The interstellar clouds called molecular clouds are _________.
answer
the cool clouds in which stars form
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What is a protostar?
answer
a star that is still in the process of forming
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Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
answer
thermal pressure
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Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
answer
infrared
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What kind of gas cloud is most likely to give birth to stars?
answer
a cold, dense gas cloud
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Which of the following phenomena is not commonly associated with the star formation process?
answer
intense ultraviolet radiation coming from a protostar
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When does a protostar become a main-sequence star?
answer
when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
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Approximately what core temperature is required before hydrogen fusion can begin in a star?
answer
10 million K
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Which star spends the longest time in the protostellar phase of life?
answer
a 1 solar mass star
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What is the approximate range of masses that newborn main sequence stars can have?
answer
0.1 to 150 solar masses
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The vast majority of stars in a newly formed star cluster are _________.
answer
less massive than the Sun
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Which of the following statements about brown dwarfs is not true?
answer
Brown dwarfs eventually collapse to become white dwarfs.
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What can we learn about a star from a life track on an H-R diagram?
answer
the surface temperature and luminosity the star will have at each stage of its life
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Which of the following lists the stages of life for a low-mass star in the correct order?
answer
protostar, main-sequence star, red giant, planetary nebula, white dwarf
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What happens when a main-sequence star exhausts its core hydrogen fuel supply?
answer
The core shrinks while the rest of the star expands.
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The main source of energy for a star as it grows in size to become a red giant is _________.
answer
hydrogen fusion in a shell surrounding the central core
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The overall helium fusion reaction is:
answer
Three helium nuclei fuse to form one carbon nucleus.
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What is a helium flash?
answer
The sudden onset of helium fusion in the core of a low-mass star
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What is a planetary nebula?
answer
Gas ejected from a low-mass star in the final stage of its life
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The ultimate fate of our Sun is to _________.
answer
become a white dwarf that will slowly cool with time
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What is the CNO cycle?
answer
a set of steps by which four hydrogen nuclei fuse into one helium nucleus
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In order to predict whether a star will eventually fuse oxygen into a heavier element, you mainly want to know what fact about the star?
answer
its mass
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Why is iron significant to understanding how a supernova occurs?
answer
Iron cannot release energy either by fission or fusion.
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After a supernova explosion, the remains of the stellar core _________.
answer
may be either a neutron star or a black hole
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Why is Supernova 1987A particularly important to astronomers?
answer
It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes.
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Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
answer
The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
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Where does gold (the element) come from?
answer
it is produced during the supernova explosions of high-mass stars
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Which two processes can generate energy to help a star maintain its internal thermal pressure?
answer
nuclear fusion and gravitational contraction
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Our Sun is considered to be a _________.
answer
low-mass star
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Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?
answer
The cloud must radiate much of its thermal energy.
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Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's angular momentum?
answer
the onset of core hydrogen fusion
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Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?
answer
A main-sequence star is hotter and dimmer than it was as a protostar.
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Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?
answer
A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
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We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
answer
Brown dwarfs would outnumber all ordinary stars.
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Where would a brown dwarf be located on an H-R diagram?
answer
below and to the right of the lowest part of the main sequence
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Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?
answer
H-R diagrams of globular clusters
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Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?
answer
Fusion reactions are producing energy at a greater rate in the red giant.
question
Carbon fusion occur in high-mass stars but not in low-mass stars because _________.
answer
the cores of low-mass stars never get hot enough for carbon fusion
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Which of the following statements about various stages of core nuclear burning (hydrogen, helium, carbon, and so on) in a high-mass star is not true?
answer
Each successive stage lasts for approximately the same amount of time.
question
Which event marks the beginning of a supernova?
answer
The sudden collapse of an iron core into a compact ball of neutrons
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Suppose that the star Betelgeuse (the upper left shoulder of Orion) were to supernova tomorrow (as seen here on Earth). What would it look like to the naked eye?
answer
Betelgeuse would remain a dot of light, but would suddenly become so bright that, for a few weeks, we'd be able to see this dot in the daytime.
question
Observations show that elements with atomic mass numbers divisible by 4 (such as oxygen-16, neon-20, and magnesium-24) tend to be more abundant in the universe than elements with atomic mass numbers in between. Why do we think this is the case?
answer
At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions.
question
A spinning neutron star has been observed at the center of a _________.
answer
supernova remnant
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Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?
answer
Planetary nebula formation is more common.
question
You discover a binary star system in which one star is a 15 M Sun main-sequence star and the other is a 10 M Sun giant. How do we think that a star system such as this might have come to exist?
answer
The giant must once have been the more massive star, but is now less massive because it transferred some of its mass to its companion.
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The sun formed, probably along with other stars, within a large ?
answer
molecular cloud
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A ? consists of two stars that orbit each other every few days
answer
close binary
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A ? is a "star" so small in mass that its core never gets hot enough to sustain nuclear fusion reactions
answer
brown dwarf
question
Most of the gas remaining from the process of star formation is swept into interstellar space by a ?
answer
protostellar wind
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As a protostar's internal temperature increases, its growing ? helps slow its contraction due to gravity
answer
thermal pressure
question
Planets may form within the ? that surrounds a forming star
answer
protostellar disk
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Mass can be lost through a ? of material ejected along a protostar's axis of rotation
answer
jet
question
The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____.
answer
oxygen
question
According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________.
answer
1/1000 that of hydrogen
question
According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20?
answer
iron
question
According to current understanding, the two most abundant elements in the universe were made __________.
answer
in the Big Bang
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