2) Reflection nebulae 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.
3) 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.
4) Briefly describe the star-gas-star cycle.
5) 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.
6) Explain why in space, “no one can hear you scream.”
7) What produces the striking red, blue, and black colors of ionization nebulae?
8) Astronomers observe huge bubbles of hot gas, some over a thousand light-years across, in the
Milky Way. What is their cause?
9) 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.
10) Why do spiral arms have a blue color?
11) How would our view of our galaxy be different if we were located in the central bulge? What
about if we were located in a halo globular cluster?
15.5 Mastering Astronomy Reading Quiz
1) 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.
2) What affects the average orbital speed of a star in our galaxy?
A) the star’s mass and the mass of the galaxy
B) the mass of the galaxy outside the star’s orbit and the size of its orbit
C) the mass and age of the star
D) the mass of the galaxy inside the star’s orbit and the size of its orbit
3) The Sun’s location in the Milky Way Galaxy is ________.
A) very near the galactic center
B) in the galactic disk, slightly over halfway out from the center
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
4) 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 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
5) 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
6) 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.
7) 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
8) Elements heavier than hydrogen and helium constitute about ________ of the mass of the
interstellar medium.
A) 0.002%
B) 2%
C) 70%
D) 98%
9) 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.
10) What are cosmic rays?
A) subatomic particles that travel close to the speed of light
B) another name for gamma rays and X-rays
C) fast-moving dust particles in the interstellar medium
D) the highest-energy form of light
E) lasers used as weapons
11) The primary way that we observe the atomic hydrogen that makes up most of the interstellar
gas in the Milky Way is with ________.
A) radio telescopes observing at a wavelength of 21 centimeters
B) ground-based visible-light telescopes
C) space-based ultraviolet telescopes
D) X-ray telescopes
12) Which of the following molecules is the most abundant in molecular clouds?
A) H2
B) CO
C) H2O
D) NH3
13) Interstellar dust consists mostly of ________.
A) microscopic particles of carbon and silicon
B) ozone “smog”
C) hydrogen and helium atoms
D) tiny grains of water ice
E) the same tiny particles found in household dust
14) Which of the following models best explains why our galaxy has spiral arms?
A) The spiral arms are highlighted by star formation caused by density waves propagating
through the disk of the galaxy.
B) The spiral arms are composed of groups of stars that are bound together by gravity and
therefore always stay together as the galaxy rotates.
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) No model can explain the existence of the arms, which rotate with the galaxy like the fins of a
giant pinwheel toy.
15) What do we mean by a protogalactic cloud?
A) a cloud of hydrogen and helium that contracts to become a galaxy
B) a term once used historically to refer to any galaxy
C) the cloud-like halo that surrounds the disks of spiral galaxies
D) a cloud of gas that was once a galaxy
16) Most stars in the Milky Way’s halo are ________.
A) very old
B) found inside molecular clouds
C) very young
D) blue or white in color
17) 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 colorful cloud of gas that glows because it is heated by light from nearby hot stars
D) a name sometimes used to describe spiral galaxies besides the Milky Way
18) How do halo stars differ from disk stars?
A) Halo stars orbit the galactic center with many different inclinations, while disk stars all orbit
in nearly the same plane.
B) Halo stars remain stationary, unlike disk stars that orbit the galactic center.
C) Halo stars explode as supernovae much more frequently than disk stars.
D) Halo stars orbit the center of the galaxy at much lower speeds than disk stars.
19) Where does most star formation occur in the Milky Way Galaxy?
A) in the spiral arms
B) everywhere throughout the galactic disk
C) in the central bulge
D) within the halo
20) Based on observations, which of the following statements about stars in the Milky Way is
generally true?
A) The older the star, the lower its abundance of heavy elements.
B) The older the star, the bluer its color.
C) The older the star, the higher its abundance of heavy elements.
D) The younger the star, the higher its mass.
E) The less massive the star, the older it is.
21) What kind of object do we think lies at the center of the Milky Way Galaxy?
A) a 3 to 4 million solar mass black hole
B) a gigantic X-ray binary system
C) a dense cluster of young, hot stars
D) an enormous collection of dark matter, explaining why we detect no light at all from the
galactic center
15.6 Mastering Astronomy Concept Quiz
1) If we could view our galaxy from a distance of several hundred thousand light-years, it would
appear ________.
A) as a flattened disk with a central bulge and spiral arms
B) as a faintly glowing band of light stretching all the way around the sky
C) as a faint smudge barely visible to the unaided eye
D) as a single, bright star
2) How does the interstellar medium affect our view of most of the galaxy?
A) It prevents us from seeing most of the visible and ultraviolet light from the galactic disk.
B) It absorbs all wavelengths of light.
C) It produces so much visible light that it blocks our view of anything beyond it.
D) It has no effect on visible-light observations, but prevents us from studying the galactic center
with radio waves or X-rays.
