Download PREVIEW-Reading Quiz 06 - Chapter 12

Reading Quiz 06 - Chapter 12
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Started: Sep 26 at 10:18am
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Reading Quiz 06 - Chapter 12 of Understanding Our Universe
Question 1
1 pts
Each of the following supports the statement, "This reasoning tells us that mass determines the structure of a star and its place on the main sequence," except one.
Which one should be tossed out as being non-supportive?
Higher mass means a stronger force of gravity compressing the star's interior.
As a more luminous star ages, its mass increases due to the excess energy it is producing.
The higher rate of nuclear fusion means the star will be a more luminous star.
The greater force of gravity means the star must produce more outward pressure through nuclear fusion.
The higher the luminosity of a star on the main sequence, the shorter its main sequence lifetime.
Question 2
1 pts
Regulus (constellation Leo) and Barnard's Star (constellation Ophiuchus) are both main sequence stars. Regulus has a mass about 25 times that of
Barnard's Star, but will live a much shorter time on the main sequence. Why is that?
Regulus has a much smaller fusion core than Barnard's Star and thus has less hydrogen to fuse.
Because it is fully convective throughout its interior, Barnard's star can fuse its entire supply of hydrogen.
Although Regulus is more massive, it is a rare star that has a much lower fusion rate.
Regulus must have a much higher rate of fusion to support itself and runs out of core hydrogen faster.
Question 3
1 pts
We are going to add some extra explanation of what degeneracy means in a star.
"Quantum mechanics restricts the number of electrons that can have low energy. Basically, each electron must occupy its own energy state. When electrons are
packed together, as they are in a white dwarf, the number of available low energy states is too small and many electrons are forced into high energy states. When
this happens the electrons are said to be degenerate. These high energy electrons make a significant contribution to the pressure. Because the pressure arises
from this quantum mechanical effect, it is insensitive to temperature, i.e., the pressure doesn't go down as the star cools. This pressure is known as electron
degeneracy pressure and it is the force that supports white dwarf stars against their own gravity."
This means only 2 electrons in the lowest energy state (as in the helium atom depicted here, except electrons and nuclei are separated in a
white dwarf) and only 8 in the next energy state. Quantum mechanics dictates how many electrons can be packed together. What happens to
the degenerate core as more and more mass (helium ash) is piled onto it?
Shrinkage of the core to a smaller radius means the force of gravity is stronger.
The smaller, more massive core produces higher pressure.
Heat coming from the contracting degenerate core increases the shell nuclear fusion.
These statements all describe events happening when the helium core becomes degenerate.
As more ash is piled onto the core, it actually shrinks.
Question 4
Where does the helium flash occur in the figure shown above (which, once it occurs, actually breaks the cycle)?
Between E and A
Between C and D
Between D and E
Between B and C
Between A and B
1 pts
Question 5
1 pts
Why do stars become much more luminous while at the same time their surface temperatures are dropping (eventually reaching a minimum temperature) as they move
first up the red giant branch and, later, up the asymptotic giant branch? [Hint: Closely study Figure 12.8.]
For a sunlike star, the flash expands the core and reduces the star’s luminosity, sending it onto the horizontal branch of the H–R diagram.
These stars become more luminous because the degenerate helium or carbon cores continue to grow due to particle pressure.
Stars follow the Stefan-Boltzmann law: The higher luminosities generated by the shell fusion makes the star expand enormously, and the increase in
radius offsets the decrease in temperature.
These stars have cores that are expanding and giving off heat due to conservation of energy.
Question 6
1 pts
Consider the cores of sun-like stars that are "leaving" the main sequence to the cores of sun-like stars that are "leaving" the horizontal branch. Which one of the
following choices does not state a difference?
The helium-fusing core of a horizontal branch star had much higher temperatures.
Cores of stars leaving the main sequence have helium ash dumped on them; horizontal branch stars, carbon ash.
The core of a star leaving the horizontal branch does not experience a "carbon flash."
The carbon core of the horizonta branch star never becomes electron degenerate.
Question 7
1 pts
What characteristic of a giant or an asymptotic giant branch star lead to their eventual loss of around 50% of their mass?
Because the surface temperatures of these stars are so low, dust forms along with the gas and gets ejected.
The vast majority of these giant stars are part of a binary system and mass ends up being dumped on the other star.
These giant stars have convection occurring throughout their interior, and this motion causes gas ejection.
These stars are so huge that the force of gravity at their surfaces is less than the outward thermal pressure.
Question 8
1 pts
Refer back to the sections about light in Chapter 10 of our textbook. Planetary nebulae provide excellent examples of the range colors that can be produced by
processes occurring in atoms. (The colors displayed for the images of the planetary nebulae in the text may, in fact, be the actual colors.) What kind of spectra do we
expect to see from planetary nebulae?
Emission
Continuous, absorption, and emission on top of each other.
Continuous
Absorption
Question 9
1 pts
The hot carbon core of the once sun-like star is about the size of Earth but extremely hot - 100,000 Kelvin or more. What does this imply about the source of energy,
the photons, that is exciting the gases of planetary nebulae?
The radiation from the white dwarf follow Wien's Law: Ultraviolet photons are being created and they excite the gases.
Many trillions of photons at radio wavelengths are created every second.
The white dwarf is spinning exceedingly fast and that excites the gases of the nebulae.
The high velocities and thus kinetic energy of the ejected gases cause them to glow.
Question 10
1 pts
If we were able to observe a cluster of stars from when the stars were 5 billion years old until they were 12 billion years old, what would we observe about the overall
luminosities and temperatures of the stars that are still on the main sequence as the cluster aged?
Both the luminosities and temperatures of stars would decrease as lower mass stars are all that are left on the main sequence.
The stars on the main sequence would still represent the entire range of luminosities and temperatures as the cluster aged.
Stars still on the main sequence would be those having the highest luminosities but lowest temperatures.
The temperatures of the stars would get higher while their luminosities decreased.
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