Download The correct answers are written in bold, italic and underlined. The

Comins DEU 3e Ch 09 Quiz 2
The correct answers are written in bold, italic and underlined.
The most important questions to study for the exam are highlighted.
1. Which physical parameter uniquely fixes a star's location on the zero-age main
sequence (ZAMS) on the Hertzsprung-Russell diagram when it reaches this stage of its
evolution?
• Its chemical composition
• Its mass
• Its age
The most massive stars are the most luminous, while less massive stars are distributed
down the ZAMS.
2. On the main sequence of the Hertsprung-Russell diagram of a very young cluster,
where will the most massive stars be found?
• At the very bottom of the main sequence, massive stars being cool because of
their great mass
• At the very top of the main sequence, massive stars being very hot and active
• In the middle of the main sequence, with very hot but less massive stars
positioned higher and massive red giant stars positioned lower
Very massive stars evolve very quickly and generate the most energy and therefore exist
for only a short time.
3. If three stars of different mass are formed at the same time in a cluster, which one will
last longest?
• The small-mass star because its hydrogen-burning nuclear processes will
proceed very slowly, if at all
• The intermediate-mass star because nuclear processes proceed rather slowly, and
there is enough mass to sustain the star for a long time
• The high-mass star because these stars have enormous resources of nuclear fuel
available to them and can last a very long time
A low-mass star has less fuel than a high-mass star, but it uses that fuel much more
slowly and in fact lasts for a longer time than the high-mass star.
4. What happens after hydrogen fusion ceases in the core of a star?
• The surface layers expand as the expanding core pushes them outward.
• The surface layers contract as the core beneath them contracts.
• The core contracts and the surface of the star expands outward.
The core contracts because nuclear fusion becomes less active and cannot generate
enough pressure to oppose gravity. However, an intense source of hydrogen fusion begins
in a shell around the core, and this causes the layers between it and the surface to expand,
pushing the surface outward.
5. When the Sun evolves into a red giant star, its intrinsic brightness or luminosity,
compared to its present brightness, will be
• about the same, the effect of the increase in size offsetting that of the decrease in
its temperature.
• much higher, the effect of its increasing size being much greater than that of its
decreasing temperature.
• much lower, the effect of the decrease in temperature being much greater than that
of its increasing size.
The red giant Sun will be 2000 times brighter than the present Sun.
6. As an isolated star evolves, which of the following will NOT occur?
• The chemical composition of the star changes.
• The star loses mass to space.
• The star gains mass because of nuclear fusion reactions.
Isolated stars do not gain mass. Fusion reactions convert lighter elements into heavier
elements, but each heavier nucleus has less mass than the sum of the lighter nuclei that
combined to form it. The difference in mass has been converted into energy, which
ultimately is radiated from the star, so the star in fact loses mass through fusion reactions.
7. In the core of a red giant star of mass smaller than 2 solar masses, the behavior of the
gas is governed by electron quantum mechanical effects that produce electron degeneracy
pressure. The pressure of this gas
• does not increase as its temperature increases.
• depends on temperature, an increase in temperature leading to an increase in
pressure.
• decreases inversely as the temperature increases.
This electron degeneracy pressure leads to the helium flash in low-mass red giants.
8. The "helium flash" occurs in a 2-solar-mass star when
• the core temperature becomes sufficiently high for helium fusion for the first
time.
• the core heats up from helium fusion and the gas does not expand to cool down
and stabilize the temperature because the gas properties are controlled by
degenerate electrons.
• convection within the star carries helium to the surface, where it shows its own
characteristic spectrum for the first time.
Degenerate pressure of electrons is independent of temperature, so heating the gas does
not produce expansion that, in a normal gas, would cool and stabilize the gas temperature
like a thermostat.
9. A globular cluster of stars is judged to be
• young because it contains only young, red stars that have not yet reached the
hydrogen fusion stage of evolution.
• intermediate in age because there are many evolved red stars in the cluster, but
there is no evidence of any remnant dust or gas from the early stages of star
formation.
• old because it contains no hot and blue, luminous main-sequence stars.
These are the oldest systems in the universe, containing only highly evolved stars.
10. The Hertzsprung-Russell diagram of a globular cluster contains stars along a line
called the horizontal branch. These stars are:
• stars converting helium to carbon in their cores.
• main-sequence stars.
• pre-helium-flash stars converting hydrogen to helium in a shell around the core.
Horizontal branch stars are post-helium-flash stars with a helium-burning core and a
hydrogen-burning shell.
11. If we were to be able to watch the evolution of a cluster of stars from their birth to a
time when many of them were well evolved and plotted the positions of stars on a
Hertzsprung-Russell diagram, what would happen to the main sequence as time
progressed?
• The main sequence would slowly move across the Hertzsprung-Russell diagram
from left to right, from high to low temperatures.
• The main sequence would become progressively shorter as low-mass stars
disappeared from the lower, red end.
• The main sequence would become progressively shorter, with stars
disappearing from its upper end.
Massive, very luminous stars spend only a very short time on the main sequence before
evolving toward the giant/supergiant phase. The age of a cluster can be estimated by
noting the position of the upper end of the remaining main sequence on its HertsprungRussell diagram.
12. Which of the following methods is the most accurate for measuring the age of a star
cluster?
• Determining the turn-off point from the main sequence in its HertzsprungRussell diagram
• Measuring the amount of gas and dust in the cluster
• Observing the strength of the metal lines in the spectra of its stars
Brighter stars on the main sequence are more massive, so they have shorter lives. Stars at
the turn-off point are just finishing their main-sequence lives, so the age of the cluster is
equal to the main-sequence lifetime of those stars. This age is known from stellar
evolution calculations for all points on the main sequence.
13. What is the fundamental difference between RR Lyrae variable stars and Cepheid
variable stars?
• Their evolutionary stage after main-sequence: RR Lyrae stars are helium coreburning stars, and Cepheids are red giants that have not yet ignited helium in their
cores.
• Their masses: Cepheids are high-mass stars, and RR Lyrae stars are low-mass
stars.
• Their ages: Cepheids are post-main-sequence stars, and RR Lyrae stars are still on
the main sequence.
Both Cepheids and RR Lyrae stars are post-main-sequence, post-helium-ignition stars
and differ primarily in their masses.
14. The reason Cepheid variable stars pulsate in size is that
• ionization below the surface traps heat when the star contracts, and this heat
gives an extra push as the star expands.
• convection currents below the surface generate seismic waves that make the star
oscillate in size.
• nuclear reactions in the hydrogen-burning shell intensify when the star contracts,
and the extra heat gives the gas a push as it expands.
The star is like the gasoline engine in a car in the sense that extra heat is put into the gas
during compression, and this drives the outward expansion more strongly than just by
rebound alone.
15. Cepheid variable stars are important because we can
• measure their distances by measuring only their periods and apparent
magnitudes.
• find their absolute magnitudes after measuring only their apparent magnitudes.
• measure their distances by measuring only their periods and absolute magnitudes.
Their periods and the period-luminosity law give us absolute magnitudes, and from these
absolute magnitudes and their apparent magnitudes, we can then find their distances.