Question 1

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?

Accepted Answer

Each successive stage of core nuclear burning in a high-mass star lasts for a progressively shorter amount of time. The hydrogen burning stage lasts the longest, while the stages that create heavier elements burn for increasingly shorter periods.

Question 2

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage does the star have an inert (non-burning) carbon core surrounded by shells of helium and hydrogen burning?

Accepted Answer

In the life cycle of a 1 solar mass star, the stage with an inert carbon core surrounded by shells of helium and hydrogen burning corresponds to the asymptotic giant branch (AGB) phase, which is typically labeled as stage viii.

Question 3

Which event marks the beginning of a supernova?

Accepted Answer

The sudden collapse of an iron core into a compact ball of neutrons is what marks the beginning of a supernova. This collapse is triggered by the inability of the core to support its own weight through fusion reactions, leading to a runaway gravitational collapse that releases enormous amounts of energy in the form of neutrinos and shock waves. This energy drives the explosive ejection of the outer layers of the star, resulting in a supernova. The other options - helium burning after a helium flash, neon burning in an extremely massive star, and the CNO cycle - are all processes that occur during the later stages of stellar evolution and are not directly related to the onset of a supernova.

Question 4

Where would a brown dwarf be located on an H-R diagram?

Accepted Answer

Brown dwarfs are not massive enough to sustain fusion reactions, so they cool and dim with time, falling below the main sequence. Therefore, they would be located below and to the right of the lowest part of the main sequence on an H-R diagram.

Question 5

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?

Accepted Answer

Betelgeuse, if it were to supernova, would indeed become significantly brighter, but it would still appear as a point of light to the naked eye. Its brightness could increase to the point where it would be visible during the daytime for a period. Supernovae do not cause stars to visually expand to the size of the Moon to the naked eye, nor do they result in the immediate visible expansion of a gas cloud or the star disappearing instantly.

Question 6

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. What will happen to the star after stage viii?

Accepted Answer

After stage viii, the star will enter the final stage of its life cycle and eject its outer layers to form a planetary nebula. The core will then shrink and become a white dwarf. This is the typical fate of a low to medium mass star like the one shown in the diagram. Option B is incorrect because carbon fusion does not occur in low to medium mass stars. Option C is also incorrect because supernovae are only produced by more massive stars, and the diagram shows a 1 solar mass star. Option D is incorrect because stage viii (the red giant stage) is not a stable state, and the star will not remain in this stage for another 10 billion years.

Question 7

Carbon fusion occur in high-mass stars but not in low-mass stars because ________.

Accepted Answer

Carbon fusion requires extremely high temperatures and pressures, which are only reached in the cores of high-mass stars. Low-mass stars do not have sufficient mass or gravity to achieve the necessary conditions for carbon fusion.

Question 8

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage does the star have an inert (non-burning) helium core?

Accepted Answer

During stage iv, the star has an inert helium core. This stage corresponds to the red giant phase where hydrogen shell burning occurs around the inert helium core.

Question 9

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?

Accepted Answer

If the general trends observed in stellar birth masses apply to brown dwarfs, it implies that lower mass objects are more common than higher mass objects in the universe. Since brown dwarfs are lower in mass than stars, this would mean they are more numerous than ordinary stars.

Question 10

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. Which stage lasts the longest?

Accepted Answer

Stage iii (the main sequence stage) lasts the longest for a 1 solar mass star.

Question 11

Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?

Accepted Answer

A red giant is a star that has exhausted most of the hydrogen fuel in its core and has moved into a later stage of its evolution. As the core contracts, outer layers of the star expand and cool, which makes the red giant larger and more luminous. While a main sequence star is still fusing hydrogen in its core, a red giant is fusing heavier elements like helium and carbon. These fusion reactions produce energy at a greater rate, making the red giant more luminous than a main sequence star of the same mass.

Question 12

Which of the following types of data provide evidence that helps us understand the life tracks of low-mass stars?

Accepted Answer

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Question 13

A spinning neutron star has been observed at the center of a ________.

Accepted Answer

A spinning neutron star is commonly found at the center of a supernova remnant, which is the leftover debris after a massive star has exploded. The intense explosion can trigger the collapse of the star's core into a dense, rapidly rotating neutron star or pulsar. Planetary nebula, red supergiants, and protostars are not typically associated with the existence of neutron stars.

Question 14

You discover a binary star system in which one star is a 15 MSᵤn main-sequence star and the other is a 10 MSᵤn giant. How do we think that a star system such as this might have come to exist?

Accepted Answer

The giant star must have once been the more massive one, but it transferred some of its mass to its companion, causing the current mass difference between the two stars. This is known as mass transfer.

Question 15

Which is more common: a star blows up as a supernova, or a star forms a planetary nebula/white dwarf system?

Accepted Answer

While supernovae may be more spectacular and get more of the limelight, planetary nebula formation is actually more common. It is estimated that more than half of all stars will eventually undergo this process, while only a small percentage will end in a supernova.

Question 16

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?

Accepted Answer

At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions, which often result in elements with atomic mass numbers divisible by 4.

Question 17

This diagram represents the life track of a 1 solar mass star. Refer to the life stages labeled with roman numerals. During which stage is the star's energy supplied by primarily by gravitational contraction?

Accepted Answer

In stage ii (protostar phase), the star's energy is primarily supplied by gravitational contraction. The gravitational energy is converted into thermal energy and heats up the protostar until it becomes hot enough for nuclear fusion to occur in stage iii (main sequence phase). In all other stages, the energy is supplied by nuclear fusion.

Quiz 12: Star Stuff