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Assignment 8
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
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1. If an astronomer wants to find and identify as many stars as possible in a star cluster that has recently formed near the surface of a giant molecular cloud (such as the Trapezium cluster in the Orion Nebula), what instrument would be best for her to use?
a. an infra­red telescope (and camera)
b. an x­ray telescope
c. a very good camera attached to a large reflector telescope on the ground
d. an ultraviolet spectrograph attached to the Hubble Space Telescope
e. the instruments that were part of the Compton Gamma Ray Observatory
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2. The Orion Nebula is
a. a distant galaxy of stars and raw material
b. a small disk of gas and dust surrounding a single star that was recently formed
c. a cloud of gas and dust illuminated by the light of newly formed stars within it
d. the remnant of a star that exploded several thousand years ago
e. an illusion caused by activity in the Earth's upper atmosphere
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3. Astronomers call a clump of matter that is contracting to become a star
a. a Herbig­Haro object
b. a giant molecular cloud
c. a planet
d. a protostar
e. a main sequence star
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4. If you want to find stars that are just being born, where are the best places to search?
a. in HII regions
b. in giant molecular clouds
c. in regions of ultra­hot interstellar hydrogen gas
d. in the disks around massive stars that were just recently formed
e. Hollywood
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5. Astronomers believe that disks of material will form around protostars that are spinning. Which of the following observed phenomena is a good indication of the presence of a disk around a protostar?
a. HII regions
b. huge pillars of dust, like those seen by the Hubble in the Eagle Nebula
c. jets and Herbig­Haro objects
d. the presence of molecules in giant clouds of gas and dust
e. gamma ray bursts seen in the direction of giant molecular clouds
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6. Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star?
a. when it starts to contract rapidly from a cloud of gas and dust
b. when it first becomes visible inside its dust cloud
c. when a wind is observed coming from its surface
d. when nuclear fusion reactions begin inside its core
e. when it glows with infrared radiation
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7. Why do all stars spend most of their lives on the main sequence?
a. because the neutrinos created inside the Sun do not carry any energy away with them
b. because during this stage the star contracts from enormous size to a relatively small ball; this takes a long time
c. because the fuel for energy production in this stage of the star's life is hydrogen; and that is an element every star has lots and lots of
d. because in this stage, the processes inside the star do not generate any energy; thus the star can continue in this stage indefinitely
e. this is an unsolved problem in astronomy, which is an important project for the world's largest telescopes to work on
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8. When a star settles down to a stable existence as a main­sequence star, what characteristics determines where on the main sequence in an H­R diagram the star will fall?
a. its mass
b. the fraction of its atmosphere that consists of hydrogen
c. whether it is located on the outer regions or the central regions of the molecular cloud that gave it birth
d. the speed and direction of its rotation
e. the size of the disk around it
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9. Which of the following types of stars will spend the longest time (the greatest number of years) on the main sequence?
a. O
b. A
c. G
d. K
e. you can't fool me; every star spends about the same number of years on the main sequence
____ 10. How long a main sequence star remains on the main sequence in the H­R diagram depends most strongly on
a. its initial composition
b. the number of companion stars or planets orbiting it
c. its radial velocity (as measured from the spectrum)
d. its mass
e. its ability to fuse the element carbon into some other element
____ 11. When the outer layers of a star like the Sun expand, and it becomes a giant, which way does it move on the H­
R diagram?
a. toward the upper right
b. toward the upper left
c. toward the lower right
d. toward the lower left
e. it moves horizontally, but stays on the main sequence
____ 12. As a star becomes a giant, its outer layers are expanding. Where does the energy for expanding these layers come from?
a. from the fusion of helium into carbon in the core
b. from the long­term fusion of hydrogen into helium in the core
c. from an explosion in the core
d. from a magnetic dynamo effect in the star's outer layers, caused by a much stronger magnetic field inside the star
e. from the fusion of hydrogen into helium in a shell around the core
____ 13. The event in the life of a star that begins its expansion into a giant is
a. the core reaches a temperature of ten million degrees
b. as much as 90% of the star explodes violently
c. almost all the hydrogen in its core that was hot enough for fusion has been turned into helium
d. the star's internal structure reaches equilibrium for the first time in its life
e. it reaches the stage that astronomers call the zero­age main sequence
____ 14. When the core of a star reaches a temperature of about 100 million degrees (K), something new happens in the core. What is this new event?
a. hydrogen nuclei begin to fuse into heavy hydrogen (deuterium) and then into helium
b. two helium nuclei begin fusing of beryllium (element number 4)
c. the core explodes
d. three helium nuclei begin fusing carbon (element number 6)
e. the fusion of iron (element 26)
____ 15. After it experiences a "helium flash" a star like the Sun will have a brief period of stability, fusing helium into carbon (and sometimes oxygen). During this brief stable stage, the star
a. gets to be even larger in diameter than it was as a red giant
b. returns to the position on the H­R diagram that the star had in its main­sequence stage
c. increases tremendously in luminosity
d. is able to fuse many of the heaviest elements (such as iron and gold) in its superhot core
e. none of the above
____ 16. If you trace back the history of a carbon atom in your little finger through all of cosmic history, where did this atom most likely originate?