3) About what percentage of the mass of a molecular cloud is in the form of dust?
A) 1%
B) 10%
C) 50%
D) 98%
4) 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 have a higher fraction of elements heavier than hydrogen and helium.
B) It should be higher in mass.
C) It should be much brighter.
D) It should orbit the galactic center at a much higher rate of speed.
E) All of the above would be expected.
5) 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) It is reasonable because we already know that interstellar dust obscures our view through the
disk of the galaxy.
C) The 10% figure is too low because most of the mass of the galaxy is in the form of interstellar
dust.
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.
6) The most common constituent of gas in the disk of the Milky Way Galaxy is ________.
A) atomic hydrogen
B) molecular hydrogen
C) helium
D) water vapor
7) 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 about the same, but the percentage of elements heavier than
hydrogen and helium will have risen from the current 2% to more than 50%.
C) The total amount of gas will be much greater, since many stars will undergo supernovae
between now and then.
D) 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.
8) Over time, the star-gas-star cycle leads the gas in the Milky Way to ________.
A) have a greater abundance of heavy elements
B) have a lower abundance of heavy elements
C) become denser and have a greater abundance of heavy elements
D) become denser and hotter
9) 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 Chandra X-ray Observatory
B) the Keck I telescope on the summit of Mauna Kea
C) the SOFIA airborne infrared observatory
D) the Hubble Space Telescope
10) What happens after many generations of the star-gas-star cycle?
A) The amounts of heavy elements and gas available for star formation stay the same: it’s a
cycle.
B) The amount of heavy elements in the galaxy goes up, and so does the amount of gas available
for star formation.
C) The amount of heavy elements in the galaxy goes down, and so does the amount of gas
available for star formation.
D) The amount of heavy elements in the galaxy goes up while the amount of gas available for
star formation goes down.
11) All the following types of objects are found almost exclusively in the disk of the Milky Way
except ________.
A) globular clusters
B) young stars
C) X-ray binaries
D) high-mass, red supergiant stars
12) Red and orange stars are found evenly spread throughout the galactic disk, and blue stars are
typically found ________.
A) only in or near star-forming clouds
B) in the halo
C) only in the central bulge
D) also evenly spread throughout the galactic disk
13) Which of the following statements comparing halo stars to our Sun is not true?
A) Most stars in the halo have either died or are in their final stages of life, while the Sun is only
in about the middle of its lifetime.
B) Most stars in the halo have cooler surface temperatures than the Sun.
C) Most stars in the halo are less luminous than the Sun.
D) Most stars in the halo contain a much lower percentage of heavy elements than the Sun.
14) 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 probably a halo star that is currently passing through the disk.
B) It is a very young star, recently formed.
C) It has been pushed to high speed by the shock wave from a nearby supernova.
D) It is a very high mass star.
15) Where is most of the dark matter in our galaxy thought to exist?
A) in the massive black hole at the galactic center
B) in small, compact objects in the halo of the galaxy
C) spread throughout the disk in the form of neutral hydrogen
D) spread throughout the halo of the galaxy
16) Spiral arms appear bright because ________.
A) they contain more hot young stars than other parts of the disk
B) they contain far more stars than other parts of the galactic disk
C) they contain more molecular clouds than other parts of the disk
D) they are the only places where we find stars within the disk of the galaxy
17) How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
A) The stars that formed first could orbit the center of the galaxy in any direction at any
inclination.
B) The stars that formed first eventually settled into a galactic disk, circling the center of the
galaxy.
C) The protogalactic cloud gradually formed stars, starting from the center of the galaxy working
outwards.
D) The protogalactic cloud gradually formed stars, starting from the outer edges of the spiral
arms and working inward.
18) If we could watch spiral arms from a viewpoint looking down on the plane of our galaxy,
over 500 million years, what would we see happen?
A) Stars will move through the spiral arms, bunching up closer as they pass through. Young hot
stars will form and die within the arms before having a chance to move out.
B) The spiral arms will seem to “wind up,” to wrap more and more tightly around the center of
the galaxy.
C) The spiral arms will eventually dissipate and fade away, since they are a temporary
phenomenon that should only last for a million years or so.
D) The spiral arms will eventually unwind, as centripetal forces send the stars flying outwards
into intergalactic space.
19) What is the best evidence for an extremely massive black hole in the center of the Milky
Way?
A) The orbits of stars in the center of the galaxy indicate the presence of an object about 4
million times the mass of the Sun in a region no larger than our solar system.
B) Huge amounts of X-rays are pouring out of the center of the galaxy.
C) The center of our galaxy hosts a pulsar that is spinning so fast that it could only be a black
hole.
D) We observe stars vanishing in the center of the galaxy as they are sucked into the black hole.
20) 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 thought to harbor a black hole of about 4 million solar masses.
C) It is a source of X-ray emission that we have observed with telescopes in space.
D) It is a source of bright radio emission.