Chapter 17 Homework

The main illustration in the video shows the life track of a one-solar mass star. Each point along this track represents __________.

the star's surface temperature and luminosity at one time in its life

The opening graph in the video shows the measured relative abundances of elements in the galaxy. What is the 4th most abundant element?

Carbon

What element(s) should you expect to be most abundant in the star when it reaches Stage 6?

Carbon

When it dies, a high-mass star explodes as a

supernova.

Overall, careful study of the patterns revealed in the graph of measured element abundances has allowed scientists to __________.

test and validate models of how elements are produced by stars.

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

the cores of low-mass stars never get hot enough for carbon fusion

When a star exhausts its core fusion fuel so that the core begins to contract, which of the following can stop the contraction?

- degeneracy pressure - fusion of another element

Consider the six stages labeled on (Figure 1) at left, showing the life track of a one-solar-mass star. During which of the labeled stages is the star generating energy through hydrogen fusion in its central core?

1

At which of the first five labeled stages is the star in energy balance?

1 and 4

According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________.

1/1000 that of hydrogen

During which of the labeled stages on (Figure 1) at left, is the star generating energy through helium fusion in its central core (along with hydrogen fusion in a surrounding shell)?

4

The star expels its outer layers into space almost immediately upon the end of Stage _____.

5

This Hubble Space Telescope photo shows a planetary nebula. What is the white dot in the center (indicated by the arrow)?

A white dwarf

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?

At the end of a high-mass star's life, it produces new elements through a series of helium capture reactions

Which of the following statements about various stages of core nuclear fusion (hydrogen, helium, carbon, and so on) in a high-mass star is not true?

Each successive stage lasts for approximately the same amount of time

The following figures show various stages during the life of a star with the same mass as the Sun. Rank the stages based on when they occur, from first to last.

First Stage - contracting cloud of gas and dust - protostar - main-sequence G star - red giant - planetary nebula - white dwarf

Provided following are various stages during the life of a high-mass star. Rank the stages based on when they occur, from first to last.

First Stage - contracting cloud of gas and dust - protostar - main-sequence O star - red supergiant - supernova - neutron star

Assume that all four H-R diagrams below represent a star in different stages of its life, after it starts to fuse hydrogen in its core. Rank the HR diagrams based on when each stage occurs, from first to last.

First Stage - star in the line - star barely above line - star above the line - star below the line

Provided following are various elements that can be produced during fusion in the core of a high mass main sequence star. Rank these elements based on when they are produced, from first to last.

First produced - helium - carbon - oxygen - iron

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

Fusion reactions are producing energy at a greater rate in the red giant.

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

H-R diagrams of globular clusters

Listed following are characteristics that describe either high-mass or low-mass stars. Match these characteristics to the appropriate category.

High-Mass Stars - late in life fuse carbon into heavier elements - have higher fusion rate during main-sequence life - end life as a supernova Low-Mass Stars - final corpse is a white dwarf - the sun is an example - have longer lifetimes - end life has a planetary nebula

According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20?

Iron

Why is iron significant to understanding how a supernova occurs?

Iron cannot release energy either by fission or fusion

In what way is iron unique among the elements?

It has the lowest mass per nuclear particle

What is a helium flash?

It is the sudden onset of helium fusion in the core of a low-mass star

Tidal forces are important to the Algol system today but were not important when both stars were still on the main sequence. Why not?

Main-sequence stars in a system like the Algol system are small compared to their physical separation.

Is a protostar in energy balance? Why or why not?

No, because its core generates less energy than its surface radiates into space.

Suppose that hydrogen, rather than iron, had the lowest mass per nuclear particle. Which of the following would be true?

Nuclear fusion could not power stars

The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____

Oxygen

When the core of a star like the Sun uses up its supply of hydrogen for fusion, the core begins to ________.

Shrink and heat

Which statement accurately describes the length of Stage 1 compared to Stages 2 through 5? (Ignore Stage 6 for this question.)

Stage 1 lasts nearly 10 times as long as Stages 2 through 5 combined.

In Part D, you saw that elements with even atomic numbers tend to be more abundant than neighboring elements with odd atomic numbers. What nuclear process explains why this is the case?

Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus.

The observational data for the element abundances agree quite well with what we expect based on our current understanding of nuclear fusion and stellar evolution. But imagine the data had turned out to be different. Which of the following differences, if it had actually been observed, would have forced us to rethink our entire picture of stellar evolution?

The abundance of elements heavier than uranium turned out to be greater than the abundance of carbon.

Choose the correct explanation of how do red giants manufacture carbon-rich dust grains, and why are these important to life.

The convection in a low-mass star in its final stages of life dredges up carbon from the core and brings it to the surface. Because the carbon can then be lost via the stellar winds, these stars seed the interstellar medium with carbon, including the carbon that is used for life on Earth.

What happens when a main-sequence star exhausts its core hydrogen fuel supply?

The core shrinks while the rest of the star expands.

What event brings the forming star into energy balance

The onset of nuclear fusion in the core

Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Which of the following statements about this star is most likely to be true?

The star will explode as a supernova within a few days.

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?

The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.

The fundamental lesson that we learn from the life track is that the star's outward appearance at any moment in its life reflects __________.

The way it is generating energy in its core

Based on the diagram, which of the following statements best describes the observed pattern of abundances for elements with an atomic number between 6 and 20?

There is a general trend of decreasing abundance with increasing atomic number, but elements with even atomic numbers tend to be more abundant than those with odd atomic numbers.

How are low-mass red giant stars important to our existence?

These stars manufactured most of the carbon atoms in our bodies.

According to our modern understanding of the origin of elements, why are hydrogen and helium so much more abundant than any other elements?

They were produced in the Big Bang.

Choose the correct explanation of whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false).

This statement is not sensible. If the Sun had been born as a high-mass star 4.5 billion years ago, it would have exploded as a supernova a long time ago.

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false) and choose the correct explanation.

This statement is sensible. The 2.5MSun red giant had to be more massive than its companion at some point in the past in order for it to be more advanced in its evolutionary state than its companion.

Notice that nitrogen is less abundant than either carbon or oxygen. This is an example of the more general observation that __________.

elements with even atomic numbers are more abundant than the elements with odd atomic numbers that come between them

As the video shows, the star begins its life from a clump of gas that heats up as it contracts. Where does the energy that heats the cloud come from?

gravitational potential energy that the cloud loses as it shrinks in size

The final stage of core fusion in a low-mass star is

helium fusion.

Stars with mass greater than about 8 solar masses are considered

high-mass stars.

The primary source of energy for a star as it grows in size to become a red giant is ________

hydrogen fusion in a shell surrounding the central core

The energy that drives the expansion of a star into a supergiant or red giant comes from ________

hydrogen shell fusion

According to current understanding, the two most abundant elements in the universe were made _________

in the Big Bang Submit

A star that is expanding (into a subgiant and then a giant) has a(n)

inert helium core.

To predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?

its mass

Stars with masses less than about two solar masses are considered __________

low-mass stars

Stars that are fusing hydrogen in their cores are

main-sequence stars.

When it dies, a low-mass star expels a

planetary nebula.

Which of the following lists the stages of life for a low-mass star in the correct order?

protostar, main-sequence star, red giant, planetary nebula, white dwarf