a. it was made in the big bang
b. it was fused from hydrogen atoms during the main­sequence stage of a star's life long before the Sun existed
c. it was made from smaller nuclei in the hot core of the Earth when our planet first formed
d. it was fused from 3 helium nuclei in the core of a red giant star long before the Sun existed
e. it came from Bayonne, New Jersey (where a lot of carbon­rich soot originates)
____ 17. Why can a star with a mass like our Sun not fuse (produce) further elements beyond carbon and oxygen?
a. because there are no elements heavier than those two; they are the heaviest nuclei in nature
b. because they just cannot get hot enough for the fusion of heavier nuclei
c. because all such stars explode before they can make any other elements
d. because all such elements become radioactive and their nuclei break apart rather quickly
e. because the cores of such stars get too hot for further types of fusion to be able to happen
____ 18. Many names used by astronomers are misleading or outdated. A good example is the term planetary nebula, which astronomers use to refer to:
a. a solar system in formation
b. the remains of an exploded high­mass star
c. a region of gas and dust where new planets have recently formed
d. the shell let go by a dying low­mass star
e. a globular cluster, which looks like a planet through very small telescopes
____ 19. When stars become giants, which of the following does NOT usually happen?
a. their outer envelopes expand significantly
b. they lose a significant amount of mass from their outside layers
c. their surface temperatures become lower than before
d. their overall luminosities increase
e. their mass grows significantly as they incorporate planets and interstellar matter near the star
____ 20. Really massive stars differ from stars with masses like the Sun in that they
a. go through all the stages of their lives more slowly
b. do not really go through a main sequence stage in their lives
c. can fuse elements beyond carbon and oxygen in their hot central regions
d. are no longer forming in the Galaxy; they only formed very early in the Galaxy's history
e. they are significantly less luminous after the main sequence stage is over
____ 21. In a planetary nebula, the shell of expelled material is glowing intensely. What is the main source of energy for this glow?
a. friction, as the atoms of the expelled shell rub against each other
b. the explosion of the dying star
c. ultraviolet radiation from the hot star at the center
d. the fusion of hydrogen into helium in the shell
e. the change of electrons and protons into neutrons
____ 22. After the core of a massive star becomes a neutron star, the rest of the star's material
a. falls inward very slowly, taking billions of years to get really compressed
b. makes a planetary nebula, which gently moves outward from the center
c. is vaporized by the incredible heat of the dying star and evaporates
d. explodes outward as a supernova
e. continues regular fusion and returns to the main sequence
____ 23. If most stars are low­mass stars, and low­mass stars typically eject a planetary nebula, why then do astronomers see relatively few planetary nebulae in the sky?
a. planetary nebulae can only be detected through their faint radio emissions
b. planetary nebulae are only visible when a planet surrounding the star plows through the ejected material, causing it to glow
c. planetary nebulae expand rapidly and soon become too faint to be visible
d. planetary nebulae quickly fall back onto the star produced them
e. while most stars are low­mass when they are born, throughout their lives they gather more and more material; so few stars are low­mass when they die
____ 24. When a single star with a mass equal to the Sun dies, it will become a
a. white dwarf
b. neutron star
c. black hole
d. pulsar
e. burster
____ 25. Which of the following stages will the Sun definitely go through as it gets older?
a. red giant
b. source of a planetary nebula
c. white dwarf
d. black dwarf
e. all the above
____ 26. Which of the following stages will our own Sun go through:
a. spending a long time on the main sequence
b. expanding to become red giant
c. eventually fusing helium into carbon
d. giving off a planetary nebula
e. all of the above
____ 27. A white dwarf, compared to a main sequence star with the same mass, would always be:
a. redder in color
b. smaller in diameter
c. the same size
d. younger in age
e. less massive
____ 28. Which of the following stages can only occur in the life of a low­mass star (whose final mass is less than 1.4 times the mass of the Sun)?
a. proto­star
b. main­sequence
c. red giant
d. white dwarf
e. supernova
____ 29. The most stable element in the universe (the one that doesn't "like" to undergo either nuclear fusion or fission) is:
a. hydrogen
b. carbon
c. uranium
d. technetium
e. iron
____ 30. What incident in a star's life sets off the very quick chain of events that leads to a supernova explosion?
a. swelling up to become a red giant
b. the fusion of iron
c. helium begins to fuse into carbon
d. an event horizon forms
e. the star's core becomes degenerate and the electrons don't allow further compression
Assignment 8
Answer Section
MULTIPLE CHOICE
1. ANS: A
3. ANS: D
5. ANS: C
7. ANS: C
9. ANS: D
11. ANS: A
13. ANS: C
15. ANS: E
17. ANS: B
19. ANS: E
21. ANS: C
23. ANS: C
25. ANS: E
27. ANS: B
29. ANS: E