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

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

Question 76

How do disk stars orbit the center of the galaxy?

A) They all orbit in roughly the same plane and in the same direction.
B) They have orbits randomly inclined and in different directions relative to the galactic center.
C) They follow spiral paths along the spiral arms.
D) 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.

Question 77

What do we mean by the interstellar medium?

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

Question 78

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

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

Question 79

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

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

Question 80

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