Comins DEU 3e Ch 09 Quiz 2
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 Hertsprung-
Russell 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 Hertzsprung-
Russell 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.